==========
Nim Manual
==========

:Authors: Andreas Rumpf, Zahary Karadjov
:Version: |nimversion|

.. default-role:: code
.. include:: rstcommon.rst
.. contents::


> "Complexity" seems to be a lot like "energy": you can transfer it from the
> end-user to one/some of the other players, but the total amount seems to remain
> pretty much constant for a given task. -- Ran


About this document
===================

**Note**: This document is a draft! Several of Nim's features may need more
precise wording. This manual is constantly evolving into a proper specification.

**Note**: The experimental features of Nim are
covered [here](manual_experimental.html).

**Note**: Assignments, moves, and destruction are specified in
the [destructors](destructors.html) document.


This document describes the lexis, the syntax, and the semantics of the Nim language.

To learn how to compile Nim programs and generate documentation see
the [Compiler User Guide](nimc.html) and the [DocGen Tools Guide](docgen.html).

The language constructs are explained using an extended BNF, in which `(a)*`
means 0 or more `a`'s, `a+` means 1 or more `a`'s, and `(a)?` means an
optional *a*. Parentheses may be used to group elements.

`&` is the lookahead operator; `&a` means that an `a` is expected but
not consumed. It will be consumed in the following rule.

The `|`, `/` symbols are used to mark alternatives and have the lowest
precedence. `/` is the ordered choice that requires the parser to try the
alternatives in the given order. `/` is often used to ensure the grammar
is not ambiguous.

Non-terminals start with a lowercase letter, abstract terminal symbols are in
UPPERCASE. Verbatim terminal symbols (including keywords) are quoted
with `'`. An example:

    ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)?

The binary `^*` operator is used as a shorthand for 0 or more occurrences
separated by its second argument; likewise `^+` means 1 or more
occurrences: `a ^+ b` is short for `a (b a)*`
and `a ^* b` is short for `(a (b a)*)?`. Example:

    arrayConstructor = '[' expr ^* ',' ']'

Other parts of Nim, like scoping rules or runtime semantics, are
described informally.




Definitions
===========

Nim code specifies a computation that acts on a memory consisting of
components called `locations`:idx:. A variable is basically a name for a
location. Each variable and location is of a certain `type`:idx:. The
variable's type is called `static type`:idx:, the location's type is called
`dynamic type`:idx:. If the static type is not the same as the dynamic type,
it is a super-type or subtype of the dynamic type.

An `identifier`:idx: is a symbol declared as a name for a variable, type,
procedure, etc. The region of the program over which a declaration applies is
called the `scope`:idx: of the declaration. Scopes can be nested. The meaning
of an identifier is determined by the smallest enclosing scope in which the
identifier is declared unless overloading resolution rules suggest otherwise.

An expression specifies a computation that produces a value or location.
Expressions that produce locations are called `l-values`:idx:. An l-value
can denote either a location or the value the location contains, depending on
the context.

A Nim `program`:idx: consists of one or more text `source files`:idx: containing
Nim code. It is processed by a Nim `compiler`:idx: into an `executable`:idx:.
The nature of this executable depends on the compiler implementation; it may,
for example, be a native binary or JavaScript source code.

In a typical Nim program, most of the code is compiled into the executable.
However, some code may be executed at
`compile-time`:idx:. This can include constant expressions, macro definitions,
and Nim procedures used by macro definitions. Most of the Nim language is
supported at compile-time, but there are some restrictions -- see [Restrictions
on Compile-Time Execution] for
details. We use the term `runtime`:idx: to cover both compile-time execution
and code execution in the executable.

The compiler parses Nim source code into an internal data structure called the
`abstract syntax tree`:idx: (`AST`:idx:). Then, before executing the code or
compiling it into the executable, it transforms the AST through
`semantic analysis`:idx:. This adds semantic information such as expression types,
identifier meanings, and in some cases expression values. An error detected
during semantic analysis is called a `static error`:idx:. Errors described in
this manual are static errors when not otherwise specified.

A `panic`:idx: is an error that the implementation detects
and reports at runtime. The method for reporting such errors is via
*raising exceptions* or *dying with a fatal error*. However, the implementation
provides a means to disable these `runtime checks`:idx:. See the section
[Pragmas] for details.

Whether a panic results in an exception or in a fatal error is
implementation specific. Thus, the following program is invalid; even though the
code purports to catch the `IndexDefect` from an out-of-bounds array access, the
compiler may instead choose to allow the program to die with a fatal error.

  ```nim
  var a: array[0..1, char]
  let i = 5
  try:
    a[i] = 'N'
  except IndexDefect:
    echo "invalid index"
  ```

The current implementation allows switching between these different behaviors
via `--panics:on|off`:option:. When panics are turned on, the program dies with a
panic, if they are turned off the runtime errors are turned into
exceptions. The benefit of `--panics:on`:option: is that it produces smaller binary
code and the compiler has more freedom to optimize the code.

An `unchecked runtime error`:idx: is an error that is not guaranteed to be
detected and can cause the subsequent behavior of the computation to
be arbitrary. Unchecked runtime errors cannot occur if only `safe`:idx:
language features are used and if no runtime checks are disabled.

A `constant expression`:idx: is an expression whose value can be computed during
a semantic analysis of the code in which it appears. It is never an l-value and
never has side effects. Constant expressions are not limited to the capabilities
of semantic analysis, such as constant folding; they can use all Nim language
features that are supported for compile-time execution. Since constant
expressions can be used as an input to semantic analysis (such as for defining
array bounds), this flexibility requires the compiler to interleave semantic
analysis and compile-time code execution.

It is mostly accurate to picture semantic analysis proceeding top to bottom and
left to right in the source code, with compile-time code execution interleaved
when necessary to compute values that are required for subsequent semantic
analysis. We will see much later in this document that macro invocation not only
requires this interleaving, but also creates a situation where semantic analysis
does not entirely proceed top to bottom and left to right.


Lexical Analysis
================

Encoding
--------

All Nim source files are in the UTF-8 encoding (or its ASCII subset). Other
encodings are not supported. Any of the standard platform line termination
sequences can be used - the Unix form using ASCII LF (linefeed), the Windows
form using the ASCII sequence CR LF (return followed by linefeed), or the old
Macintosh form using the ASCII CR (return) character. All of these forms can be
used equally, regardless of the platform.


Indentation
-----------

Nim's standard grammar describes an `indentation sensitive`:idx: language.
This means that all the control structures are recognized by indentation.
Indentation consists only of spaces; tabulators are not allowed.

The indentation handling is implemented as follows: The lexer annotates the
following token with the preceding number of spaces; indentation is not
a separate token. This trick allows parsing of Nim with only 1 token of
lookahead.

The parser uses a stack of indentation levels: the stack consists of integers
counting the spaces. The indentation information is queried at strategic
places in the parser but ignored otherwise: The pseudo-terminal `IND{>}`
denotes an indentation that consists of more spaces than the entry at the top
of the stack; `IND{=}` an indentation that has the same number of spaces. `DED`
is another pseudo terminal that describes the *action* of popping a value
from the stack, `IND{>}` then implies to push onto the stack.

With this notation we can now easily define the core of the grammar: A block of
statements (simplified example):

    ifStmt = 'if' expr ':' stmt
             (IND{=} 'elif' expr ':' stmt)*
             (IND{=} 'else' ':' stmt)?

    simpleStmt = ifStmt / ...

    stmt = IND{>} stmt ^+ IND{=} DED  # list of statements
         / simpleStmt                 # or a simple statement



Comments
--------

Comments start anywhere outside a string or character literal with the
hash character `#`.
Comments consist of a concatenation of `comment pieces`:idx:. A comment piece
starts with `#` and runs until the end of the line. The end of line characters
belong to the piece. If the next line only consists of a comment piece with
no other tokens between it and the preceding one, it does not start a new
comment:


  ```nim
  i = 0     # This is a single comment over multiple lines.
    # The lexer merges these two pieces.
    # The comment continues here.
  ```


`Documentation comments`:idx: are comments that start with two `##`.
Documentation comments are tokens; they are only allowed at certain places in
the input file as they belong to the syntax tree.


Multiline comments
------------------

Starting with version 0.13.0 of the language Nim supports multiline comments.
They look like:

  ```nim
  #[Comment here.
  Multiple lines
  are not a problem.]#
  ```

Multiline comments support nesting:

  ```nim
  #[  #[ Multiline comment in already
     commented out code. ]#
  proc p[T](x: T) = discard
  ]#
  ```

Multiline documentation comments also exist and support nesting too:

  ```nim
  proc foo =
    ##[Long documentation comment
       here.
    ]##
  ```

You can also use the [discard statement](#statements-and-expressions-discard-statement) together with
[triple quoted string literals](#lexical-analysis-triple-quoted-string-literals) to create multiline comments:

  ```nim
  discard """ You can have any Nim code text commented
  out inside this with no indentation restrictions.
        yes("May I ask a pointless question?") """
  ```

This was how multiline comments were done before version 0.13.0,
and it is used to provide specifications to [testament](testament.html#writing-unit-tests) test framework.


Identifiers & Keywords
----------------------

Identifiers in Nim can be any string of letters, digits
and underscores, with the following restrictions:

* begins with a letter
* does not end with an underscore `_`
* two immediate following underscores `__` are not allowed:

  ```
  letter ::= 'A'..'Z' | 'a'..'z' | '\x80'..'\xff'
  digit ::= '0'..'9'
  IDENTIFIER ::= letter ( ['_'] (letter | digit) )*
  ```

Currently, any Unicode character with an ordinal value > 127 (non-ASCII) is
classified as a `letter` and may thus be part of an identifier but later
versions of the language may assign some Unicode characters to belong to the
operator characters instead.

The following keywords are reserved and cannot be used as identifiers:

  ```nim file="keywords.txt"
  ```

Some keywords are unused; they are reserved for future developments of the
language.


Identifier equality
-------------------

Two identifiers are considered equal if the following algorithm returns true:

  ```nim
  proc sameIdentifier(a, b: string): bool =
    a[0] == b[0] and
      a.replace("_", "").toLowerAscii == b.replace("_", "").toLowerAscii
  ```

That means only the first letters are compared in a case-sensitive manner. Other
letters are compared case-insensitively within the ASCII range and underscores are ignored.

This rather unorthodox way to do identifier comparisons is called
`partial case-insensitivity`:idx: and has some advantages over the conventional
case sensitivity:

It allows programmers to mostly use their own preferred
spelling style, be it humpStyle or snake_style, and libraries written
by different programmers cannot use incompatible conventions.
A Nim-aware editor or IDE can show the identifiers as preferred.
Another advantage is that it frees the programmer from remembering
the exact spelling of an identifier. The exception with respect to the first
letter allows common code like `var foo: Foo` to be parsed unambiguously.

Note that this rule also applies to keywords, meaning that `notin` is
the same as `notIn` and `not_in` (all-lowercase version (`notin`, `isnot`)
is the preferred way of writing keywords).

Historically, Nim was a fully `style-insensitive`:idx: language. This meant that
it was not case-sensitive and underscores were ignored and there was not even a
distinction between `foo` and `Foo`.


Keywords as identifiers
-----------------------

If a keyword is enclosed in backticks it loses its keyword property and becomes an ordinary identifier.

Examples

  ```nim
  var `var` = "Hello Stropping"
  ```

  ```nim
  type Obj = object
    `type`: int

  let `object` = Obj(`type`: 9)
  assert `object` is Obj
  assert `object`.`type` == 9

  var `var` = 42
  let `let` = 8
  assert `var` + `let` == 50

  const `assert` = true
  assert `assert`
  ```


String literals
---------------

Terminal symbol in the grammar: `STR_LIT`.

String literals can be delimited by matching double quotes, and can
contain the following `escape sequences`:idx:\ :

==================         ===================================================
  Escape sequence          Meaning
==================         ===================================================
  ``\p``                   platform specific newline: CRLF on Windows,
                           LF on Unix
  ``\r``, ``\c``           `carriage return`:idx:
  ``\n``, ``\l``           `line feed`:idx: (often called `newline`:idx:)
  ``\f``                   `form feed`:idx:
  ``\t``                   `tabulator`:idx:
  ``\v``                   `vertical tabulator`:idx:
  ``\\``                   `backslash`:idx:
  ``\"``                   `quotation mark`:idx:
  ``\'``                   `apostrophe`:idx:
  ``\`` '0'..'9'+          `character with decimal value d`:idx:;
                           all decimal digits directly
                           following are used for the character
  ``\a``                   `alert`:idx:
  ``\b``                   `backspace`:idx:
  ``\e``                   `escape`:idx: `[ESC]`:idx:
  ``\x`` HH                `character with hex value HH`:idx:;
                           exactly two hex digits are allowed
  ``\u`` HHHH              `unicode codepoint with hex value HHHH`:idx:;
                           exactly four hex digits are allowed
  ``\u`` {H+}              `unicode codepoint`:idx:;
                           all hex digits enclosed in `{}` are used for
                           the codepoint
==================         ===================================================


Strings in Nim may contain any 8-bit value, even embedded zeros. However,
some operations may interpret the first binary zero as a terminator.


Triple quoted string literals
-----------------------------

Terminal symbol in the grammar: `TRIPLESTR_LIT`.

String literals can also be delimited by three double quotes `"""` ... `"""`.
Literals in this form may run for several lines, may contain `"` and do not
interpret any escape sequences.
For convenience, when the opening `"""` is followed by a newline (there may
be whitespace between the opening `"""` and the newline),
the newline (and the preceding whitespace) is not included in the string. The
ending of the string literal is defined by the pattern `"""[^"]`, so this:

  ```nim
  """"long string within quotes""""
  ```

Produces:

    "long string within quotes"


Raw string literals
-------------------

Terminal symbol in the grammar: `RSTR_LIT`.

There are also raw string literals that are preceded with the
letter `r` (or `R`) and are delimited by matching double quotes (just
like ordinary string literals) and do not interpret the escape sequences.
This is especially convenient for regular expressions or Windows paths:

  ```nim
  var f = openFile(r"C:\texts\text.txt") # a raw string, so ``\t`` is no tab
  ```

To produce a single `"` within a raw string literal, it has to be doubled:

  ```nim
  r"a""b"
  ```

Produces:

    a"b

`r""""` is not possible with this notation, because the three leading
quotes introduce a triple quoted string literal. `r"""` is the same
as `"""` since triple quoted string literals do not interpret escape
sequences either.


Generalized raw string literals
-------------------------------

Terminal symbols in the grammar: `GENERALIZED_STR_LIT`,
`GENERALIZED_TRIPLESTR_LIT`.

The construct `identifier"string literal"` (without whitespace between the
identifier and the opening quotation mark) is a
generalized raw string literal. It is a shortcut for the construct
`identifier(r"string literal")`, so it denotes a routine call with a
raw string literal as its only argument. Generalized raw string literals
are especially convenient for embedding mini languages directly into Nim
(for example regular expressions).

The construct `identifier"""string literal"""` exists too. It is a shortcut
for `identifier("""string literal""")`.


Character literals
------------------

Character literals are enclosed in single quotes `''` and can contain the
same escape sequences as strings - with one exception: the platform
dependent `newline`:idx: (``\p``)
is not allowed as it may be wider than one character (it can be the pair
CR/LF). Here are the valid `escape sequences`:idx: for character
literals:

==================         ===================================================
  Escape sequence          Meaning
==================         ===================================================
  ``\r``, ``\c``           `carriage return`:idx:
  ``\n``, ``\l``           `line feed`:idx:
  ``\f``                   `form feed`:idx:
  ``\t``                   `tabulator`:idx:
  ``\v``                   `vertical tabulator`:idx:
  ``\\``                   `backslash`:idx:
  ``\"``                   `quotation mark`:idx:
  ``\'``                   `apostrophe`:idx:
  ``\`` '0'..'9'+          `character with decimal value d`:idx:;
                           all decimal digits directly
                           following are used for the character
  ``\a``                   `alert`:idx:
  ``\b``                   `backspace`:idx:
  ``\e``                   `escape`:idx: `[ESC]`:idx:
  ``\x`` HH                `character with hex value HH`:idx:;
                           exactly two hex digits are allowed
==================         ===================================================

A character is not a Unicode character but a single byte.

Rationale: It enables the efficient support of `array[char, int]` or
`set[char]`.

The `Rune` type can represent any Unicode character.
`Rune` is declared in the [unicode module](unicode.html).

A character literal that does not end in `'` is interpreted as `'` if there
is a preceding backtick token. There must be no whitespace between the preceding
backtick token and the character literal. This special case ensures that a declaration
like ``proc `'customLiteral`(s: string)`` is valid. ``proc `'customLiteral`(s: string)``
is the same as ``proc `'\''customLiteral`(s: string)``.

See also [custom numeric literals].


Numeric literals
----------------

Numeric literals have the form:

    hexdigit = digit | 'A'..'F' | 'a'..'f'
    octdigit = '0'..'7'
    bindigit = '0'..'1'
    unary_minus = '-' # See the section about unary minus
    HEX_LIT = unary_minus? '0' ('x' | 'X' ) hexdigit ( ['_'] hexdigit )*
    DEC_LIT = unary_minus? digit ( ['_'] digit )*
    OCT_LIT = unary_minus? '0' 'o' octdigit ( ['_'] octdigit )*
    BIN_LIT = unary_minus? '0' ('b' | 'B' ) bindigit ( ['_'] bindigit )*

    INT_LIT = HEX_LIT
            | DEC_LIT
            | OCT_LIT
            | BIN_LIT

    INT8_LIT = INT_LIT ['\''] ('i' | 'I') '8'
    INT16_LIT = INT_LIT ['\''] ('i' | 'I') '16'
    INT32_LIT = INT_LIT ['\''] ('i' | 'I') '32'
    INT64_LIT = INT_LIT ['\''] ('i' | 'I') '64'

    UINT_LIT = INT_LIT ['\''] ('u' | 'U')
    UINT8_LIT = INT_LIT ['\''] ('u' | 'U') '8'
    UINT16_LIT = INT_LIT ['\''] ('u' | 'U') '16'
    UINT32_LIT = INT_LIT ['\''] ('u' | 'U') '32'
    UINT64_LIT = INT_LIT ['\''] ('u' | 'U') '64'

    exponent = ('e' | 'E' ) ['+' | '-'] digit ( ['_'] digit )*
    FLOAT_LIT = unary_minus? digit (['_'] digit)* (('.' digit (['_'] digit)* [exponent]) |exponent)
    FLOAT32_SUFFIX = ('f' | 'F') ['32']
    FLOAT32_LIT = HEX_LIT '\'' FLOAT32_SUFFIX
                | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT32_SUFFIX
    FLOAT64_SUFFIX = ( ('f' | 'F') '64' ) | 'd' | 'D'
    FLOAT64_LIT = HEX_LIT '\'' FLOAT64_SUFFIX
                | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT64_SUFFIX

    CUSTOM_NUMERIC_LIT = (FLOAT_LIT | INT_LIT) '\'' CUSTOM_NUMERIC_SUFFIX

    # CUSTOM_NUMERIC_SUFFIX is any Nim identifier that is not
    # a pre-defined type suffix.


As can be seen in the productions, numeric literals can contain underscores
for readability. Integer and floating-point literals may be given in decimal (no
prefix), binary (prefix `0b`), octal (prefix `0o`), and hexadecimal
(prefix `0x`) notation.

The fact that the unary minus `-` in a number literal like `-1` is considered
to be part of the literal is a late addition to the language. The rationale is that
an expression `-128'i8` should be valid and without this special case, this would
be impossible -- `128` is not a valid `int8` value, only `-128` is.

For the `unary_minus` rule there are further restrictions that are not covered
in the formal grammar. For `-` to be part of the number literal the immediately
preceding character has to be in the
set `{' ', '\t', '\n', '\r', ',', ';', '(', '[', '{'}`. This set was designed to
cover most cases in a natural manner.

In the following examples, `-1` is a single token:

  ```nim
  echo -1
  echo(-1)
  echo [-1]
  echo 3,-1

  "abc";-1
  ```

In the following examples, `-1` is parsed as two separate tokens
(as `-`:tok: `1`:tok:):

  ```nim
  echo x-1
  echo (int)-1
  echo [a]-1
  "abc"-1
  ```


The suffix starting with an apostrophe ('\'') is called a
`type suffix`:idx:. Literals without a type suffix are of an integer type
unless the literal contains a dot or `E|e` in which case it is of
type `float`. This integer type is `int` if the literal is in the range
`low(int32)..high(int32)`, otherwise it is `int64`.
For notational convenience, the apostrophe of a type suffix
is optional if it is not ambiguous (only hexadecimal floating-point literals
with a type suffix can be ambiguous).


The pre-defined type suffixes are:

=================    =========================
  Type Suffix        Resulting type of literal
=================    =========================
  `'i8`              int8
  `'i16`             int16
  `'i32`             int32
  `'i64`             int64
  `'u`               uint
  `'u8`              uint8
  `'u16`             uint16
  `'u32`             uint32
  `'u64`             uint64
  `'f`               float32
  `'d`               float64
  `'f32`             float32
  `'f64`             float64
=================    =========================

Floating-point literals may also be in binary, octal or hexadecimal
notation:
`0B0_10001110100_0000101001000111101011101111111011000101001101001001'f64`
is approximately 1.72826e35 according to the IEEE floating-point standard.

Literals must match the datatype, for example, `333'i8` is an invalid literal.
Non-base-10 literals are used mainly for flags and bit pattern representations,
therefore the checking is done on bit width and not on value range.
Hence: 0b10000000'u8 == 0x80'u8 == 128, but, 0b10000000'i8 == 0x80'i8 == -1
instead of causing an overflow error.


### Custom numeric literals

If the suffix is not predefined, then the suffix is assumed to be a call
to a proc, template, macro or other callable identifier that is passed the
string containing the literal. The callable identifier needs to be declared
with a special ``'`` prefix:

  ```nim
  import std/strutils
  type u4 = distinct uint8 # a 4-bit unsigned integer aka "nibble"
  proc `'u4`(n: string): u4 =
    # The leading ' is required.
    result = (parseInt(n) and 0x0F).u4

  var x = 5'u4
  ```

More formally, a custom numeric literal `123'custom` is transformed
to r"123".`'custom` in the parsing step. There is no AST node kind that
corresponds to this transformation. The transformation naturally handles
the case that additional parameters are passed to the callee:

  ```nim
  import std/strutils
  type u4 = distinct uint8 # a 4-bit unsigned integer aka "nibble"
  proc `'u4`(n: string; moreData: int): u4 =
    result = (parseInt(n) and 0x0F).u4

  var x = 5'u4(123)
  ```

Custom numeric literals are covered by the grammar rule named `CUSTOM_NUMERIC_LIT`.
A custom numeric literal is a single token.


Operators
---------

Nim allows user defined operators. An operator is any combination of the
following characters:

       =     +     -     *     /     <     >
       @     $     ~     &     %     |
       !     ?     ^     .     :     \

(The grammar uses the terminal OPR to refer to operator symbols as
defined here.)

These keywords are also operators:
`and or not xor shl shr div mod in notin is isnot of as from`.

`.`:tok:, `=`:tok:, `:`:tok:, `::`:tok: are not available as general operators; they
are used for other notational purposes.

`*:` is as a special case treated as the two tokens `*`:tok: and `:`:tok:
(to support `var v*: T`).

The `not` keyword is always a unary operator, `a not b` is parsed
as `a(not b)`, not as `(a) not (b)`.

Unicode Operators
-----------------

These Unicode operators are also parsed as operators:

    ∙ ∘ × ★ ⊗ ⊘ ⊙ ⊛ ⊠ ⊡ ∩ ∧ ⊓   # same priority as * (multiplication)
    ± ⊕ ⊖ ⊞ ⊟ ∪ ∨ ⊔             # same priority as + (addition)


Unicode operators can be combined with non-Unicode operator
symbols. The usual precedence extensions then apply, for example, `⊠=` is an
assignment like operator just like `*=` is.

No Unicode normalization step is performed.


Other tokens
------------

The following strings denote other tokens:

    `   (    )     {    }     [    ]    ,  ;   [.    .]  {.   .}  (.  .)  [:


The `slice`:idx: operator `..`:tok: takes precedence over other tokens that
contain a dot: `{..}` are the three tokens `{`:tok:, `..`:tok:, `}`:tok:
and not the two tokens `{.`:tok:, `.}`:tok:.


Syntax
======

This section lists Nim's standard syntax. How the parser handles
the indentation is already described in the [Lexical Analysis] section.

Nim allows user-definable operators.
Binary operators have 11 different levels of precedence.



Associativity
-------------

Binary operators whose first character is `^` are right-associative, all
other binary operators are left-associative.

  ```nim
  proc `^/`(x, y: float): float =
    # a right-associative division operator
    result = x / y
  echo 12 ^/ 4 ^/ 8 # 24.0 (4 / 8 = 0.5, then 12 / 0.5 = 24.0)
  echo 12  / 4  / 8 # 0.375 (12 / 4 = 3.0, then 3 / 8 = 0.375)
  ```

Precedence
----------

Unary operators always bind stronger than any binary
operator: `$a + b` is `($a) + b` and not `$(a + b)`.

If a unary operator's first character is `@` it is a `sigil-like`:idx:
operator which binds stronger than a `primarySuffix`: `@x.abc` is parsed
as `(@x).abc` whereas `$x.abc` is parsed as `$(x.abc)`.


For binary operators that are not keywords, the precedence is determined by the
following rules:

Operators ending in either `->`, `~>` or `=>` are called
`arrow like`:idx:, and have the lowest precedence of all operators.

If the operator ends with `=` and its first character is none of
`<`, `>`, `!`, `=`, `~`, `?`, it is an *assignment operator* which
has the second-lowest precedence.

Otherwise, precedence is determined by the first character.


================  =======================================================  ==================  ===============
Precedence level    Operators                                              First character     Terminal symbol
================  =======================================================  ==================  ===============
 10 (highest)                                                              `$  ^`              OP10
  9               `*    /    div   mod   shl  shr  %`                      `*  %  \  /`        OP9
  8               `+    -`                                                 `+  -  ~  |`        OP8
  7               `&`                                                      `&`                 OP7
  6               `..`                                                     `.`                 OP6
  5               `==  <= < >= > !=  in notin is isnot not of as from`     `=  <  >  !`        OP5
  4               `and`                                                                        OP4
  3               `or xor`                                                                     OP3
  2                                                                        `@  :  ?`           OP2
  1               *assignment operator* (like `+=`, `*=`)                                      OP1
  0 (lowest)      *arrow like operator* (like `->`, `=>`)                                      OP0
================  =======================================================  ==================  ===============


Whether an operator is used as a prefix operator is also affected by preceding
whitespace (this parsing change was introduced with version 0.13.0):

  ```nim
  echo $foo
  # is parsed as
  echo($foo)
  ```


Spacing also determines whether `(a, b)` is parsed as an argument list
of a call or whether it is parsed as a tuple constructor:

  ```nim
  echo(1, 2) # pass 1 and 2 to echo
  ```

  ```nim
  echo (1, 2) # pass the tuple (1, 2) to echo
  ```

Dot-like operators
------------------

Terminal symbol in the grammar: `DOTLIKEOP`.

Dot-like operators are operators starting with `.`, but not with `..`, for e.g. `.?`;
they have the same precedence as `.`, so that `a.?b.c` is parsed as `(a.?b).c` instead of `a.?(b.c)`.


Grammar
-------

The grammar's start symbol is `module`.

.. include:: grammar.txt
   :literal:



Order of evaluation
===================

Order of evaluation is strictly left-to-right, inside-out as it is typical for most others
imperative programming languages:

  ```nim  test = "nim c $1"
  var s = ""

  proc p(arg: int): int =
    s.add $arg
    result = arg

  discard p(p(1) + p(2))

  doAssert s == "123"
  ```


Assignments are not special, the left-hand-side expression is evaluated before the
right-hand side:

  ```nim  test = "nim c $1"
  var v = 0
  proc getI(): int =
    result = v
    inc v

  var a, b: array[0..2, int]

  proc someCopy(a: var int; b: int) = a = b

  a[getI()] = getI()

  doAssert a == [1, 0, 0]

  v = 0
  someCopy(b[getI()], getI())

  doAssert b == [1, 0, 0]
  ```


Rationale: Consistency with overloaded assignment or assignment-like operations,
`a = b` can be read as `performSomeCopy(a, b)`.


However, the concept of "order of evaluation" is only applicable after the code
was normalized: The normalization involves template expansions and argument
reorderings that have been passed to named parameters:

  ```nim  test = "nim c $1"
  var s = ""

  proc p(): int =
    s.add "p"
    result = 5

  proc q(): int =
    s.add "q"
    result = 3

  # Evaluation order is 'b' before 'a' due to template
  # expansion's semantics.
  template swapArgs(a, b): untyped =
    b + a

  doAssert swapArgs(p() + q(), q() - p()) == 6
  doAssert s == "qppq"

  # Evaluation order is not influenced by named parameters:
  proc construct(first, second: int) =
    discard

  # 'p' is evaluated before 'q'!
  construct(second = q(), first = p())

  doAssert s == "qppqpq"
  ```


Rationale: This is far easier to implement than hypothetical alternatives.


Constants and Constant Expressions
==================================

A `constant`:idx: is a symbol that is bound to the value of a constant
expression. Constant expressions are restricted to depend only on the following
categories of values and operations, because these are either built into the
language or declared and evaluated before semantic analysis of the constant
expression:

* literals
* built-in operators
* previously declared constants and compile-time variables
* previously declared macros and templates
* previously declared procedures that have no side effects beyond
  possibly modifying compile-time variables

A constant expression can contain code blocks that may internally use all Nim
features supported at compile time (as detailed in the next section below).
Within such a code block, it is possible to declare variables and then later
read and update them, or declare variables and pass them to procedures that
modify them. However, the code in such a block must still adhere to the
restrictions listed above for referencing values and operations outside the
block.

The ability to access and modify compile-time variables adds flexibility to
constant expressions that may be surprising to those coming from other
statically typed languages. For example, the following code echoes the beginning
of the Fibonacci series **at compile-time**. (This is a demonstration of
flexibility in defining constants, not a recommended style for solving this
problem.)

  ```nim  test = "nim c $1"
  import std/strformat

  var fibN {.compileTime.}: int
  var fibPrev {.compileTime.}: int
  var fibPrevPrev {.compileTime.}: int

  proc nextFib(): int =
    result = if fibN < 2:
      fibN
    else:
      fibPrevPrev + fibPrev
    inc(fibN)
    fibPrevPrev = fibPrev
    fibPrev = result

  const f0 = nextFib()
  const f1 = nextFib()

  const displayFib = block:
    const f2 = nextFib()
    var result = fmt"Fibonacci sequence: {f0}, {f1}, {f2}"
    for i in 3..12:
      add(result, fmt", {nextFib()}")
    result

  static:
    echo displayFib
  ```


Restrictions on Compile-Time Execution
======================================

Nim code that will be executed at compile time cannot use the following
language features:

* methods
* closure iterators
* the `cast` operator
* reference (pointer) types
* FFI

The use of wrappers that use FFI and/or `cast` is also disallowed. Note that
these wrappers include the ones in the standard libraries.

Some or all of these restrictions are likely to be lifted over time.


Types
=====

All expressions have a type that is known during semantic analysis. Nim
is statically typed. One can declare new types, which is in essence defining
an identifier that can be used to denote this custom type.

These are the major type classes:

* ordinal types (consist of integer, bool, character, enumeration
  (and subranges thereof) types)
* floating-point types
* string type
* structured types
* reference (pointer) type
* procedural type
* generic type


Ordinal types
-------------
Ordinal types have the following characteristics:

- Ordinal types are countable and ordered. This property allows the operation
  of functions such as `inc`, `ord`, and `dec` on ordinal types to
  be defined.
- Ordinal types have a smallest possible value, accessible with `low(type)`.
  Trying to count further down than the smallest value produces a panic or
  a static error.
- Ordinal types have a largest possible value, accessible with `high(type)`.
  Trying to count further up than the largest value produces a panic or
  a static error.

Integers, bool, characters, and enumeration types (and subranges of these
types) belong to ordinal types.

A distinct type is an ordinal type if its base type is an ordinal type.


Pre-defined integer types
-------------------------
These integer types are pre-defined:

`int`
: the generic signed integer type; its size is platform-dependent and has the
  same size as a pointer. This type should be used in general. An integer
  literal that has no type suffix is of this type if it is in the range
  `low(int32)..high(int32)` otherwise the literal's type is `int64`.

`int`\ XX
: additional signed integer types of XX bits use this naming scheme
  (example: int16 is a 16-bit wide integer).
  The current implementation supports `int8`, `int16`, `int32`, `int64`.
  Literals of these types have the suffix 'iXX.

`uint`
: the generic `unsigned integer`:idx: type; its size is platform-dependent and
  has the same size as a pointer. An integer literal with the type
  suffix `'u` is of this type.

`uint`\ XX
: additional unsigned integer types of XX bits use this naming scheme
  (example: uint16 is a 16-bit wide unsigned integer).
  The current implementation supports `uint8`, `uint16`, `uint32`,
  `uint64`. Literals of these types have the suffix 'uXX.
  Unsigned operations all wrap around; they cannot lead to over- or
  underflow errors.


In addition to the usual arithmetic operators for signed and unsigned integers
(`+ - *` etc.) there are also operators that formally work on *signed*
integers but treat their arguments as *unsigned*: They are mostly provided
for backwards compatibility with older versions of the language that lacked
unsigned integer types. These unsigned operations for signed integers use
the `%` suffix as convention:


======================   ======================================================
operation                meaning
======================   ======================================================
`a +% b`                 unsigned integer addition
`a -% b`                 unsigned integer subtraction
`a *% b`                 unsigned integer multiplication
`a /% b`                 unsigned integer division
`a %% b`                 unsigned integer modulo operation
`a <% b`                 treat `a` and `b` as unsigned and compare
`a <=% b`                treat `a` and `b` as unsigned and compare
======================   ======================================================

`Automatic type conversion`:idx: is performed in expressions where different
kinds of integer types are used: the smaller type is converted to the larger.

A `narrowing type conversion`:idx: converts a larger to a smaller type (for
example `int32 -> int16`). A `widening type conversion`:idx: converts a
smaller type to a larger type (for example `int16 -> int32`). In Nim only
widening type conversions are *implicit*:

  ```nim
  var myInt16 = 5i16
  var myInt: int
  myInt16 + 34     # of type `int16`
  myInt16 + myInt  # of type `int`
  myInt16 + 2i32   # of type `int32`
  ```

However, `int` literals are implicitly convertible to a smaller integer type
if the literal's value fits this smaller type and such a conversion is less
expensive than other implicit conversions, so `myInt16 + 34` produces
an `int16` result.

For further details, see [Convertible relation].


Subrange types
--------------
A subrange type is a range of values from an ordinal or floating-point type (the base
type). To define a subrange type, one must specify its limiting values -- the
lowest and highest value of the type. For example:

  ```nim
  type
    Subrange = range[0..5]
    PositiveFloat = range[0.0..Inf]
    Positive* = range[1..high(int)] # as defined in `system`
  ```


`Subrange` is a subrange of an integer which can only hold the values 0
to 5. `PositiveFloat` defines a subrange of all positive floating-point values.
NaN does not belong to any subrange of floating-point types.
Assigning any other value to a variable of type `Subrange` is a
panic (or a static error if it can be determined during
semantic analysis). Assignments from the base type to one of its subrange types
(and vice versa) are allowed.

A subrange type has the same size as its base type (`int` in the
Subrange example).


Pre-defined floating-point types
--------------------------------

The following floating-point types are pre-defined:

`float`
: the generic floating-point type; its size used to be platform-dependent,
  but now it is always mapped to `float64`.
  This type should be used in general.

`float`\ XX
: an implementation may define additional floating-point types of XX bits using
  this naming scheme (example: `float64` is a 64-bit wide float). The current
  implementation supports `float32` and `float64`. Literals of these types
  have the suffix 'fXX.


Automatic type conversion in expressions with different kinds of floating-point
types is performed: See [Convertible relation] for further details. Arithmetic
performed on floating-point types follows the IEEE standard. Integer types are
not converted to floating-point types automatically and vice versa.

The IEEE standard defines five types of floating-point exceptions:

* Invalid: operations with mathematically invalid operands,
  for example 0.0/0.0, sqrt(-1.0), and log(-37.8).
* Division by zero: divisor is zero and dividend is a finite nonzero number,
  for example 1.0/0.0.
* Overflow: operation produces a result that exceeds the range of the exponent,
  for example MAXDOUBLE+0.0000000000001e308.
* Underflow: operation produces a result that is too small to be represented
  as a normal number, for example, MINDOUBLE * MINDOUBLE.
* Inexact: operation produces a result that cannot be represented with infinite
  precision, for example, 2.0 / 3.0, log(1.1) and 0.1 in input.

The IEEE exceptions are either ignored during execution or mapped to the
Nim exceptions: `FloatInvalidOpDefect`:idx:, `FloatDivByZeroDefect`:idx:,
`FloatOverflowDefect`:idx:, `FloatUnderflowDefect`:idx:,
and `FloatInexactDefect`:idx:.
These exceptions inherit from the `FloatingPointDefect`:idx: base class.

Nim provides the pragmas `nanChecks`:idx: and `infChecks`:idx: to control
whether the IEEE exceptions are ignored or trap a Nim exception:

  ```nim
  {.nanChecks: on, infChecks: on.}
  var a = 1.0
  var b = 0.0
  echo b / b # raises FloatInvalidOpDefect
  echo a / b # raises FloatOverflowDefect
  ```

In the current implementation `FloatDivByZeroDefect` and `FloatInexactDefect`
are never raised. `FloatOverflowDefect` is raised instead of
`FloatDivByZeroDefect`.
There is also a `floatChecks`:idx: pragma that is a short-cut for the
combination of `nanChecks` and `infChecks` pragmas. `floatChecks` are
turned off as default.

The only operations that are affected by the `floatChecks` pragma are
the `+`, `-`, `*`, `/` operators for floating-point types.

An implementation should always use the maximum precision available to evaluate
floating-point values during semantic analysis; this means expressions like
`0.09'f32 + 0.01'f32 == 0.09'f64 + 0.01'f64` that are evaluating during
constant folding are true.


Boolean type
------------
The boolean type is named `bool`:idx: in Nim and can be one of the two
pre-defined values `true` and `false`. Conditions in `while`,
`if`, `elif`, `when`-statements need to be of type `bool`.

This condition holds:

  ```nim
  ord(false) == 0 and ord(true) == 1
  ```

The operators `not, and, or, xor, <, <=, >, >=, !=, ==` are defined
for the bool type. The `and` and `or` operators perform short-cut
evaluation. Example:

  ```nim
  while p != nil and p.name != "xyz":
    # p.name is not evaluated if p == nil
    p = p.next
  ```


The size of the bool type is one byte.


Character type
--------------
The character type is named `char` in Nim. Its size is one byte.
Thus, it cannot represent a UTF-8 character, but a part of it.

The `Rune` type is used for Unicode characters, it can represent any Unicode
character. `Rune` is declared in the [unicode module](unicode.html).




Enumeration types
-----------------
Enumeration types define a new type whose values consist of the ones
specified. The values are ordered. Example:

  ```nim
  type
    Direction = enum
      north, east, south, west
  ```


Now the following holds:

  ```nim
  ord(north) == 0
  ord(east) == 1
  ord(south) == 2
  ord(west) == 3

  # Also allowed:
  ord(Direction.west) == 3
  ```

The implied order is: north < east < south < west. The comparison operators can be used
with enumeration types. Instead of `north` etc., the enum value can also
be qualified with the enum type that it resides in, `Direction.north`.

For better interfacing to other programming languages, the fields of enum
types can be assigned an explicit ordinal value. However, the ordinal values
have to be in ascending order. A field whose ordinal value is not
explicitly given is assigned the value of the previous field + 1.

An explicit ordered enum can have *holes*:

  ```nim
  type
    TokenType = enum
      a = 2, b = 4, c = 89 # holes are valid
  ```

However, it is then not ordinal anymore, so it is impossible to use these
enums as an index type for arrays. The procedures `inc`, `dec`, `succ`
and `pred` are not available for them either.


The compiler supports the built-in stringify operator `$` for enumerations.
The stringify's result can be controlled by explicitly giving the string
values to use:

  ```nim
  type
    MyEnum = enum
      valueA = (0, "my value A"),
      valueB = "value B",
      valueC = 2,
      valueD = (3, "abc")
  ```

As can be seen from the example, it is possible to both specify a field's
ordinal value and its string value by using a tuple. It is also
possible to only specify one of them.

An enum can be marked with the `pure` pragma so that its fields are
added to a special module-specific hidden scope that is only queried
as the last attempt. Only non-ambiguous symbols are added to this scope.
But one can always access these via type qualification written
as `MyEnum.value`:

  ```nim
  type
    MyEnum {.pure.} = enum
      valueA, valueB, valueC, valueD, amb

    OtherEnum {.pure.} = enum
      valueX, valueY, valueZ, amb


  echo valueA # MyEnum.valueA
  echo amb    # Error: Unclear whether it's MyEnum.amb or OtherEnum.amb
  echo MyEnum.amb # OK.
  ```

Enum value names are overloadable, much like routines. If both of the enums
`T` and `U` have a member named `foo`, then the identifier `foo` corresponds
to a choice between `T.foo` and `U.foo`. During overload resolution,
the correct type of `foo` is decided from the context. If the type of `foo` is
ambiguous, a static error will be produced.

  ```nim  test = "nim c $1"

  type
    E1 = enum
      value1,
      value2
    E2 = enum
      value1,
      value2 = 4

  const
    Lookuptable = [
      E1.value1: "1",
      # no need to qualify value2, known to be E1.value2
      value2: "2"
    ]

  proc p(e: E1) =
    # disambiguation in 'case' statements:
    case e
    of value1: echo "A"
    of value2: echo "B"

  p value2
  ```

In some cases, ambiguity of enums is resolved depending on the relation
between the current scope and the scope the enums were defined in.

  ```nim
  # a.nim
  type Foo* = enum abc

  # b.nim
  import a
  type Bar = enum abc
  echo abc is Bar # true

  block:
    type Baz = enum abc
    echo abc is Baz # true
  ```

To implement bit fields with enums see [Bit fields].


String type
-----------
All string literals are of the type `string`. A string in Nim is very
similar to a sequence of characters. However, strings in Nim are both
zero-terminated and have a length field. One can retrieve the length with the
builtin `len` procedure; the length never counts the terminating zero.

The terminating zero cannot be accessed unless the string is converted
to the `cstring` type first. The terminating zero assures that this
conversion can be done in O(1) and without any allocations.

The assignment operator for strings always copies the string.
The `&` operator concatenates strings.

Most native Nim types support conversion to strings with the special `$` proc.
When calling the `echo` proc, for example, the built-in stringify operation
for the parameter is called:

  ```nim
  echo 3 # calls `$` for `int`
  ```

Whenever a user creates a specialized object, implementation of this procedure
provides for `string` representation.

  ```nim
  type
    Person = object
      name: string
      age: int

  proc `$`(p: Person): string = # `$` always returns a string
    result = p.name & " is " &
            $p.age & # we *need* the `$` in front of p.age which
                     # is natively an integer to convert it to
                     # a string
            " years old."
  ```

While `$p.name` can also be used, the `$` operation on a string does
nothing. Note that we cannot rely on automatic conversion from an `int` to
a `string` like we can for the `echo` proc.

Strings are compared by their lexicographical order. All comparison operators
are available. Strings can be indexed like arrays (lower bound is 0). Unlike
arrays, they can be used in case statements:

  ```nim
  case paramStr(i)
  of "-v": incl(options, optVerbose)
  of "-h", "-?": incl(options, optHelp)
  else: write(stdout, "invalid command line option!\n")
  ```

Per convention, all strings are UTF-8 strings, but this is not enforced. For
example, when reading strings from binary files, they are merely a sequence of
bytes. The index operation `s[i]` means the i-th *char* of `s`, not the
i-th *unichar*. The iterator `runes` from the [unicode module](unicode.html)
can be used for iteration over all Unicode characters.


cstring type
------------

The `cstring` type meaning `compatible string` is the native representation
of a string for the compilation backend. For the C backend the `cstring` type
represents a pointer to a zero-terminated char array
compatible with the type `char*` in ANSI C. Its primary purpose lies in easy
interfacing with C. The index operation `s[i]` means the i-th *char* of
`s`; however no bounds checking for `cstring` is performed making the
index operation unsafe.

A Nim `string` is implicitly convertible
to `cstring` for convenience. If a Nim string is passed to a C-style
variadic proc, it is implicitly converted to `cstring` too:

  ```nim
  proc printf(formatstr: cstring) {.importc: "printf", varargs,
                                    header: "<stdio.h>".}

  printf("This works %s", "as expected")
  ```

Even though the conversion is implicit, it is not *safe*: The garbage collector
does not consider a `cstring` to be a root and may collect the underlying
memory. For this reason, the implicit conversion will be removed in future
releases of the Nim compiler. Certain idioms like conversion of a `const` string
to `cstring` are safe and will remain to be allowed.

A `$` proc is defined for cstrings that returns a string. Thus, to get a nim
string from a cstring:

  ```nim
  var str: string = "Hello!"
  var cstr: cstring = str
  var newstr: string = $cstr
  ```

`cstring` literals shouldn't be modified.

  ```nim
  var x = cstring"literals"
  x[1] = 'A' # This is wrong!!!
  ```

If the `cstring` originates from a regular memory (not read-only memory),
it can be modified:

  ```nim
  var x = "123456"
  prepareMutation(x) # call `prepareMutation` before modifying the strings
  var s: cstring = cstring(x)
  s[0] = 'u' # This is ok
  ```

`cstring` values may also be used in case statements like strings.

Structured types
----------------
A variable of a structured type can hold multiple values at the same
time. Structured types can be nested to unlimited levels. Arrays, sequences,
tuples, objects, and sets belong to the structured types.

Array and sequence types
------------------------
Arrays are a homogeneous type, meaning that each element in the array has the
same type. Arrays always have a fixed length specified as a constant expression
(except for open arrays). They can be indexed by any ordinal type.
A parameter `A` may be an *open array*, in which case it is indexed by
integers from 0 to `len(A)-1`. An array expression may be constructed by the
array constructor `[]`. The element type of this array expression is
inferred from the type of the first element. All other elements need to be
implicitly convertible to this type.

An array type can be defined using the `array[size, T]` syntax, or using
`array[lo..hi, T]` for arrays that start at an index other than zero.

Sequences are similar to arrays but of dynamic length which may change
during runtime (like strings). Sequences are implemented as growable arrays,
allocating pieces of memory as items are added. A sequence `S` is always
indexed by integers from 0 to `len(S)-1` and its bounds are checked.
Sequences can be constructed by the array constructor `[]` in conjunction
with the array to sequence operator `@`. Another way to allocate space for a
sequence is to call the built-in `newSeq` procedure.

A sequence may be passed to a parameter that is of type *open array*.

Example:

  ```nim
  type
    IntArray = array[0..5, int] # an array that is indexed with 0..5
    IntSeq = seq[int] # a sequence of integers
  var
    x: IntArray
    y: IntSeq
  x = [1, 2, 3, 4, 5, 6]  # [] is the array constructor
  y = @[1, 2, 3, 4, 5, 6] # the @ turns the array into a sequence

  let z = [1.0, 2, 3, 4] # the type of z is array[0..3, float]
  ```

The lower bound of an array or sequence may be received by the built-in proc
`low()`, the higher bound by `high()`. The length may be
received by `len()`. `low()` for a sequence or an open array always returns
0, as this is the first valid index.
One can append elements to a sequence with the `add()` proc or the `&`
operator, and remove (and get) the last element of a sequence with the
`pop()` proc.

The notation `x[i]` can be used to access the i-th element of `x`.

Arrays are always bounds checked (statically or at runtime). These
checks can be disabled via pragmas or invoking the compiler with the
`--boundChecks:off`:option: command-line switch.

An array constructor can have explicit indexes for readability:

  ```nim
  type
    Values = enum
      valA, valB, valC

  const
    lookupTable = [
      valA: "A",
      valB: "B",
      valC: "C"
    ]
  ```

If an index is left out, `succ(lastIndex)` is used as the index
value:

  ```nim
  type
    Values = enum
      valA, valB, valC, valD, valE

  const
    lookupTable = [
      valA: "A",
      "B",
      valC: "C",
      "D", "e"
    ]
  ```



Open arrays
-----------

Often fixed size arrays turn out to be too inflexible; procedures should
be able to deal with arrays of different sizes. The `openarray`:idx: type
allows this; it can only be used for parameters. Open arrays are always
indexed with an `int` starting at position 0. The `len`, `low`
and `high` operations are available for open arrays too. Any array with
a compatible base type can be passed to an open array parameter, the index
type does not matter. In addition to arrays, sequences can also be passed
to an open array parameter.

The `openarray` type cannot be nested: multidimensional open arrays are not
supported because this is seldom needed and cannot be done efficiently.

  ```nim
  proc testOpenArray(x: openArray[int]) = echo repr(x)

  testOpenArray([1,2,3])  # array[]
  testOpenArray(@[1,2,3]) # seq[]
  ```

Varargs
-------

A `varargs` parameter is an open array parameter that additionally
allows a variable number of arguments to be passed to a procedure. The compiler
converts the list of arguments to an array implicitly:

  ```nim
  proc myWriteln(f: File, a: varargs[string]) =
    for s in items(a):
      write(f, s)
    write(f, "\n")

  myWriteln(stdout, "abc", "def", "xyz")
  # is transformed to:
  myWriteln(stdout, ["abc", "def", "xyz"])
  ```

This transformation is only done if the `varargs` parameter is the
last parameter in the procedure header. It is also possible to perform
type conversions in this context:

  ```nim
  proc myWriteln(f: File, a: varargs[string, `$`]) =
    for s in items(a):
      write(f, s)
    write(f, "\n")

  myWriteln(stdout, 123, "abc", 4.0)
  # is transformed to:
  myWriteln(stdout, [$123, $"abc", $4.0])
  ```

In this example `$` is applied to any argument that is passed to the
parameter `a`. (Note that `$` applied to strings is a nop.)

Note that an explicit array constructor passed to a `varargs` parameter is
not wrapped in another implicit array construction:

  ```nim
  proc takeV[T](a: varargs[T]) = discard

  takeV([123, 2, 1]) # takeV's T is "int", not "array of int"
  ```


`varargs[typed]` is treated specially: It matches a variable list of arguments
of arbitrary type but *always* constructs an implicit array. This is required
so that the builtin `echo` proc does what is expected:

  ```nim
  proc echo*(x: varargs[typed, `$`]) {...}

  echo @[1, 2, 3]
  # prints "@[1, 2, 3]" and not "123"
  ```


Unchecked arrays
----------------
The `UncheckedArray[T]` type is a special kind of `array` where its bounds
are not checked. This is often useful to implement customized flexibly sized
arrays. Additionally, an unchecked array is translated into a C array of
undetermined size:

  ```nim
  type
    MySeq = object
      len, cap: int
      data: UncheckedArray[int]
  ```

Produces roughly this C code:

  ```C
  typedef struct {
    NI len;
    NI cap;
    NI data[];
  } MySeq;
  ```

The base type of the unchecked array may not contain any GC'ed memory but this
is currently not checked.

**Future directions**: GC'ed memory should be allowed in unchecked arrays and
there should be an explicit annotation of how the GC is to determine the
runtime size of the array.



Tuples and object types
-----------------------
A variable of a tuple or object type is a heterogeneous storage
container.
A tuple or object defines various named *fields* of a type. A tuple also
defines a lexicographic *order* of the fields. Tuples are meant to be
heterogeneous storage types with few abstractions. The `()` syntax
can be used to construct tuples. The order of the fields in the constructor
must match the order of the tuple's definition. Different tuple-types are
*equivalent* if they specify the same fields of the same type in the same
order. The *names* of the fields also have to be the same.

  ```nim
  type
    Person = tuple[name: string, age: int] # type representing a person:
                                           # it consists of a name and an age.
  var person: Person
  person = (name: "Peter", age: 30)
  assert person.name == "Peter"
  # the same, but less readable:
  person = ("Peter", 30)
  assert person[0] == "Peter"
  assert Person is (string, int)
  assert (string, int) is Person
  assert Person isnot tuple[other: string, age: int] # `other` is a different identifier
  ```

A tuple with one unnamed field can be constructed with the parentheses and a
trailing comma:

  ```nim
  proc echoUnaryTuple(a: (int,)) =
    echo a[0]

  echoUnaryTuple (1,)
  ```


In fact, a trailing comma is allowed for every tuple construction.

The implementation aligns the fields for the best access performance. The alignment
is compatible with the way the C compiler does it.

For consistency  with `object` declarations, tuples in a `type` section
can also be defined with indentation instead of `[]`:

  ```nim
  type
    Person = tuple   # type representing a person
      name: string   # a person consists of a name
      age: Natural   # and an age
  ```

Objects provide many features that tuples do not. Objects provide inheritance
and the ability to hide fields from other modules. Objects with inheritance
enabled have information about their type at runtime so that the `of` operator
can be used to determine the object's type. The `of` operator is similar to
the `instanceof` operator in Java.

  ```nim
  type
    Person = object of RootObj
      name*: string   # the * means that `name` is accessible from other modules
      age: int        # no * means that the field is hidden

    Student = ref object of Person # a student is a person
      id: int                      # with an id field

  var
    student: Student
    person: Person
  assert(student of Student) # is true
  assert(student of Person) # also true
  ```

Object fields that should be visible from outside the defining module have to
be marked by `*`. In contrast to tuples, different object types are
never *equivalent*, they are nominal types whereas tuples are structural.
Objects that have no ancestor are implicitly `final` and thus have no hidden
type information. One can use the `inheritable` pragma to
introduce new object roots apart from `system.RootObj`.

  ```nim
  type
    Person = object # example of a final object
      name*: string
      age: int

    Student = ref object of Person # Error: inheritance only works with non-final objects
      id: int
  ```

The assignment operator for tuples and objects copies each component.
The methods to override this copying behavior are described [here][type
bound operators].


Object construction
-------------------

Objects can also be created with an `object construction expression`:idx: that
has the syntax `T(fieldA: valueA, fieldB: valueB, ...)` where `T` is
an `object` type or a `ref object` type:

  ```nim
  type
    Student = object
      name: string
      age: int
    PStudent = ref Student
  var a1 = Student(name: "Anton", age: 5)
  var a2 = PStudent(name: "Anton", age: 5)
  # this also works directly:
  var a3 = (ref Student)(name: "Anton", age: 5)
  # not all fields need to be mentioned, and they can be mentioned out of order:
  var a4 = Student(age: 5)
  ```

Note that, unlike tuples, objects require the field names along with their values.
For a `ref object` type `system.new` is invoked implicitly.


Object variants
---------------
Often an object hierarchy is an overkill in certain situations where simple variant
types are needed. Object variants are tagged unions discriminated via an
enumerated type used for runtime type flexibility, mirroring the concepts of
*sum types* and *algebraic data types (ADTs)* as found in other languages.

An example:

  ```nim
  # This is an example of how an abstract syntax tree could be modelled in Nim
  type
    NodeKind = enum  # the different node types
      nkInt,          # a leaf with an integer value
      nkFloat,        # a leaf with a float value
      nkString,       # a leaf with a string value
      nkAdd,          # an addition
      nkSub,          # a subtraction
      nkIf            # an if statement
    Node = ref NodeObj
    NodeObj = object
      case kind: NodeKind  # the `kind` field is the discriminator
      of nkInt: intVal: int
      of nkFloat: floatVal: float
      of nkString: strVal: string
      of nkAdd, nkSub:
        leftOp, rightOp: Node
      of nkIf:
        condition, thenPart, elsePart: Node

  # create a new case object:
  var n = Node(kind: nkIf, condition: nil)
  # accessing n.thenPart is valid because the `nkIf` branch is active:
  n.thenPart = Node(kind: nkFloat, floatVal: 2.0)

  # the following statement raises an `FieldDefect` exception, because
  # n.kind's value does not fit and the `nkString` branch is not active:
  n.strVal = ""

  # invalid: would change the active object branch:
  n.kind = nkInt

  var x = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4),
                            rightOp: Node(kind: nkInt, intVal: 2))
  # valid: does not change the active object branch:
  x.kind = nkSub
  ```

As can be seen from the example, an advantage to an object hierarchy is that
no casting between different object types is needed. Yet, access to invalid
object fields raises an exception.

The syntax of `case` in an object declaration follows closely the syntax of
the `case` statement: The branches in a `case` section may be indented too.

In the example, the `kind` field is called the `discriminator`:idx:\: For
safety, its address cannot be taken and assignments to it are restricted: The
new value must not lead to a change of the active object branch. Also, when the
fields of a particular branch are specified during object construction, the
corresponding discriminator value must be specified as a constant expression.

Instead of changing the active object branch, replace the old object in memory
with a new one completely:

  ```nim
  var x = Node(kind: nkAdd, leftOp: Node(kind: nkInt, intVal: 4),
                            rightOp: Node(kind: nkInt, intVal: 2))
  # change the node's contents:
  x[] = NodeObj(kind: nkString, strVal: "abc")
  ```


Starting with version 0.20 `system.reset` cannot be used anymore to support
object branch changes as this never was completely memory safe.

As a special rule, the discriminator kind can also be bounded using a `case`
statement. If possible values of the discriminator variable in a
`case` statement branch are a subset of discriminator values for the selected
object branch, the initialization is considered valid. This analysis only works
for immutable discriminators of an ordinal type and disregards `elif`
branches. For discriminator values with a `range` type, the compiler
checks if the entire range of possible values for the discriminator value is
valid for the chosen object branch.

A small example:

  ```nim
  let unknownKind = nkSub

  # invalid: unsafe initialization because the kind field is not statically known:
  var y = Node(kind: unknownKind, strVal: "y")

  var z = Node()
  case unknownKind
  of nkAdd, nkSub:
    # valid: possible values of this branch are a subset of nkAdd/nkSub object branch:
    z = Node(kind: unknownKind, leftOp: Node(), rightOp: Node())
  else:
    echo "ignoring: ", unknownKind

  # also valid, since unknownKindBounded can only contain the values nkAdd or nkSub
  let unknownKindBounded = range[nkAdd..nkSub](unknownKind)
  z = Node(kind: unknownKindBounded, leftOp: Node(), rightOp: Node())
  ```


cast uncheckedAssign
--------------------

Some restrictions for case objects can be disabled via a `{.cast(uncheckedAssign).}` section:

  ```nim  test="nim c $1"
  type
    TokenKind* = enum
      strLit, intLit
    Token = object
      case kind*: TokenKind
      of strLit:
        s*: string
      of intLit:
        i*: int64

  proc passToVar(x: var TokenKind) = discard

  var t = Token(kind: strLit, s: "abc")

  {.cast(uncheckedAssign).}:
    # inside the 'cast' section it is allowed to pass 't.kind' to a 'var T' parameter:
    passToVar(t.kind)

    # inside the 'cast' section it is allowed to set field 's' even though the
    # constructed 'kind' field has an unknown value:
    t = Token(kind: t.kind, s: "abc")

    # inside the 'cast' section it is allowed to assign to the 't.kind' field directly:
    t.kind = intLit
  ```

Default values for object fields
--------------------------------

Object fields are allowed to have a constant default value. The type of field can be omitted if a default value is given.

```nim test
type
  Foo = object
    a: int = 2
    b: float = 3.14
    c = "I can have a default value"

  Bar = ref object
    a: int = 2
    b: float = 3.14
    c = "I can have a default value"
```

The explicit initialization uses these defaults which includes an `object` created with an object construction expression or the procedure `default`; a `ref object` created with an object construction expression or the procedure `new`; an array or a tuple with a subtype which has a default created with the procedure `default`.


```nim test
type
  Foo = object
    a: int = 2
    b = 3.0
  Bar = ref object
    a: int = 2
    b = 3.0

block: # created with an object construction expression
  let x = Foo()
  assert x.a == 2 and x.b == 3.0

  let y = Bar()
  assert y.a == 2 and y.b == 3.0

block: # created with an object construction expression
  let x = default(Foo)
  assert x.a == 2 and x.b == 3.0

  let y = default(array[1, Foo])
  assert y[0].a == 2 and y[0].b == 3.0

  let z = default(tuple[x: Foo])
  assert z.x.a == 2 and z.x.b == 3.0

block: # created with the procedure `new`
  let y = new Bar
  assert y.a == 2 and y.b == 3.0
```

Set type
--------

.. include:: sets_fragment.txt

Reference and pointer types
---------------------------
References (similar to pointers in other programming languages) are a
way to introduce many-to-one relationships. This means different references can
point to and modify the same location in memory (also called `aliasing`:idx:).

Nim distinguishes between `traced`:idx: and `untraced`:idx: references.
Untraced references are also called *pointers*. Traced references point to
objects of a garbage-collected heap, untraced references point to
manually allocated objects or objects somewhere else in memory. Thus,
untraced references are *unsafe*. However, for certain low-level operations
(accessing the hardware) untraced references are unavoidable.

Traced references are declared with the **ref** keyword, untraced references
are declared with the **ptr** keyword. In general, a `ptr T` is implicitly
convertible to the `pointer` type.

An empty subscript `[]` notation can be used to de-refer a reference,
the `addr` procedure returns the address of an item. An address is always
an untraced reference.
Thus, the usage of `addr` is an *unsafe* feature.

The `.` (access a tuple/object field operator)
and `[]` (array/string/sequence index operator) operators perform implicit
dereferencing operations for reference types:

  ```nim
  type
    Node = ref NodeObj
    NodeObj = object
      le, ri: Node
      data: int

  var
    n: Node
  new(n)
  n.data = 9
  # no need to write n[].data; in fact n[].data is highly discouraged!
  ```

In order to simplify structural type checking, recursive tuples are not valid:

  ```nim
  # invalid recursion
  type MyTuple = tuple[a: ref MyTuple]
  ```

Likewise `T = ref T` is an invalid type.

As a syntactical extension, `object` types can be anonymous if
declared in a type section via the `ref object` or `ptr object` notations.
This feature is useful if an object should only gain reference semantics:

  ```nim
  type
    Node = ref object
      le, ri: Node
      data: int
  ```


To allocate a new traced object, the built-in procedure `new` has to be used.
To deal with untraced memory, the procedures `alloc`, `dealloc` and
`realloc` can be used. The documentation of the [system](system.html) module
contains further information.


Nil
---

If a reference points to *nothing*, it has the value `nil`. `nil` is the
default value for all `ref` and `ptr` types. The `nil` value can also be
used like any other literal value. For example, it can be used in an assignment
like `myRef = nil`.

Dereferencing `nil` is an unrecoverable fatal runtime error (and not a panic).

A successful dereferencing operation `p[]` implies that `p` is not nil. This
can be exploited by the implementation to optimize code like:

  ```nim
  p[].field = 3
  if p != nil:
    # if p were nil, `p[]` would have caused a crash already,
    # so we know `p` is always not nil here.
    action()
  ```

Into:

  ```nim
  p[].field = 3
  action()
  ```


*Note*: This is not comparable to C's "undefined behavior" for
dereferencing NULL pointers.


Mixing GC'ed memory with `ptr`
--------------------------------

Special care has to be taken if an untraced object contains traced objects like
traced references, strings, or sequences: in order to free everything properly,
the built-in procedure `reset` has to be called before freeing the untraced
memory manually:

  ```nim
  type
    Data = tuple[x, y: int, s: string]

  # allocate memory for Data on the heap:
  var d = cast[ptr Data](alloc0(sizeof(Data)))

  # create a new string on the garbage collected heap:
  d.s = "abc"

  # tell the GC that the string is not needed anymore:
  reset(d.s)

  # free the memory:
  dealloc(d)
  ```

Without the `reset` call the memory allocated for the `d.s` string would
never be freed. The example also demonstrates two important features for
low-level programming: the `sizeof` proc returns the size of a type or value
in bytes. The `cast` operator can circumvent the type system: the compiler
is forced to treat the result of the `alloc0` call (which returns an untyped
pointer) as if it would have the type `ptr Data`. Casting should only be
done if it is unavoidable: it breaks type safety and bugs can lead to
mysterious crashes.

**Note**: The example only works because the memory is initialized to zero
(`alloc0` instead of `alloc` does this): `d.s` is thus initialized to
binary zero which the string assignment can handle. One needs to know low-level
details like this when mixing garbage-collected data with unmanaged memory.

.. XXX finalizers for traced objects


Procedural type
---------------
A procedural type is internally a pointer to a procedure. `nil` is
an allowed value for a variable of a procedural type.

Examples:

  ```nim
  proc printItem(x: int) = ...

  proc forEach(c: proc (x: int) {.cdecl.}) =
    ...

  forEach(printItem)  # this will NOT compile because calling conventions differ
  ```


  ```nim
  type
    OnMouseMove = proc (x, y: int) {.closure.}

  proc onMouseMove(mouseX, mouseY: int) =
    # has default calling convention
    echo "x: ", mouseX, " y: ", mouseY

  proc setOnMouseMove(mouseMoveEvent: OnMouseMove) = discard

  # ok, 'onMouseMove' has the default calling convention, which is compatible
  # to 'closure':
  setOnMouseMove(onMouseMove)
  ```


A subtle issue with procedural types is that the calling convention of the
procedure influences the type compatibility: procedural types are only
compatible if they have the same calling convention. As a special extension,
a procedure of the calling convention `nimcall` can be passed to a parameter
that expects a proc of the calling convention `closure`.

Nim supports these `calling conventions`:idx:\:

`nimcall`:idx:
:   is the default convention used for a Nim **proc**. It is the
    same as `fastcall`, but only for C compilers that support `fastcall`.

`closure`:idx:
:   is the default calling convention for a **procedural type** that lacks
    any pragma annotations. It indicates that the procedure has a hidden
    implicit parameter (an *environment*). Proc vars that have the calling
    convention `closure` take up two machine words: One for the proc pointer
    and another one for the pointer to implicitly passed environment.

`stdcall`:idx:
:   This is the stdcall convention as specified by Microsoft. The generated C
    procedure is declared with the `__stdcall` keyword.

`cdecl`:idx:
:   The cdecl convention means that a procedure shall use the same convention
    as the C compiler. Under Windows the generated C procedure is declared with
    the `__cdecl` keyword.

`safecall`:idx:
:   This is the safecall convention as specified by Microsoft. The generated C
    procedure is declared with the `__safecall` keyword. The word *safe*
    refers to the fact that all hardware registers shall be pushed to the
    hardware stack.

`inline`:idx:
:   The inline convention means the caller should not call the procedure,
    but inline its code directly. Note that Nim does not inline, but leaves
    this to the C compiler; it generates `__inline` procedures. This is
    only a hint for the compiler: it may completely ignore it, and
    it may inline procedures that are not marked as `inline`.

`noinline`:idx:
:   The backend compiler may inline procedures that are not marked as `inline`.
    The noinline convention prevents it.

`fastcall`:idx:
:   Fastcall means different things to different C compilers. One gets whatever
    the C `__fastcall` means.

`thiscall`:idx:
:   This is the thiscall calling convention as specified by Microsoft, used on
    C++ class member functions on the x86 architecture.

`syscall`:idx:
:   The syscall convention is the same as `__syscall`:c: in C. It is used for
    interrupts.

`noconv`:idx:
:   The generated C code will not have any explicit calling convention and thus
    use the C compiler's default calling convention. This is needed because
    Nim's default calling convention for procedures is `fastcall` to
    improve speed.

Most calling conventions exist only for the Windows 32-bit platform.

The default calling convention is `nimcall`, unless it is an inner proc (a
proc inside of a proc). For an inner proc an analysis is performed whether it
accesses its environment. If it does so, it has the calling convention
`closure`, otherwise it has the calling convention `nimcall`.


Distinct type
-------------

A `distinct` type is a new type derived from a `base type`:idx: that is
incompatible with its base type. In particular, it is an essential property
of a distinct type that it **does not** imply a subtype relation between it
and its base type. Explicit type conversions from a distinct type to its
base type and vice versa are allowed. See also `distinctBase` to get the
reverse operation.

A distinct type is an ordinal type if its base type is an ordinal type.


### Modeling currencies

A distinct type can be used to model different physical `units`:idx: with a
numerical base type, for example. The following example models currencies.

Different currencies should not be mixed in monetary calculations. Distinct
types are a perfect tool to model different currencies:

  ```nim
  type
    Dollar = distinct int
    Euro = distinct int

  var
    d: Dollar
    e: Euro

  echo d + 12
  # Error: cannot add a number with no unit and a `Dollar`
  ```

Unfortunately, `d + 12.Dollar` is not allowed either,
because `+` is defined for `int` (among others), not for `Dollar`. So
a `+` for dollars needs to be defined:

  ```nim
  proc `+` (x, y: Dollar): Dollar =
    result = Dollar(int(x) + int(y))
  ```

It does not make sense to multiply a dollar with a dollar, but with a
number without unit; and the same holds for division:

  ```nim
  proc `*` (x: Dollar, y: int): Dollar =
    result = Dollar(int(x) * y)

  proc `*` (x: int, y: Dollar): Dollar =
    result = Dollar(x * int(y))

  proc `div` ...
  ```

This quickly gets tedious. The implementations are trivial and the compiler
should not generate all this code only to optimize it away later - after all
`+` for dollars should produce the same binary code as `+` for ints.
The pragma `borrow`:idx: has been designed to solve this problem; in principle,
it generates the above trivial implementations:

  ```nim
  proc `*` (x: Dollar, y: int): Dollar {.borrow.}
  proc `*` (x: int, y: Dollar): Dollar {.borrow.}
  proc `div` (x: Dollar, y: int): Dollar {.borrow.}
  ```

The `borrow` pragma makes the compiler use the same implementation as
the proc that deals with the distinct type's base type, so no code is
generated.

But it seems all this boilerplate code needs to be repeated for the `Euro`
currency. This can be solved with [templates].

  ```nim  test = "nim c $1"
  template additive(typ: typedesc) =
    proc `+` *(x, y: typ): typ {.borrow.}
    proc `-` *(x, y: typ): typ {.borrow.}

    # unary operators:
    proc `+` *(x: typ): typ {.borrow.}
    proc `-` *(x: typ): typ {.borrow.}

  template multiplicative(typ, base: typedesc) =
    proc `*` *(x: typ, y: base): typ {.borrow.}
    proc `*` *(x: base, y: typ): typ {.borrow.}
    proc `div` *(x: typ, y: base): typ {.borrow.}
    proc `mod` *(x: typ, y: base): typ {.borrow.}

  template comparable(typ: typedesc) =
    proc `<` * (x, y: typ): bool {.borrow.}
    proc `<=` * (x, y: typ): bool {.borrow.}
    proc `==` * (x, y: typ): bool {.borrow.}

  template defineCurrency(typ, base: untyped) =
    type
      typ* = distinct base
    additive(typ)
    multiplicative(typ, base)
    comparable(typ)

  defineCurrency(Dollar, int)
  defineCurrency(Euro, int)
  ```


The borrow pragma can also be used to annotate the distinct type to allow
certain builtin operations to be lifted:

  ```nim
  type
    Foo = object
      a, b: int
      s: string

    Bar {.borrow: `.`.} = distinct Foo

  var bb: ref Bar
  new bb
  # field access now valid
  bb.a = 90
  bb.s = "abc"
  ```

Currently, only the dot accessor can be borrowed in this way.


### Avoiding SQL injection attacks

An SQL statement that is passed from Nim to an SQL database might be
modeled as a string. However, using string templates and filling in the
values is vulnerable to the famous `SQL injection attack`:idx:\:

  ```nim
  import std/strutils

  proc query(db: DbHandle, statement: string) = ...

  var
    username: string

  db.query("SELECT FROM users WHERE name = '$1'" % username)
  # Horrible security hole, but the compiler does not mind!
  ```

This can be avoided by distinguishing strings that contain SQL from strings
that don't. Distinct types provide a means to introduce a new string type
`SQL` that is incompatible with `string`:

  ```nim
  type
    SQL = distinct string

  proc query(db: DbHandle, statement: SQL) = ...

  var
    username: string

  db.query("SELECT FROM users WHERE name = '$1'" % username)
  # Static error: `query` expects an SQL string!
  ```


It is an essential property of abstract types that they **do not** imply a
subtype relation between the abstract type and its base type. Explicit type
conversions from `string` to `SQL` are allowed:

  ```nim
  import std/[strutils, sequtils]

  proc properQuote(s: string): SQL =
    # quotes a string properly for an SQL statement
    return SQL(s)

  proc `%` (frmt: SQL, values: openarray[string]): SQL =
    # quote each argument:
    let v = values.mapIt(properQuote(it))
    # we need a temporary type for the type conversion :-(
    type StrSeq = seq[string]
    # call strutils.`%`:
    result = SQL(string(frmt) % StrSeq(v))

  db.query("SELECT FROM users WHERE name = '$1'".SQL % [username])
  ```

Now we have compile-time checking against SQL injection attacks. Since
`"".SQL` is transformed to `SQL("")` no new syntax is needed for nice
looking `SQL` string literals. The hypothetical `SQL` type actually
exists in the library as the [SqlQuery type](db_common.html#SqlQuery) of
modules like [db_sqlite](db_sqlite.html).


Auto type
---------

The `auto` type can only be used for return types and parameters. For return
types it causes the compiler to infer the type from the routine body:

  ```nim
  proc returnsInt(): auto = 1984
  ```

For parameters it currently creates implicitly generic routines:

  ```nim
  proc foo(a, b: auto) = discard
  ```

Is the same as:

  ```nim
  proc foo[T1, T2](a: T1, b: T2) = discard
  ```

However, later versions of the language might change this to mean "infer the
parameters' types from the body". Then the above `foo` would be rejected as
the parameters' types can not be inferred from an empty `discard` statement.


Type relations
==============

The following section defines several relations on types that are needed to
describe the type checking done by the compiler.


Type equality
-------------

Nim uses structural type equivalence for most types. Only for objects,
enumerations and distinct types and for generic types name equivalence is used.


Subtype relation
----------------

If object `a` inherits from `b`, `a` is a subtype of `b`.

This subtype relation is extended to the types `var`, `ref`, `ptr`.
If `A` is a subtype of `B` and `A` and `B` are `object` types then:

- `var A` is a subtype of `var B`
- `ref A` is a subtype of `ref B`
- `ptr A` is a subtype of `ptr B`.

**Note**: One of the above pointer-indirections is required for assignment from
a subtype to its parent type to prevent "object slicing".


Convertible relation
--------------------

A type `a` is **implicitly** convertible to type `b` iff the following
algorithm returns true:

  ```nim
  proc isImplicitlyConvertible(a, b: PType): bool =
    if isSubtype(a, b):
      return true
    if isIntLiteral(a):
      return b in {int8, int16, int32, int64, int, uint, uint8, uint16,
                   uint32, uint64, float32, float64}
    case a.kind
    of int:     result = b in {int32, int64}
    of int8:    result = b in {int16, int32, int64, int}
    of int16:   result = b in {int32, int64, int}
    of int32:   result = b in {int64, int}
    of uint:    result = b in {uint32, uint64}
    of uint8:   result = b in {uint16, uint32, uint64}
    of uint16:  result = b in {uint32, uint64}
    of uint32:  result = b in {uint64}
    of float32: result = b in {float64}
    of float64: result = b in {float32}
    of seq:
      result = b == openArray and typeEquals(a.baseType, b.baseType)
    of array:
      result = b == openArray and typeEquals(a.baseType, b.baseType)
      if a.baseType == char and a.indexType.rangeA == 0:
        result = b == cstring
    of cstring, ptr:
      result = b == pointer
    of string:
      result = b == cstring
    of proc:
      result = typeEquals(a, b) or compatibleParametersAndEffects(a, b)
  ```

We used the predicate `typeEquals(a, b)` for the "type equality" property
and the predicate `isSubtype(a, b)` for the "subtype relation".
`compatibleParametersAndEffects(a, b)` is currently not specified.

Implicit conversions are also performed for Nim's `range` type
constructor.

Let `a0`, `b0` of type `T`.

Let `A = range[a0..b0]` be the argument's type, `F` the formal
parameter's type. Then an implicit conversion from `A` to `F`
exists if `a0 >= low(F) and b0 <= high(F)` and both `T` and `F`
are signed integers or if both are unsigned integers.


A type `a` is **explicitly** convertible to type `b` iff the following
algorithm returns true:

  ```nim
  proc isIntegralType(t: PType): bool =
    result = isOrdinal(t) or t.kind in {float, float32, float64}

  proc isExplicitlyConvertible(a, b: PType): bool =
    result = false
    if isImplicitlyConvertible(a, b): return true
    if typeEquals(a, b): return true
    if a == distinct and typeEquals(a.baseType, b): return true
    if b == distinct and typeEquals(b.baseType, a): return true
    if isIntegralType(a) and isIntegralType(b): return true
    if isSubtype(a, b) or isSubtype(b, a): return true
  ```

The convertible relation can be relaxed by a user-defined type
`converter`:idx:.

  ```nim
  converter toInt(x: char): int = result = ord(x)

  var
    x: int
    chr: char = 'a'

  # implicit conversion magic happens here
  x = chr
  echo x # => 97
  # one can use the explicit form too
  x = chr.toInt
  echo x # => 97
  ```

The type conversion `T(a)` is an L-value if `a` is an L-value and
`typeEqualsOrDistinct(T, typeof(a))` holds.


Assignment compatibility
------------------------

An expression `b` can be assigned to an expression `a` iff `a` is an
`l-value` and `isImplicitlyConvertible(b.typ, a.typ)` holds.


Overload resolution
===================

In a call `p(args)` where `p` may refer to more than one
candidate, it is said to be a symbol choice. Overload resolution will attempt to
find the best candidate, thus transforming the symbol choice into a resolved symbol.
The routine `p` that matches best is selected following a series of trials explained below. 
In order: Category matching, Hierarchical Order Comparison, and finally, Complexity Analysis.

If multiple candidates match equally well after all trials have been tested, the ambiguity 
is reported during semantic analysis.

First Trial: Category matching
--------------------------------

Every arg in `args` needs to match and there are multiple different categories of matches.
Let `f` be the formal parameter's type and `a` the type of the argument.

1. Exact match: `a` and `f` are of the same type.
2. Literal match: `a` is an integer literal of value `v`
   and `f` is a signed or unsigned integer type and `v` is in `f`'s
   range. Or:  `a` is a floating-point literal of value `v`
   and `f` is a floating-point type and `v` is in `f`'s
   range.
3. Generic match: `f` is a generic type and `a` matches, for
   instance `a` is `int` and `f` is a generic (constrained) parameter
   type (like in `[T]` or `[T: int|char]`).
4. Subrange or subtype match: `a` is a `range[T]` and `T`
   matches `f` exactly. Or: `a` is a subtype of `f`.
5. Integral conversion match: `a` is convertible to `f` and `f` and `a`
   is some integer or floating-point type.
6. Conversion match: `a` is convertible to `f`, possibly via a user
   defined `converter`.

Each operand may fall into one of the categories above; the operand's
highest priority category. The list above is in order or priority.
If a candidate has more priority matches than all other candidates, it is selected as the
resolved symbol.

For example, if a candidate with one exact match is compared to a candidate with multiple
generic matches and zero exact matches, the candidate with an exact match will win.

Below is a pseudocode interpretation of category matching, `count(p, m)` counts the number 
of matches of the matching category `m` for the routine `p`.

A routine `p` matches better than a routine `q` if the following
algorithm returns true:

  ```nim
  for each matching category m in ["exact match", "literal match",
                                  "generic match", "subtype match",
                                  "integral match", "conversion match"]:
    if count(p, m) > count(q, m): return true
    elif count(p, m) == count(q, m):
      discard "continue with next category m"
    else:
      return false
  return "ambiguous"
  ```

Second Trial: Hierarchical Order Comparison
----------------------------------------------

The hierarchical order of a type is analogous to its relative specificity. Consider the type defined:

```nim
type A[T] = object
```

Matching formals for this type include `T`, `object`, `A`, `A[...]` and `A[C]` where `C` is a concrete type, `A[...]`
is a generic typeclass composition and `T` is an unconstrained generic type variable. This list is in order of 
specificity with respect to `A` as each subsequent category narrows the set of types that are members of their match set.

In this trial, the formal parameters of candidates are compared in order (1st parameter, 2nd parameter, etc.) to search for
a candidate that has an unrivaled specificity. If such a formal parameter is found, the candidate it belongs to is chosen 
as the resolved symbol.

Third Trial: Complexity Analysis
----------------------------------

A slight clarification: While category matching digests all the formal parameters of a candidate at once (order doesn't matter),
specificity comparison and complexity analysis operate on each formal parameter at a time. The following
is the final trial to disambiguate a symbol choice when a pair of formal parameters have the same hierarchical order.

The complexity of a type is essentially its number of modifiers and depth of shape. The definition with the *highest*
complexity wins. Consider the following types:

```nim
type
  A[T] = object
  B[T, H] = object
```

Note: The below examples are not exhaustive.

We shall say that:

1. `A[T]` has a higher complexity than `A`
2. `var A[T]` has a higher complexity than `A[T]`
3. `A[A[T]]` has a higher complexity than `A[T]`
4. `B[T, H]` has a higher complexity than `A[T]` (`A` and `B` are not compatible here, but convoluted versions of this exist)
5. `B[ptr T, H]` has a higher complexity than `B[T, H]`

Some Examples
---------------

  ```nim
  proc takesInt(x: int) = echo "int"
  proc takesInt[T](x: T) = echo "T"
  proc takesInt(x: int16) = echo "int16"

  takesInt(4) # "int"
  var x: int32
  takesInt(x) # "T"
  var y: int16
  takesInt(y) # "int16"
  var z: range[0..4] = 0
  takesInt(z) # "T"
  ```


If the argument `a` matches both the parameter type `f` of `p`
and `g` of `q` via a subtyping relation, the inheritance depth is taken
into account:

  ```nim
  type
    A = object of RootObj
    B = object of A
    C = object of B

  proc p(obj: A) =
    echo "A"

  proc p(obj: B) =
    echo "B"

  var c = C()
  # not ambiguous, calls 'B', not 'A' since B is a subtype of A
  # but not vice versa:
  p(c)

  proc pp(obj: A, obj2: B) = echo "A B"
  proc pp(obj: B, obj2: A) = echo "B A"

  # but this is ambiguous:
  pp(c, c)
  ```


Likewise, for generic matches, the most specialized generic type (that still
matches) is preferred:

  ```nim
  proc gen[T](x: ref ref T) = echo "ref ref T"
  proc gen[T](x: ref T) = echo "ref T"
  proc gen[T](x: T) = echo "T"

  var ri: ref int
  gen(ri) # "ref T"
  ```

Type variables match
----------------------

When overload resolution is considering candidates, the type variable's definition
is not overlooked as it is used to define the formal parameter's type via variable substitution.

For example:
```nim
type A
proc p[T: A](param: T)
proc p[T: object](param: T)
```

These signatures are not ambiguous for a concrete type of `A` even though the formal parameters match ("T" == "T").
Instead `T` is treated as a variable in that (`T` ?= `T`) depending on the bound type of `T` at the time of
overload resolution.


Overloading based on 'var T'
--------------------------------------

If the formal parameter `f` is of type `var T`
in addition to the ordinary type checking,
the argument is checked to be an `l-value`:idx:.
`var T` matches better than just `T` then.

  ```nim
  proc sayHi(x: int): string =
    # matches a non-var int
    result = $x
  proc sayHi(x: var int): string =
    # matches a var int
    result = $(x + 10)

  proc sayHello(x: int) =
    var m = x # a mutable version of x
    echo sayHi(x) # matches the non-var version of sayHi
    echo sayHi(m) # matches the var version of sayHi

  sayHello(3) # 3
              # 13
  ```


Lazy type resolution for untyped
--------------------------------

**Note**: An `unresolved`:idx: expression is an expression for which no symbol
lookups and no type checking have been performed.

Since templates and macros that are not declared as `immediate` participate
in overloading resolution, it's essential to have a way to pass unresolved
expressions to a template or macro. This is what the meta-type `untyped`
accomplishes:

  ```nim
  template rem(x: untyped) = discard

  rem unresolvedExpression(undeclaredIdentifier)
  ```

A parameter of type `untyped` always matches any argument (as long as there is
any argument passed to it).

But one has to watch out because other overloads might trigger the
argument's resolution:

  ```nim
  template rem(x: untyped) = discard
  proc rem[T](x: T) = discard

  # undeclared identifier: 'unresolvedExpression'
  rem unresolvedExpression(undeclaredIdentifier)
  ```

`untyped` and `varargs[untyped]` are the only metatype that are lazy in this sense, the other
metatypes `typed` and `typedesc` are not lazy.


Varargs matching
----------------

See [Varargs].


iterable
--------

A called `iterator` yielding type `T` can be passed to a template or macro via
a parameter typed as `untyped` (for unresolved expressions) or the type class
`iterable` or `iterable[T]` (after type checking and overload resolution).

  ```nim
  iterator iota(n: int): int =
    for i in 0..<n: yield i

  template toSeq2[T](a: iterable[T]): seq[T] =
    var ret: seq[T]
    assert a.typeof is T
    for ai in a: ret.add ai
    ret

  assert iota(3).toSeq2 == @[0, 1, 2]
  assert toSeq2(5..7) == @[5, 6, 7]
  assert not compiles(toSeq2(@[1,2])) # seq[int] is not an iterable
  assert toSeq2(items(@[1,2])) == @[1, 2] # but items(@[1,2]) is
  ```


Overload disambiguation
=======================

For routine calls "overload resolution" is performed. There is a weaker form of
overload resolution called *overload disambiguation* that is performed when an
overloaded symbol is used in a context where there is additional type information
available. Let `p` be an overloaded symbol. These contexts are:

- In a function call `q(..., p, ...)` when the corresponding formal parameter
  of `q` is a `proc` type. If `q` itself is overloaded then the cartesian product
  of every interpretation of `q` and `p` must be considered.
- In an object constructor `Obj(..., field: p, ...)` when `field` is a `proc`
  type. Analogous rules exist for array/set/tuple constructors.
- In a declaration like `x: T = p` when `T` is a `proc` type.

As usual, ambiguous matches produce a compile-time error.

Named argument overloading
--------------------------

Routines with the same type signature can be called individually if
a parameter has different names between them.

  ```Nim
  proc foo(x: int) =
    echo "Using x: ", x
  proc foo(y: int) =
    echo "Using y: ", y

  foo(x = 2) # Using x: 2
  foo(y = 2) # Using y: 2
  ```

Not supplying the parameter name in such cases results in an
ambiguity error.

Concepts
=========

Concepts are a mechanism for users to define custom type classes that match other
types based on a given set of bindings.

```nim
type
  Comparable = concept # Atomic concept
    proc cmp(a, b: Self): int
  Indexable[I, T] = concept # Container concept
    proc `[]`(x: Self; at: I): T
    proc `[]=`(x: var Self; at: I; newVal: T)
    proc len(x: Self): I
  Index = concept
    proc inc(x: var Self)
    proc `<`(a, b: Self): bool
proc sort*[I: Index; T: Comparable](x: var Indexable[I, T])
```

In the above example, `Comparable` and `Indexable` are types that will match any type that
can can bind each definition declared in the concept body. The special `Self` type defined
in the concept body refers to the type being matched, also called the "implementation" of 
the concept. Implementations that match the concept are generic matches, and the concept 
typeclasses themselves work in a similar way to generic type variables in that they are never
concrete types themselves (even if they have concrete type parameters such as `Indexable[int, int]`)
and expressions like `typeof(x)` in the body of `proc sort` from the above example will return the 
type of the implementation, not the concept typeclass. Concepts are useful for providing information
to the compiler in generic contexts, most notably for generic type checking, and as a tool for 
[Overload resolution]. Generic type checking is forthcoming, so this will only explain overload
resolution for now.

In the example above, "atomic" and "container" concepts are mentioned. These kinds of concept
are determined by the generic type variables of the concept. Atomic concepts` definitions contain
only concrete types, and the `Self` type is inferred to be concrete. Container types are the same,
under the condition that their generic variables are bound to concrete types and substituted appropriately.
The programmer is free to define a concept that breaks these concreteness rules, thus making a "gray" concept:

```nim
type
  Processor = concept
    proc process[T](s: Self; data: T)
```

The above concept does not have generic variables, and its definition contains `T` which is not concrete.
This kind of concept may disrupt the compiler's ability to type check generic contexts, but it is useful for
overload resolution. The difference between `Indexable[I, T]` and `Processor` is that a given implementation
is effectively described as an instantiation of `Indexable` (as in `Indexable[int, int]`) whereas a `Processor`
concept describes an implementation designed to handle multiple different types of data `T`.

Concept overload resolution
-----------------------------

When an operand's type is being matched to a concept, the operand's type  is set as the "potential
implementation". For each definition in the concept body, overload resolution is performed by substituting `Self`
for the potential implementation to try and find a match for each definition. If this succeeds, the concept 
matches. Implementations do not need to exactly match the definitions in the concept. For example:

```nim
type
  C1 = concept
    proc p(s: Self; x: int)
  Implementation = object

proc p(x: Implementation; y: SomeInteger)
proc spring(x: C1)
spring(Implementation())
```
This will bind because `p(Implementation(), 0)` will bind. Conversely, container types will bind to
less specific definitions if the generic constraints and bindings allow it, as per usual generic matching.

Things start to get more complicated when overload resolution starts "Hierarchical Order Comparison"
I.E. specificity comparison as per [Overload resolution]. In this state the compiler may be comparing
all kinds of types and typeclasses with concepts as defined in the `proc` definitions of each overload.
This leads to confusing and impractical behavior in most situations, so the rules are simplified. They are:

1. if a concept is being compared with `T` or any type that accepts all other types (`auto`) the concept
is more specific
2. if the concept is being compared with another concept the result is deferred to [Concept subset matching]
3. in any other case the concept is less specific then it's competitor 

Currently, the concept evaluation mechanism evaluates to a successful match on the first acceptable candidate
for each defined binding. This has a couple of notable effects:

- generic parameters are fulfilled by the first candidate match even if other candidates would also match and bind different parameters
- inheritable objects match as they do in normal overload resolution except the "depth" is not accounted for, because that would require calculating the minimum depth of any matching binding


Concept subset matching
-------------------------

This type of matching is simple. When comparing concepts `C1` and `C2`, if all valid implementations of `C1`
are also valid implementations of `C2` but not vice versa then `C1` is a subset of `C2`. This means that
`C1` will match to `C2` and therefore the disambiguation process will prefer `C2` as it is more specific.
If neither of them are subsets of one another, then the disambiguation proceeds to complexity analysis
and the concept with the most definitions wins, if any. No definite winner is an ambiguity error at
compile time.

Recursive concepts
------------------

Concepts can reference themselves in their definitions, enabling recursive type constraints.
This is useful for matching `distinct` types that should inherit traits from their base type:

```nim
import std/typetraits

type
  PrimitiveBase = SomeNumber | bool | ptr | pointer | enum

  # Matches PrimitiveBase directly, or any distinct type whose base is Primitive
  Primitive = concept x
    x is PrimitiveBase or distinctBase(x) is Primitive

  # Application: a handle type that should be treated like a primitive
  Handle = distinct int
  SpecialHandle = distinct Handle

assert int is Primitive
assert Handle is Primitive
assert SpecialHandle is Primitive  # works through 2 levels
assert not (string is Primitive)
```

Concepts can also be mutually recursive (co-dependent):

```nim
type
  Serializable = concept
    proc serialize(s: Self; writer: var Writer)
  Writer = concept
    proc write(w: var Self; data: Serializable)
```

The compiler uses cycle detection to handle these cases without infinite recursion.

Statements and expressions
==========================

Nim uses the common statement/expression paradigm: Statements do not
produce a value in contrast to expressions. However, some expressions are
statements.

Statements are separated into `simple statements`:idx: and
`complex statements`:idx:.
Simple statements are statements that cannot contain other statements like
assignments, calls, or the `return` statement; complex statements can
contain other statements. To avoid the `dangling else problem`:idx:, complex
statements always have to be indented. The details can be found in the grammar.


Statement list expression
-------------------------

Statements can also occur in an expression context that looks
like `(stmt1; stmt2; ...; ex)`. This is called
a statement list expression or `(;)`. The type
of `(stmt1; stmt2; ...; ex)` is the type of `ex`. All the other statements
must be of type `void`. (One can use `discard` to produce a `void` type.)
`(;)` does not introduce a new scope.


Discard statement
-----------------

Example:

  ```nim
  proc p(x, y: int): int =
    result = x + y

  discard p(3, 4) # discard the return value of `p`
  ```

The `discard` statement evaluates its expression for side-effects and
throws the expression's resulting value away, and should only be used
when ignoring this value is known not to cause problems.

Ignoring the return value of a procedure without using a discard statement is
a static error.

The return value can be ignored implicitly if the called proc/iterator has
been declared with the `discardable`:idx: pragma:

  ```nim
  proc p(x, y: int): int {.discardable.} =
    result = x + y

  p(3, 4) # now valid
  ```

however the discardable pragma does not work on templates as templates substitute the AST in place. For example:

  ```nim
  {.push discardable .}
  template example(): string = "https://nim-lang.org"
  {.pop.}

  example()
  ```

This template will resolve into "https://nim-lang.org" which is a string literal and since {.discardable.} doesn't apply to literals, the compiler will error.

An empty `discard` statement is often used as a null statement:

  ```nim
  proc classify(s: string) =
    case s[0]
    of SymChars, '_': echo "an identifier"
    of '0'..'9': echo "a number"
    else: discard
  ```


Void context
------------

In a list of statements, every expression except the last one needs to have the
type `void`. In addition to this rule an assignment to the builtin `result`
symbol also triggers a mandatory `void` context for the subsequent expressions:

  ```nim
  proc invalid*(): string =
    result = "foo"
    "invalid"  # Error: value of type 'string' has to be discarded
  ```

  ```nim
  proc valid*(): string =
    let x = 317
    "valid"
  ```


Var statement
-------------

Var statements declare new local and global variables and
initialize them. A comma-separated list of variables can be used to specify
variables of the same type:

  ```nim
  var
    a: int = 0
    x, y, z: int
  ```

If an initializer is given, the type can be omitted: the variable is then of the
same type as the initializing expression. Variables are always initialized
with a default value if there is no initializing expression. The default
value depends on the type and is always a zero in binary.

============================    ==============================================
Type                            default value
============================    ==============================================
any integer type                0
any float                       0.0
char                            '\\0'
bool                            false
ref or pointer type             nil
procedural type                 nil
sequence                        `@[]`
string                          `""`
`tuple[x: A, y: B, ...]`        (zeroDefault(A), zeroDefault(B), ...)
                                (analogous for objects)
`array[0..., T]`                `[zeroDefault(T), ...]`
`range[T]`                      default(T); this may be out of the valid range
T = enum                        `cast[T](0)`; this may be an invalid value
============================    ==============================================


The implicit initialization can be avoided for optimization reasons with the
`noinit`:idx: pragma:

  ```nim
  var
    a {.noinit.}: array[0..1023, char]
  ```

If a proc is annotated with the `noinit` pragma, this refers to its implicit
`result` variable:

  ```nim
  proc returnUndefinedValue: int {.noinit.} = discard
  ```


The implicit initialization can also be prevented by the `requiresInit`:idx:
type pragma. The compiler requires an explicit initialization for the object
and all of its fields. However, it does a `control flow analysis`:idx: to prove
the variable has been initialized and does not rely on syntactic properties:

  ```nim
  type
    MyObject {.requiresInit.} = object

  proc p() =
    # the following is valid:
    var x: MyObject
    if someCondition():
      x = a()
    else:
      x = a()
    # use x
  ```

`requiresInit` pragma can also be applied to `distinct` types.

Given the following distinct type definitions:

  ```nim
  type
    Foo = object
      x: string

    DistinctFoo {.requiresInit, borrow: `.`.} = distinct Foo
    DistinctString {.requiresInit.} = distinct string
  ```

The following code blocks will fail to compile:

  ```nim
  var foo: DistinctFoo
  foo.x = "test"
  doAssert foo.x == "test"
  ```

  ```nim
  var s: DistinctString
  s = "test"
  doAssert string(s) == "test"
  ```

But these will compile successfully:

  ```nim
  let foo = DistinctFoo(Foo(x: "test"))
  doAssert foo.x == "test"
  ```

  ```nim
  let s = DistinctString("test")
  doAssert string(s) == "test"
  ```

Let statement
-------------

A `let` statement declares new local and global `single assignment`:idx:
variables and binds a value to them. The syntax is the same as that of the `var`
statement, except that the keyword `var` is replaced by the keyword `let`.
Let variables are not l-values and can thus not be passed to `var` parameters
nor can their address be taken. They cannot be assigned new values.

For let variables, the same pragmas are available as for ordinary variables.

As `let` statements are immutable after creation they need to define a value
when they are declared. The only exception to this is if the `{.importc.}`
pragma (or any of the other `importX` pragmas) is applied, in this case the
value is expected to come from native code, typically a C/C++ `const`.

Special identifier `_` (underscore)
-----------------------------------

The identifier `_` has a special meaning in declarations.
Any definition with the name `_` will not be added to scope, meaning the
definition is evaluated, but cannot be used. As a result the name `_` can be
indefinitely redefined.

  ```nim
  let _ = 123
  echo _ # error
  let _ = 456 # compiles
  ```

Tuple unpacking
---------------

In a `var`, `let` or `const` statement tuple unpacking can be performed.
The special identifier `_` can be used to ignore some parts of the tuple:

  ```nim
  proc returnsTuple(): (int, int, int) = (4, 2, 3)

  let (x, _, z) = returnsTuple()
  ```

This is treated as syntax sugar for roughly the following:

  ```nim
  let
    tmpTuple = returnsTuple()
    x = tmpTuple[0]
    z = tmpTuple[2]
  ```

For `var` or `let` statements, if the value expression is a tuple literal,
each expression is directly expanded into an assignment without the use of
a temporary variable.

  ```nim
  let (x, y, z) = (1, 2, 3)
  # becomes
  let
    x = 1
    y = 2
    z = 3
  ```

Tuple unpacking can also be nested:

  ```nim
  proc returnsNestedTuple(): (int, (int, int), int, int) = (4, (5, 7), 2, 3)

  let (x, (_, y), _, z) = returnsNestedTuple()
  ```


Const section
-------------

A const section declares constants whose values are constant expressions:

  ```nim
  import std/[strutils]
  const
    roundPi = 3.1415
    constEval = contains("abc", 'b') # computed at compile time!
  ```

Once declared, a constant's symbol can be used as a constant expression.

The value part of a constant declaration opens a new scope for each constant,
so no symbols declared in the constant value are accessible outside of it.

  ```nim
  const foo = (var a = 1; a)
  const bar = a # error
  let baz = a # error
  ```

See [Constants and Constant Expressions] for details.

Static statement/expression
---------------------------

A static statement/expression explicitly requires compile-time execution.
Even some code that has side effects is permitted in a static block:

  ```nim
  static:
    echo "echo at compile time"
  ```

`static` can also be used like a routine.

  ```nim
  proc getNum(a: int): int = a

  # Below calls "echo getNum(123)" at compile time.
  static:
    echo getNum(123)

  # Below call evaluates the "getNum(123)" at compile time, but its
  # result gets used at run time.
  echo static(getNum(123))
  ```

There are limitations on what Nim code can be executed at compile time;
see [Restrictions on Compile-Time Execution] for details.
It's a static error if the compiler cannot execute the block at compile
time.


If statement
------------

Example:

  ```nim
  var name = readLine(stdin)

  if name == "Andreas":
    echo "What a nice name!"
  elif name == "":
    echo "Don't you have a name?"
  else:
    echo "Boring name..."
  ```

The `if` statement is a simple way to make a branch in the control flow:
The expression after the keyword `if` is evaluated, if it is true
the corresponding statements after the `:` are executed. Otherwise,
the expression after the `elif` is evaluated (if there is an
`elif` branch), if it is true the corresponding statements after
the `:` are executed. This goes on until the last `elif`. If all
conditions fail, the `else` part is executed. If there is no `else`
part, execution continues with the next statement.

In `if` statements, new scopes begin immediately after
the `if`/`elif`/`else` keywords and ends after the
corresponding *then* block.
For visualization purposes the scopes have been enclosed
in `{|  |}` in the following example:

  ```nim
  if {| (let m = input =~ re"(\w+)=\w+"; m.isMatch):
    echo "key ", m[0], " value ", m[1]  |}
  elif {| (let m = input =~ re""; m.isMatch):
    echo "new m in this scope"  |}
  else: {|
    echo "m not declared here"  |}
  ```

Case statement
--------------

Example:

  ```nim
  let line = readline(stdin)
  case line
  of "delete-everything", "restart-computer":
    echo "permission denied"
  of "go-for-a-walk":     echo "please yourself"
  elif line.len == 0:     echo "empty" # optional, must come after `of` branches
  else:                   echo "unknown command" # ditto

  # indentation of the branches is also allowed; and so is an optional colon
  # after the selecting expression:
  case readline(stdin):
    of "delete-everything", "restart-computer":
      echo "permission denied"
    of "go-for-a-walk":     echo "please yourself"
    else:                   echo "unknown command"
  ```


The `case` statement is similar to the `if` statement, but it represents
a multi-branch selection. The expression after the keyword `case` is
evaluated and if its value is in a *slicelist* the corresponding statements
(after the `of` keyword) are executed. If the value is not in any
given *slicelist*, trailing `elif` and `else` parts are executed using same
semantics as for `if` statement, and `elif` is handled just like `else: if`.
If there are no `else` or `elif` parts and not
all possible values that `expr` can hold occur in a *slicelist*, a static error occurs.
This holds only for expressions of ordinal types.
"All possible values" of `expr` are determined by `expr`'s type.
To suppress the static error an `else: discard` should be used.

Only ordinal types, floats, strings and cstrings are allowed as values
in case statements.

For non-ordinal types, it is not possible to list every possible value and so
these always require an `else` part.
An exception to this rule is for the `string` type, which currently doesn't
require a trailing `else` or `elif` branch; it's unspecified whether this will
keep working in future versions.

Because case statements are checked for exhaustiveness during semantic analysis,
the value in every `of` branch must be a constant expression.
This restriction also allows the compiler to generate more performant code.

As a special semantic extension, an expression in an `of` branch of a case
statement may evaluate to a set or array constructor; the set or array is then
expanded into a list of its elements:

  ```nim
  const
    SymChars: set[char] = {'a'..'z', 'A'..'Z', '\x80'..'\xFF'}

  proc classify(s: string) =
    case s[0]
    of SymChars, '_': echo "an identifier"
    of '0'..'9': echo "a number"
    else: echo "other"

  # is equivalent to:
  proc classify(s: string) =
    case s[0]
    of 'a'..'z', 'A'..'Z', '\x80'..'\xFF', '_': echo "an identifier"
    of '0'..'9': echo "a number"
    else: echo "other"
  ```

The `case` statement doesn't produce an l-value, so the following example
won't work:

  ```nim
  type
    Foo = ref object
      x: seq[string]

  proc get_x(x: Foo): var seq[string] =
    # doesn't work
    case true
    of true:
      x.x
    else:
      x.x

  var foo = Foo(x: @[])
  foo.get_x().add("asd")
  ```

This can be fixed by explicitly using `result` or `return`:

  ```nim
  proc get_x(x: Foo): var seq[string] =
    case true
    of true:
      result = x.x
    else:
      result = x.x
  ```


When statement
--------------

Example:

  ```nim
  when sizeof(int) == 2:
    echo "running on a 16 bit system!"
  elif sizeof(int) == 4:
    echo "running on a 32 bit system!"
  elif sizeof(int) == 8:
    echo "running on a 64 bit system!"
  else:
    echo "cannot happen!"
  ```

The `when` statement is almost identical to the `if` statement with some
exceptions:

* Each condition (`expr`) has to be a constant expression (of type `bool`).
* The statements do not open a new scope.
* The statements that belong to the expression that evaluated to true are
  translated by the compiler, the other statements are not checked for
  semantics! However, each condition is checked for semantics.

The `when` statement enables conditional compilation techniques. As
a special syntactic extension, the `when` construct is also available
within `object` definitions.


When nimvm statement
--------------------

`nimvm` is a special symbol that may be used as the expression of a
`when nimvm` statement to differentiate the execution path between
compile-time and the executable.

Example:

  ```nim
  proc someProcThatMayRunInCompileTime(): bool =
    when nimvm:
      # This branch is taken at compile time.
      result = true
    else:
      # This branch is taken in the executable.
      result = false
  const ctValue = someProcThatMayRunInCompileTime()
  let rtValue = someProcThatMayRunInCompileTime()
  assert(ctValue == true)
  assert(rtValue == false)
  ```

A `when nimvm` statement must meet the following requirements:

* Its expression must always be `nimvm`. More complex expressions are not
  allowed.
* It must not contain `elif` branches.
* It must contain an `else` branch.
* Code in branches must not affect semantics of the code that follows the
  `when nimvm` statement. E.g. it must not define symbols that are used in
  the following code.

Return statement
----------------

Example:

  ```nim
  return 40 + 2
  ```

The `return` statement ends the execution of the current procedure.
It is only allowed in procedures. If there is an `expr`, this is syntactic
sugar for:

  ```nim
  result = expr
  return result
  ```


`return` without an expression is a short notation for `return result` if
the proc has a return type. The `result`:idx: variable is always the return
value of the procedure. It is automatically declared by the compiler. As all
variables, `result` is initialized to (binary) zero:

  ```nim
  proc returnZero(): int =
    # implicitly returns 0
  ```


Yield statement
---------------

Example:

  ```nim
  yield (1, 2, 3)
  ```

The `yield` statement is used instead of the `return` statement in
iterators. It is only valid in iterators. Execution is returned to the body
of the for loop that called the iterator. Yield does not end the iteration
process, but the execution is passed back to the iterator if the next iteration
starts. See the section about iterators ([Iterators and the for statement])
for further information.


Block statement
---------------

Example:

  ```nim
  var found = false
  block myblock:
    for i in 0..3:
      for j in 0..3:
        if a[j][i] == 7:
          found = true
          break myblock # leave the block, in this case both for-loops
  echo found
  ```

The block statement is a means to group statements to a (named) `block`.
Inside the block, the `break` statement is allowed to leave the block
immediately. A `break` statement can contain a name of a surrounding
block to specify which block is to be left.


Break statement
---------------

Example:

  ```nim
  break
  ```

The `break` statement is used to leave a block immediately. If `symbol`
is given, it is the name of the enclosing block that is to be left. If it is
absent, the innermost block is left.


While statement
---------------

Example:

  ```nim
  echo "Please tell me your password:"
  var pw = readLine(stdin)
  while pw != "12345":
    echo "Wrong password! Next try:"
    pw = readLine(stdin)
  ```


The `while` statement is executed until the `expr` evaluates to false.
Endless loops are no error. `while` statements open an `implicit block`
so that they can be left with a `break` statement.


Continue statement
------------------

A `continue` statement leads to the immediate next iteration of the
surrounding loop construct. It is only allowed within a loop. A continue
statement is syntactic sugar for a nested block:

  ```nim
  while expr1:
    stmt1
    continue
    stmt2
  ```

Is equivalent to:

  ```nim
  while expr1:
    block myBlockName:
      stmt1
      break myBlockName
      stmt2
  ```


Assembler statement
-------------------

The direct embedding of assembler code into Nim code is supported
by the unsafe `asm` statement. Identifiers in the assembler code that refer to
Nim identifiers shall be enclosed in a special character which can be
specified in the statement's pragmas. The default special character is `'\`'`:

  ```nim
  {.push stackTrace:off.}
  proc addInt(a, b: int): int =
    # a in eax, and b in edx
    asm """
        mov eax, `a`
        add eax, `b`
        jno theEnd
        call `raiseOverflow`
      theEnd:
    """
  {.pop.}
  ```

If the GNU assembler is used, quotes and newlines are inserted automatically:

  ```nim
  proc addInt(a, b: int): int =
    asm """
      addl %%ecx, %%eax
      jno 1
      call `raiseOverflow`
      1:
      :"=a"(`result`)
      :"a"(`a`), "c"(`b`)
    """
  ```

Instead of:

  ```nim
  proc addInt(a, b: int): int =
    asm """
      "addl %%ecx, %%eax\n"
      "jno 1\n"
      "call `raiseOverflow`\n"
      "1: \n"
      :"=a"(`result`)
      :"a"(`a`), "c"(`b`)
    """
  ```

Using statement
---------------

The `using` statement provides syntactic convenience in modules where
the same parameter names and types are used over and over. Instead of:

  ```nim
  proc foo(c: Context; n: Node) = ...
  proc bar(c: Context; n: Node, counter: int) = ...
  proc baz(c: Context; n: Node) = ...
  ```

One can tell the compiler about the convention that a parameter of
name `c` should default to type `Context`, `n` should default to
`Node` etc.:

  ```nim
  using
    c: Context
    n: Node
    counter: int

  proc foo(c, n) = ...
  proc bar(c, n, counter) = ...
  proc baz(c, n) = ...

  proc mixedMode(c, n; x, y: int) =
    # 'c' is inferred to be of the type 'Context'
    # 'n' is inferred to be of the type 'Node'
    # But 'x' and 'y' are of type 'int'.
  ```

The `using` section uses the same indentation based grouping syntax as
a `var` or `let` section.

Note that `using` is not applied for `template` since the untyped template
parameters default to the type `system.untyped`.

Mixing parameters that should use the `using` declaration with parameters
that are explicitly typed is possible and requires a semicolon between them.


If expression
-------------

An `if` expression is almost like an if statement, but it is an expression.
This feature is similar to *ternary operators* in other languages.
Example:

  ```nim
  var y = if x > 8: 9 else: 10
  ```

An `if` expression always results in a value, so the `else` part is
required. `elif` parts are also allowed.

When expression
---------------

Just like an `if` expression, but corresponding to the `when` statement.

Case expression
---------------

The `case` expression is again very similar to the case statement:

  ```nim
  var favoriteFood = case animal
    of "dog": "bones"
    of "cat": "mice"
    elif animal.endsWith"whale": "plankton"
    else:
      echo "I'm not sure what to serve, but everybody loves ice cream"
      "ice cream"
  ```

As seen in the above example, the case expression can also introduce side
effects. When multiple statements are given for a branch, Nim will use
the last expression as the result value.

Block expression
----------------

A `block` expression is almost like a block statement, but it is an expression
that uses the last expression under the block as the value.
It is similar to the statement list expression, but the statement list expression
does not open a new block scope.

  ```nim
  let a = block:
    var fib = @[0, 1]
    for i in 0..10:
      fib.add fib[^1] + fib[^2]
    fib
  ```

Table constructor
-----------------

A table constructor is syntactic sugar for an array constructor:

  ```nim
  {"key1": "value1", "key2", "key3": "value2"}

  # is the same as:
  [("key1", "value1"), ("key2", "value2"), ("key3", "value2")]
  ```


The empty table can be written `{:}` (in contrast to the empty set
which is `{}`) which is thus another way to write the empty array
constructor `[]`. This slightly unusual way of supporting tables
has lots of advantages:

* The order of the (key,value)-pairs is preserved, thus it is easy to
  support ordered dicts with for example `{key: val}.newOrderedTable`.
* A table literal can be put into a `const` section and the compiler
  can easily put it into the executable's data section just like it can
  for arrays and the generated data section requires a minimal amount
  of memory.
* Every table implementation is treated equally syntactically.
* Apart from the minimal syntactic sugar, the language core does not need to
  know about tables.


Type conversions
----------------

Syntactically a *type conversion* is like a procedure call, but a
type name replaces the procedure name. A type conversion is always
safe in the sense that a failure to convert a type to another
results in an exception (if it cannot be determined statically).

Ordinary procs are often preferred over type conversions in Nim: For instance,
`$` is the `toString` operator by convention and `toFloat` and `toInt`
can be used to convert from floating-point to integer or vice versa.

Type conversion can also be used to disambiguate overloaded routines:

  ```nim
  proc p(x: int) = echo "int"
  proc p(x: string) = echo "string"

  let procVar = (proc(x: string))(p)
  procVar("a")
  ```

Since operations on unsigned numbers wrap around and are unchecked so are
type conversions to unsigned integers and between unsigned integers. The
rationale for this is mostly better interoperability with the C Programming
language when algorithms are ported from C to Nim.

**Note**: Historically the operations
were unchecked and the conversions were sometimes checked but starting with
the revision 1.0.4 of this document and the language implementation the
conversions too are now *always unchecked*.


Type casts
----------

*Type casts* are a crude mechanism to interpret the bit pattern of an expression
as if it would be of another type. Type casts are only needed for low-level
programming and are inherently unsafe.

  ```nim
  cast[int](x)
  ```

The target type of a cast must be a concrete type, for instance, a target type
that is a type class (which is non-concrete) would be invalid:

  ```nim
  type Foo = int or float
  var x = cast[Foo](1) # Error: cannot cast to a non concrete type: 'Foo'
  ```

Type casts should not be confused with *type conversions,* as mentioned in the
prior section. Unlike type conversions, a type cast cannot change the underlying
bit pattern of the data being cast (aside from that the size of the target type
may differ from the source type). Casting resembles *type punning* in other
languages or C++'s `reinterpret_cast`:cpp: and `bit_cast`:cpp: features.

If the size of the target type is larger than the size of the source type,
the remaining memory is zeroed.

The addr operator
-----------------
The `addr` operator returns the address of an l-value. If the type of the
location is `T`, the `addr` operator result is of the type `ptr T`. An
address is always an untraced reference. Taking the address of an object that
resides on the stack is **unsafe**, as the pointer may live longer than the
object on the stack and can thus reference a non-existing object. One can get
the address of variables. For easier interoperability with other compiled languages
such as C, retrieving the address of a `let` variable, a parameter,
or a `for` loop variable can be accomplished too:

  ```nim
  let t1 = "Hello"
  var
    t2 = t1
    t3 : pointer = addr(t2)
  echo repr(addr(t2))
  # --> ref 0x7fff6b71b670 --> 0x10bb81050"Hello"
  echo cast[ptr string](t3)[]
  # --> Hello
  # The following line also works
  echo repr(addr(t1))
  ```

The unsafeAddr operator
-----------------------

The `unsafeAddr` operator is a deprecated alias for the `addr` operator:

  ```nim
  let myArray = [1, 2, 3]
  foreignProcThatTakesAnAddr(unsafeAddr myArray)
  ```

Procedures
==========

What most programming languages call `methods`:idx: or `functions`:idx: are
called `procedures`:idx: in Nim. A procedure
declaration consists of an identifier, zero or more formal parameters, a return
value type and a block of code. Formal parameters are declared as a list of
identifiers separated by either comma or semicolon. A parameter is given a type
by `: typename`. The type applies to all parameters immediately before it,
until either the beginning of the parameter list, a semicolon separator, or an
already typed parameter, is reached. The semicolon can be used to make
separation of types and subsequent identifiers more distinct.

  ```nim
  # Using only commas
  proc foo(a, b: int, c, d: bool): int

  # Using semicolon for visual distinction
  proc foo(a, b: int; c, d: bool): int

  # Will fail: a is untyped since ';' stops type propagation.
  proc foo(a; b: int; c, d: bool): int
  ```

A parameter may be declared with a default value which is used if the caller
does not provide a value for the argument. The value will be reevaluated
every time the function is called.

  ```nim
  # b is optional with 47 as its default value.
  proc foo(a: int, b: int = 47): int
  ```

Parameters can be declared mutable and so allow the proc to modify those
arguments, by using the type modifier `var`.

  ```nim
  # "returning" a value to the caller through the 2nd argument
  # Notice that the function uses no actual return value at all (ie void)
  proc foo(inp: int, outp: var int) =
    outp = inp + 47
  ```

If the proc declaration doesn't have a body, it is a `forward`:idx: declaration.
If the proc returns a value, the procedure body can access an implicitly declared
variable named `result`:idx: that represents the return value. Procs can be
overloaded. The overloading resolution algorithm determines which proc is the
best match for the arguments. Example:

  ```nim
  proc toLower(c: char): char = # toLower for characters
    if c in {'A'..'Z'}:
      result = chr(ord(c) + (ord('a') - ord('A')))
    else:
      result = c

  proc toLower(s: string): string = # toLower for strings
    result = newString(len(s))
    for i in 0..len(s) - 1:
      result[i] = toLower(s[i]) # calls toLower for characters; no recursion!
  ```

Calling a procedure can be done in many ways:

  ```nim
  proc callme(x, y: int, s: string = "", c: char, b: bool = false) = ...

  # call with positional arguments      # parameter bindings:
  callme(0, 1, "abc", '\t', true)       # (x=0, y=1, s="abc", c='\t', b=true)
  # call with named and positional arguments:
  callme(y=1, x=0, "abd", '\t')         # (x=0, y=1, s="abd", c='\t', b=false)
  # call with named arguments (order is not relevant):
  callme(c='\t', y=1, x=0)              # (x=0, y=1, s="", c='\t', b=false)
  # call as a command statement: no () needed:
  callme 0, 1, "abc", '\t'              # (x=0, y=1, s="abc", c='\t', b=false)
  ```

A procedure may call itself recursively.


`Operators`:idx: are procedures with a special operator symbol as identifier:

  ```nim
  proc `$` (x: int): string =
    # converts an integer to a string; this is a prefix operator.
    result = intToStr(x)
  ```

Operators with one parameter are prefix operators, operators with two
parameters are infix operators. (However, the parser distinguishes these from
the operator's position within an expression.) There is no way to declare
postfix operators: all postfix operators are built-in and handled by the
grammar explicitly.

Any operator can be called like an ordinary proc with the \`opr\`
notation. (Thus an operator can have more than two parameters):

  ```nim
  proc `*+` (a, b, c: int): int =
    # Multiply and add
    result = a * b + c

  assert `*+`(3, 4, 6) == `+`(`*`(3, 4), 6)
  ```


Export marker
-------------

If a declared symbol is marked with an `asterisk`:idx: it is exported from the
current module:

  ```nim
  proc exportedEcho*(s: string) = echo s
  proc `*`*(a: string; b: int): string =
    result = newStringOfCap(a.len * b)
    for i in 1..b: result.add a

  var exportedVar*: int
  const exportedConst* = 78
  type
    ExportedType* = object
      exportedField*: int
  ```


Method call syntax
------------------

For object-oriented programming, the syntax `obj.methodName(args)` can be used
instead of `methodName(obj, args)`. The parentheses can be omitted if
there are no remaining arguments: `obj.len` (instead of `len(obj)`).

This method call syntax is not restricted to objects, it can be used
to supply any type of first argument for procedures:

  ```nim
  echo "abc".len # is the same as echo len "abc"
  echo "abc".toUpper()
  echo {'a', 'b', 'c'}.card
  stdout.writeLine("Hallo") # the same as writeLine(stdout, "Hallo")
  ```

Another way to look at the method call syntax is that it provides the missing
postfix notation.

The method call syntax conflicts with explicit generic instantiations:
`p[T](x)` cannot be written as `x.p[T]` because `x.p[T]` is always
parsed as `(x.p)[T]`.

See also: [Limitations of the method call syntax].

The `[: ]` notation has been designed to mitigate this issue: `x.p[:T]`
is rewritten by the parser to `p[T](x)`, `x.p[:T](y)` is rewritten to
`p[T](x, y)`. Note that `[: ]` has no AST representation, the rewrite
is performed directly in the parsing step.


Properties
----------
Nim has no need for *get-properties*: Ordinary get-procedures that are called
with the *method call syntax* achieve the same. But setting a value is
different; for this, a special setter syntax is needed:

  ```nim
  # Module asocket
  type
    Socket* = ref object of RootObj
      host: int # cannot be accessed from the outside of the module

  proc `host=`*(s: var Socket, value: int) {.inline.} =
    ## setter of hostAddr.
    ## This accesses the 'host' field and is not a recursive call to
    ## `host=` because the builtin dot access is preferred if it is
    ## available:
    s.host = value

  proc host*(s: Socket): int {.inline.} =
    ## getter of hostAddr
    ## This accesses the 'host' field and is not a recursive call to
    ## `host` because the builtin dot access is preferred if it is
    ## available:
    s.host
  ```

  ```nim
  # module B
  import asocket
  var s: Socket
  new s
  s.host = 34  # same as `host=`(s, 34)
  ```

A proc defined as `f=` (with the trailing `=`) is called
a `setter`:idx:. A setter can be called explicitly via the common
backticks notation:

  ```nim
  proc `f=`(x: MyObject; value: string) =
    discard

  `f=`(myObject, "value")
  ```


`f=` can be called implicitly in the pattern
`x.f = value` if and only if the type of `x` does not have a field
named `f` or if `f` is not visible in the current module. These rules
ensure that object fields and accessors can have the same name. Within the
module `x.f` is then always interpreted as field access and outside the
module it is interpreted as an accessor proc call.


Command invocation syntax
-------------------------

Routines can be invoked without the `()` if the call is syntactically
a statement. This command invocation syntax also works for
expressions, but then only a single argument may follow. This restriction
means `echo f 1, f 2` is parsed as `echo(f(1), f(2))` and not as
`echo(f(1, f(2)))`. The method call syntax may be used to provide one
more argument in this case:

  ```nim
  proc optarg(x: int, y: int = 0): int = x + y
  proc singlearg(x: int): int = 20*x

  echo optarg 1, " ", singlearg 2  # prints "1 40"

  let fail = optarg 1, optarg 8   # Wrong. Too many arguments for a command call
  let x = optarg(1, optarg 8)  # traditional procedure call with 2 arguments
  let y = 1.optarg optarg 8    # same thing as above, w/o the parenthesis
  assert x == y
  ```

The command invocation syntax also can't have complex expressions as arguments.
For example: [anonymous procedures], `if`,
`case` or `try`. Function calls with no arguments still need () to
distinguish between a call and the function itself as a first-class value.


Closures
--------

Procedures can appear at the top level in a module as well as inside other
scopes, in which case they are called nested procs. A nested proc can access
local variables from its enclosing scope and if it does so it becomes a
closure. Any captured variables are stored in a hidden additional argument
to the closure (its environment) and they are accessed by reference by both
the closure and its enclosing scope (i.e. any modifications made to them are
visible in both places). The closure environment may be allocated on the heap
or on the stack if the compiler determines that this would be safe.

### Creating closures in loops

Since closures capture local variables by reference it is often not wanted
behavior inside loop bodies. See [closureScope](
system.html#closureScope.t,untyped) and [capture](
sugar.html#capture.m,varargs[typed],untyped) for details on how to change this behavior.

Anonymous procedures
--------------------

Unnamed procedures can be used as lambda expressions to pass into other
procedures:

  ```nim
  var cities = @["Frankfurt", "Tokyo", "New York", "Kyiv"]

  cities.sort(proc (x, y: string): int =
    cmp(x.len, y.len))
  ```


Procs as expressions can appear both as nested procs and inside top-level
executable code. The  [sugar](sugar.html) module contains the `=>` macro
which enables a more succinct syntax for anonymous procedures resembling
lambdas as they are in languages like JavaScript, C#, etc.

Do notation
-----------

As a special convenience notation that keeps most elements of a
regular proc expression, the `do` keyword can be used to pass
anonymous procedures to routines:

  ```nim
  var cities = @["Frankfurt", "Tokyo", "New York", "Kyiv"]

  sort(cities) do (x, y: string) -> int:
    cmp(x.len, y.len)

  # Less parentheses using the method plus command syntax:
  cities = cities.map do (x: string) -> string:
    "City of " & x
  ```

`do` is written after the parentheses enclosing the regular proc parameters.
The proc expression represented by the `do` block is appended to the routine
call as the last argument. In calls using the command syntax, the `do` block
will bind to the immediately preceding expression rather than the command call.

`do` with a parameter list or pragma list corresponds to an anonymous `proc`,
however `do` without parameters or pragmas is treated as a normal statement
list. This allows macros to receive both indented statement lists as an
argument in inline calls, as well as a direct mirror of Nim's routine syntax.

  ```nim
  # Passing a statement list to an inline macro:
  macroResults.add quote do:
    if not `ex`:
      echo `info`, ": Check failed: ", `expString`

  # Processing a routine definition in a macro:
  rpc(router, "add") do (a, b: int) -> int:
    result = a + b
  ```

Func
----

The `func` keyword introduces a shortcut for a `noSideEffect`:idx: proc.

  ```nim
  func binarySearch[T](a: openArray[T]; elem: T): int
  ```

Is short for:

  ```nim
  proc binarySearch[T](a: openArray[T]; elem: T): int {.noSideEffect.}
  ```



Routines
--------

A routine is a symbol of kind: `proc`, `func`, `method`, `iterator`, `macro`, `template`, `converter`.

Type bound operators
--------------------

A type bound operator is a `proc` or `func` whose name starts with `=` but isn't an operator
(i.e. containing only symbols, such as `==`). These are unrelated to setters
(see [Properties]), which instead end in `=`.
A type bound operator declared for a type applies to the type regardless of whether
the operator is in scope (including if it is private).

  ```nim
  # foo.nim:
  var witness* = 0
  type Foo[T] = object
  proc initFoo*(T: typedesc): Foo[T] = discard
  proc `=destroy`[T](x: var Foo[T]) = witness.inc # type bound operator

  # main.nim:
  import foo
  block:
    var a = initFoo(int)
    doAssert witness == 0
  doAssert witness == 1
  block:
    var a = initFoo(int)
    doAssert witness == 1
    `=destroy`(a) # can be called explicitly, even without being in scope
    doAssert witness == 2
  # will still be called upon exiting scope
  doAssert witness == 3
  ```

Type bound operators are:
`=destroy`, `=copy`, `=sink`, `=trace`, `=deepcopy`, `=wasMoved`, `=dup`.

These operations can be *overridden* instead of *overloaded*. This means that
the implementation is automatically lifted to structured types. For instance,
if the type `T` has an overridden assignment operator `=`, this operator is
also used for assignments of the type `seq[T]`.

Since these operations are bound to a type, they have to be bound to a
nominal type for reasons of simplicity of implementation; this means an
overridden `deepCopy` for `ref T` is really bound to `T` and not to `ref T`.
This also means that one cannot override `deepCopy` for both `ptr T` and
`ref T` at the same time, instead a distinct or object helper type has to be
used for one pointer type.

For more details on some of those procs, see
[Lifetime-tracking hooks](destructors.html#lifetimeminustracking-hooks).

Nonoverloadable builtins
------------------------

The following built-in procs cannot be overloaded for reasons of implementation
simplicity (they require specialized semantic checking):

    declared, defined, definedInScope, compiles, sizeof,
    is, shallowCopy, getAst, astToStr, spawn, procCall

Thus, they act more like keywords than like ordinary identifiers; unlike a
keyword however, a redefinition may `shadow`:idx: the definition in
the [system](system.html) module.
From this list the following should not be written in dot
notation `x.f` since `x` cannot be type-checked before it gets passed
to `f`:

    declared, defined, definedInScope, compiles, getAst, astToStr


Var parameters
--------------
The type of a parameter may be prefixed with the `var` keyword:

  ```nim
  proc divmod(a, b: int; res, remainder: var int) =
    res = a div b
    remainder = a mod b

  var
    x, y: int

  divmod(8, 5, x, y) # modifies x and y
  assert x == 1
  assert y == 3
  ```

In the example, `res` and `remainder` are `var parameters`.
Var parameters can be modified by the procedure and the changes are
visible to the caller. The argument passed to a var parameter has to be
an l-value. Var parameters are implemented as hidden pointers. The
above example is equivalent to:

  ```nim
  proc divmod(a, b: int; res, remainder: ptr int) =
    res[] = a div b
    remainder[] = a mod b

  var
    x, y: int
  divmod(8, 5, addr(x), addr(y))
  assert x == 1
  assert y == 3
  ```

In the examples, var parameters or pointers are used to provide two
return values. This can be done in a cleaner way by returning a tuple:

  ```nim
  proc divmod(a, b: int): tuple[res, remainder: int] =
    (a div b, a mod b)

  var t = divmod(8, 5)

  assert t.res == 1
  assert t.remainder == 3
  ```

One can use `tuple unpacking`:idx: to access the tuple's fields:

  ```nim
  var (x, y) = divmod(8, 5) # tuple unpacking
  assert x == 1
  assert y == 3
  ```


**Note**: `var` parameters are never necessary for efficient parameter
passing. Since non-var parameters cannot be modified the compiler is always
free to pass arguments by reference if it considers it can speed up execution.


Var return type
---------------

A proc, converter, or iterator may return a `var` type which means that the
returned value is an l-value and can be modified by the caller:

  ```nim
  var g = 0

  proc writeAccessToG(): var int =
    result = g

  writeAccessToG() = 6
  assert g == 6
  ```

It is a static error if the implicitly introduced pointer could be
used to access a location beyond its lifetime:

  ```nim
  proc writeAccessToG(): var int =
    var g = 0
    result = g # Error!
  ```

For iterators, a component of a tuple return type can have a `var` type too:

  ```nim
  iterator mpairs(a: var seq[string]): tuple[key: int, val: var string] =
    for i in 0..a.high:
      yield (i, a[i])
  ```

In the standard library every name of a routine that returns a `var` type
starts with the prefix `m` per convention.


.. include:: manual/var_t_return.md

### Future directions

Later versions of Nim can be more precise about the borrowing rule with
a syntax like:

  ```nim
  proc foo(other: Y; container: var X): var T from container
  ```

Here `var T from container` explicitly exposes that the
location is derived from the second parameter (called
'container' in this case). The syntax `var T from p` specifies a type
`varTy[T, 2]` which is incompatible with `varTy[T, 1]`.


NRVO
----

**Note**: This section describes the current implementation. This part
of the language specification will be changed.
See https://github.com/nim-lang/RFCs/issues/230 for more information.

The return value is represented inside the body of a routine as the special
`result`:idx: variable. This allows for a mechanism much like C++'s
"named return value optimization" (`NRVO`:idx:). NRVO means that the stores
to `result` inside `p` directly affect the destination `dest`
in `let/var dest = p(args)` (definition of `dest`) and also in `dest = p(args)`
(assignment to `dest`). This is achieved by rewriting `dest = p(args)`
to `p'(args, dest)` where `p'` is a variation of `p` that returns `void` and
receives a hidden mutable parameter representing `result`.

Informally:

  ```nim
  proc p(): BigT = ...

  var x = p()
  x = p()

  # is roughly turned into:

  proc p(result: var BigT) = ...

  var x; p(x)
  p(x)
  ```


Let `T`'s be `p`'s return type. NRVO applies for `T`
if `sizeof(T) >= N` (where `N` is implementation dependent),
in other words, it applies for "big" structures.

If `p` can raise an exception, NRVO applies regardless. This can produce
observable differences in behavior:

  ```nim
  type
    BigT = array[16, int]

  proc p(raiseAt: int): BigT =
    for i in 0..high(result):
      if i == raiseAt: raise newException(ValueError, "interception")
      result[i] = i

  proc main =
    var x: BigT
    try:
      x = p(8)
    except ValueError:
      doAssert x == [0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 0]

  main()
  ```


The compiler can produce a warning in these cases, however this behavior is
turned off by default. It can be enabled for a section of code via the
`warning[ObservableStores]` and `push`/`pop` pragmas. Take the above code
as an example:

  ```nim
  {.push warning[ObservableStores]: on.}
  main()
  {.pop.}
  ```

Overloading of the subscript operator
-------------------------------------

The `[]` subscript operator for arrays/openarrays/sequences can be overloaded
for any type (with some exceptions) by defining a routine with the name `[]`.

  ```nim
  type Foo = object
    data: seq[int]
  
  proc `[]`(foo: Foo, i: int): int =
    result = foo.data[i]
  
  let foo = Foo(data: @[1, 2, 3])
  echo foo[1] # 2
  ```

Assignment to subscripts can also be overloaded by naming a routine `[]=`,
which has precedence over assigning to the result of `[]`.

  ```nim
  type Foo = object
    data: seq[int]
  
  proc `[]`(foo: Foo, i: int): int =
    result = foo.data[i]
  proc `[]=`(foo: var Foo, i: int, val: int) =
    foo.data[i] = val
  
  var foo = Foo(data: @[1, 2, 3])
  echo foo[1] # 2
  foo[1] = 5
  echo foo.data # @[1, 5, 3]
  echo foo[1] # 5
  ```

Overloads of the subscript operator cannot be applied to routine or type
symbols themselves, as this conflicts with the syntax for instantiating
generic parameters, i.e. `foo[int](1, 2, 3)` or `Foo[int]`.


Methods
=============

Procedures always use static dispatch. Methods use dynamic
dispatch. For dynamic dispatch to work on an object it should be a reference
type.

  ```nim
  type
    Expression = ref object of RootObj ## abstract base class for an expression
    Literal = ref object of Expression
      x: int
    PlusExpr = ref object of Expression
      a, b: Expression

  method eval(e: Expression): int {.base.} =
    # override this base method
    raise newException(CatchableError, "Method without implementation override")

  method eval(e: Literal): int = return e.x

  method eval(e: PlusExpr): int =
    # watch out: relies on dynamic binding
    result = eval(e.a) + eval(e.b)

  proc newLit(x: int): Literal =
    new(result)
    result.x = x

  proc newPlus(a, b: Expression): PlusExpr =
    new(result)
    result.a = a
    result.b = b

  echo eval(newPlus(newPlus(newLit(1), newLit(2)), newLit(4)))
  ```

In the example the constructors `newLit` and `newPlus` are procs
because they should use static binding, but `eval` is a method because it
requires dynamic binding.

As can be seen in the example, base methods have to be annotated with
the `base`:idx: pragma. The `base` pragma also acts as a reminder for the
programmer that a base method `m` is used as the foundation to determine all
the effects that a call to `m` might cause.


**Note**: Compile-time execution is not (yet) supported for methods.

**Note**: Starting from Nim 0.20, generic methods are deprecated.

Multi-methods
--------------

**Note:** Starting from Nim 0.20, to use multi-methods one must explicitly pass
`--multimethods:on`:option: when compiling.

In a multi-method, all parameters that have an object type are used for the
dispatching:

  ```nim  test = "nim c --multiMethods:on $1"
  type
    Thing = ref object of RootObj
    Unit = ref object of Thing
      x: int

  method collide(a, b: Thing) {.base, inline.} =
    quit "to override!"

  method collide(a: Thing, b: Unit) {.inline.} =
    echo "1"

  method collide(a: Unit, b: Thing) {.inline.} =
    echo "2"

  var a, b: Unit
  new a
  new b
  collide(a, b) # output: 2
  ```

Inhibit dynamic method resolution via procCall
-----------------------------------------------

Dynamic method resolution can be inhibited via the builtin `system.procCall`:idx:.
This is somewhat comparable to the `super`:idx: keyword that traditional OOP
languages offer.

  ```nim  test = "nim c $1"
  type
    Thing = ref object of RootObj
    Unit = ref object of Thing
      x: int

  method m(a: Thing) {.base.} =
    echo "base"

  method m(a: Unit) =
    # Call the base method:
    procCall m(Thing(a))
    echo "1"
  ```


Iterators and the for statement
===============================

The `for`:idx: statement is an abstract mechanism to iterate over the elements
of a container. It relies on an `iterator`:idx: to do so. Like `while`
statements, `for` statements open an `implicit block`:idx: so that they
can be left with a `break` statement.

The `for` loop declares iteration variables - their scope reaches until the
end of the loop body. The iteration variables' types are inferred by the
return type of the iterator.

An iterator is similar to a procedure, except that it can be called in the
context of a `for` loop. Iterators provide a way to specify the iteration over
an abstract type. The `yield` statement in the called iterator plays a key
role in the execution of a `for` loop. Whenever a `yield` statement is
reached, the data is bound to the `for` loop variables and control continues
in the body of the `for` loop. The iterator's local variables and execution
state are automatically saved between calls. Example:

  ```nim
  # this definition exists in the system module
  iterator items*(a: string): char {.inline.} =
    var i = 0
    while i < len(a):
      yield a[i]
      inc(i)

  for ch in items("hello world"): # `ch` is an iteration variable
    echo ch
  ```

The compiler generates code as if the programmer had written this:

  ```nim
  var i = 0
  while i < len(a):
    var ch = a[i]
    echo ch
    inc(i)
  ```

If the iterator yields a tuple, there can be as many iteration variables
as there are components in the tuple. The i'th iteration variable's type is
the type of the i'th component. In other words, implicit tuple unpacking in a
for loop context is supported.

Implicit items/pairs invocations
--------------------------------

If the for loop expression `e` does not denote an iterator and the for loop
has exactly 1 variable, the for loop expression is rewritten to `items(e)`;
i.e. an `items` iterator is implicitly invoked:

  ```nim
  for x in [1,2,3]: echo x
  ```

If the for loop has exactly 2 variables, a `pairs` iterator is implicitly
invoked.

Symbol lookup of the identifiers `items`/`pairs` is performed after
the rewriting step, so that all overloads of `items`/`pairs` are taken
into account.


First-class iterators
---------------------

There are 2 kinds of iterators in Nim: *inline* and *closure* iterators.
An `inline iterator`:idx: is an iterator that's always inlined by the compiler
leading to zero overhead for the abstraction, but may result in a heavy
increase in code size.

Caution: the body of a for loop over an inline iterator is inlined into
each `yield` statement appearing in the iterator code,
so ideally the code should be refactored to contain a single yield when possible
to avoid code bloat.

Inline iterators are second class citizens;
They can be passed as parameters only to other inlining code facilities like
templates, macros, and other inline iterators.

In contrast to that, a `closure iterator`:idx: can be passed around more freely:

  ```nim
  iterator count0(): int {.closure.} =
    yield 0

  iterator count2(): int {.closure.} =
    var x = 1
    yield x
    inc x
    yield x

  proc invoke(iter: iterator(): int {.closure.}) =
    for x in iter(): echo x

  invoke(count0)
  invoke(count2)
  ```

Closure iterators and inline iterators have some restrictions:

1. For now, a closure iterator cannot be executed at compile time.
2. `return` is allowed in a closure iterator but not in an inline iterator
   (but rarely useful) and ends the iteration.
3. Inline iterators cannot be recursive.
4. Neither inline nor closure iterators have the special `result` variable.

Iterators that are neither marked `{.closure.}` nor `{.inline.}` explicitly
default to being inline, but this may change in future versions of the
implementation.

The `iterator` type is always of the calling convention `closure`
implicitly; the following example shows how to use iterators to implement
a `collaborative tasking`:idx: system:

  ```nim
  # simple tasking:
  type
    Task = iterator (ticker: int)

  iterator a1(ticker: int) {.closure.} =
    echo "a1: A"
    yield
    echo "a1: B"
    yield
    echo "a1: C"
    yield
    echo "a1: D"

  iterator a2(ticker: int) {.closure.} =
    echo "a2: A"
    yield
    echo "a2: B"
    yield
    echo "a2: C"

  proc runTasks(t: varargs[Task]) =
    var ticker = 0
    while true:
      let x = t[ticker mod t.len]
      if finished(x): break
      x(ticker)
      inc ticker

  runTasks(a1, a2)
  ```

The builtin `system.finished` can be used to determine if an iterator has
finished its operation; no exception is raised on an attempt to invoke an
iterator that has already finished its work.

Note that `system.finished` is error-prone to use because it only returns
`true` one iteration after the iterator has finished:

  ```nim
  iterator mycount(a, b: int): int {.closure.} =
    var x = a
    while x <= b:
      yield x
      inc x

  var c = mycount # instantiate the iterator
  while not finished(c):
    echo c(1, 3)

  # Produces
  1
  2
  3
  0
  ```

Instead, this code has to be used:

  ```nim
  var c = mycount # instantiate the iterator
  while true:
    let value = c(1, 3)
    if finished(c): break # and discard 'value'!
    echo value
  ```

It helps to think that the iterator actually returns a
pair `(value, done)` and `finished` is used to access the hidden `done`
field.


Closure iterators are *resumable functions* and so one has to provide the
arguments to every call. To get around this limitation one can capture
parameters of an outer factory proc:

  ```nim
  proc mycount(a, b: int): iterator (): int =
    result = iterator (): int =
      var x = a
      while x <= b:
        yield x
        inc x

  let foo = mycount(1, 4)

  for f in foo():
    echo f
  ```

The call can be made more like an inline iterator with a for loop macro:

  ```nim
  import std/macros
  macro toItr(x: ForLoopStmt): untyped =
    let expr = x[0]
    let call = x[1][1] # Get foo out of toItr(foo)
    let body = x[2]
    result = quote do:
      block:
        let itr = `call`
        for `expr` in itr():
            `body`

  for f in toItr(mycount(1, 4)): # using early `proc mycount`
    echo f
  ```

Because of full backend function call apparatus involvement, closure iterator
invocation is typically higher cost than inline iterators. Adornment by
a macro wrapper at the call site like this is a possibly useful reminder.

The factory `proc`, as an ordinary procedure, can be recursive. The
above macro allows such recursion to look much like a recursive iterator
would. For example:

  ```nim
  proc recCountDown(n: int): iterator(): int =
    result = iterator(): int =
      if n > 0:
        yield n
        for e in toItr(recCountDown(n - 1)):
          yield e

  for i in toItr(recCountDown(6)): # Emits: 6 5 4 3 2 1
    echo i
  ```


See also [iterable] for passing iterators to templates and macros.

Converters
==========

A converter is like an ordinary proc except that it enhances
the "implicitly convertible" type relation (see [Convertible relation]):

  ```nim
  # bad style ahead: Nim is not C.
  converter toBool(x: int): bool = x != 0

  if 4:
    echo "compiles"
  ```


A converter can also be explicitly invoked for improved readability. Note that
implicit converter chaining is not supported: If there is a converter from
type A to type B and from type B to type C, the implicit conversion from A to C
is not provided.


Type sections
=============

Example:

  ```nim
  type # example demonstrating mutually recursive types
    Node = ref object  # an object managed by the garbage collector (ref)
      le, ri: Node     # left and right subtrees
      sym: ref Sym     # leaves contain a reference to a Sym

    Sym = object       # a symbol
      name: string     # the symbol's name
      line: int        # the line the symbol was declared in
      code: Node       # the symbol's abstract syntax tree
  ```

A type section begins with the `type` keyword. It contains multiple
type definitions. A type definition binds a type to a name. Type definitions
can be recursive or even mutually recursive. Mutually recursive types are only
possible within a single `type` section. Nominal types like `objects`
or `enums` can only be defined in a `type` section.



Exception handling
==================

Try statement
-------------

Example:

  ```nim
  # read the first two lines of a text file that should contain numbers
  # and tries to add them
  var
    f: File
  if open(f, "numbers.txt"):
    try:
      var a = readLine(f)
      var b = readLine(f)
      echo "sum: " & $(parseInt(a) + parseInt(b))
    except OverflowDefect:
      echo "overflow!"
    except ValueError, IOError:
      echo "catch multiple exceptions!"
    except CatchableError:
      echo "Catchable exception!"
    finally:
      close(f)
  ```


The statements after the `try` are executed in sequential order unless
an exception `e` is raised. If the exception type of `e` matches any
listed in an `except` clause, the corresponding statements are executed.
The statements following the `except` clauses are called
`exception handlers`:idx:.

If there is a `finally`:idx: clause, it is always executed after the
exception handlers.

The exception is *consumed* in an exception handler. However, an
exception handler may raise another exception. If the exception is not
handled, it is propagated through the call stack. This means that often
the rest of the procedure - that is not within a `finally` clause -
is not executed (if an exception occurs).


Try expression
--------------

Try can also be used as an expression; the type of the `try` branch then
needs to fit the types of `except` branches, but the type of the `finally`
branch always has to be `void`:

  ```nim test
  from std/strutils import parseInt

  let x = try: parseInt("133a")
          except ValueError: -1
          finally: echo "hi"
  ```


To prevent confusing code there is a parsing limitation; if the `try`
follows a `(` it has to be written as a one liner:

  ```nim test
  from std/strutils import parseInt
  let x = (try: parseInt("133a") except ValueError: -1)
  ```


Except clauses
--------------

Within an `except` clause it is possible to access the current exception
using the following syntax:

  ```nim
  try:
    # ...
  except IOError as e:
    # Now use "e"
    echo "I/O error: " & e.msg
  ```

Alternatively, it is possible to use `getCurrentException` to retrieve the
exception that has been raised:

  ```nim
  try:
    # ...
  except IOError:
    let e = getCurrentException()
    # Now use "e"
  ```

Note that `getCurrentException` always returns a `ref Exception`
type. If a variable of the proper type is needed (in the example
above, `IOError`), one must convert it explicitly:

  ```nim
  try:
    # ...
  except IOError:
    let e = (ref IOError)(getCurrentException())
    # "e" is now of the proper type
  ```

However, this is seldom needed. The most common case is to extract an
error message from `e`, and for such situations, it is enough to use
`getCurrentExceptionMsg`:

  ```nim
  try:
    # ...
  except CatchableError:
    echo getCurrentExceptionMsg()
  ```

Custom exceptions
-----------------

It is possible to create custom exceptions. A custom exception is a custom type:

  ```nim
  type
    LoadError* = object of Exception
  ```

Ending the custom exception's name with `Error` is recommended.

Custom exceptions can be raised just like any other exception, e.g.:

  ```nim
  raise newException(LoadError, "Failed to load data")
  ```

Defer statement
---------------

Instead of a `try finally` statement a `defer` statement can be used, which
avoids lexical nesting and offers more flexibility in terms of scoping as shown
below.

Any statements following the `defer` will be considered
to be in an implicit try block in the current block:

  ```nim  test = "nim c $1"
  proc main =
    var f = open("numbers.txt", fmWrite)
    defer: close(f)
    f.write "abc"
    f.write "def"
  ```

Is rewritten to:

  ```nim  test = "nim c $1"
  proc main =
    var f = open("numbers.txt")
    try:
      f.write "abc"
      f.write "def"
    finally:
      close(f)
  ```

When `defer` is at the outermost scope of a template/macro, its scope extends
to the block where the template/macro is called from:

  ```nim  test = "nim c $1"
  template safeOpenDefer(f, path) =
    var f = open(path, fmWrite)
    defer: close(f)

  template safeOpenFinally(f, path, body) =
    var f = open(path, fmWrite)
    try: body # without `defer`, `body` must be specified as parameter
    finally: close(f)

  block:
    safeOpenDefer(f, "/tmp/z01.txt")
    f.write "abc"
  block:
    safeOpenFinally(f, "/tmp/z01.txt"):
      f.write "abc" # adds a lexical scope
  block:
    var f = open("/tmp/z01.txt", fmWrite)
    try:
      f.write "abc" # adds a lexical scope
    finally: close(f)
  ```

Top-level `defer` statements are not supported
since it's unclear what such a statement should refer to.


Raise statement
---------------

Example:

  ```nim
  raise newException(IOError, "IO failed")
  ```

Apart from built-in operations like array indexing, memory allocation, etc.
the `raise` statement is the only way to raise an exception.

.. XXX document this better!

If no exception name is given, the current exception is `re-raised`:idx:. The
`ReraiseDefect`:idx: exception is raised if there is no exception to
re-raise. It follows that the `raise` statement *always* raises an
exception.


Exception hierarchy
-------------------

The exception tree is defined in the [system](system.html) module.
Every exception inherits from `system.Exception`. Exceptions that indicate
programming bugs inherit from `system.Defect` (which is a subtype of `Exception`)
and are strictly speaking not catchable as they can also be mapped to an operation
that terminates the whole process. If panics are turned into exceptions, these
exceptions inherit from `Defect`.

Exceptions that indicate any other runtime error that can be caught inherit from
`system.CatchableError` (which is a subtype of `Exception`).

```
Exception
|-- CatchableError
|   |-- IOError
|   |   `-- EOFError
|   |-- OSError
|   |-- ResourceExhaustedError
|   `-- ValueError
|       `-- KeyError
`-- Defect
    |-- AccessViolationDefect
    |-- ArithmeticDefect
    |   |-- DivByZeroDefect
    |   `-- OverflowDefect
    |-- AssertionDefect
    |-- DeadThreadDefect
    |-- FieldDefect
    |-- FloatingPointDefect
    |   |-- FloatDivByZeroDefect
    |   |-- FloatInvalidOpDefect
    |   |-- FloatOverflowDefect
    |   |-- FloatUnderflowDefect
    |   `-- InexactDefect
    |-- IndexDefect
    |-- NilAccessDefect
    |-- ObjectAssignmentDefect
    |-- ObjectConversionDefect
    |-- OutOfMemoryDefect
    |-- RangeDefect
    |-- ReraiseDefect
    `-- StackOverflowDefect
```

Imported exceptions
-------------------

It is possible to raise/catch imported C++ exceptions. Types imported using
`importcpp` can be raised or caught. Exceptions are raised by value and
caught by reference. Example:

  ```nim
  type
    CStdException {.importcpp: "std::exception", header: "<exception>", inheritable.} = object
      ## does not inherit from `RootObj`, so we use `inheritable` instead
    CRuntimeError {.requiresInit, importcpp: "std::runtime_error", header: "<stdexcept>".} = object of CStdException
      ## `CRuntimeError` has no default constructor => `requiresInit`
  proc what(s: CStdException): cstring {.importcpp: "((char *)#.what())".}
  proc initRuntimeError(a: cstring): CRuntimeError {.importcpp: "std::runtime_error(@)", constructor.}
  proc initStdException(): CStdException {.importcpp: "std::exception()", constructor.}

  proc fn() =
    let a = initRuntimeError("foo")
    doAssert $a.what == "foo"
    var b = ""
    try: raise initRuntimeError("foo2")
    except CStdException as e:
      doAssert e is CStdException
      b = $e.what()
    doAssert b == "foo2"

    try: raise initStdException()
    except CStdException: discard

    try: raise initRuntimeError("foo3")
    except CRuntimeError as e:
      b = $e.what()
    except CStdException:
      doAssert false
    doAssert b == "foo3"

  fn()
  ```

**Note:** `getCurrentException()` and `getCurrentExceptionMsg()` are not available
for imported exceptions from C++. One needs to use the `except ImportedException as x:` syntax
and rely on functionality of the `x` object to get exception details.


Effect system
=============

**Note**: The rules for effect tracking changed with the release of version
1.6 of the Nim compiler.


Exception tracking
------------------

Nim supports exception tracking. The `raises`:idx: pragma can be used
to explicitly define which exceptions a proc/iterator/method/converter is
allowed to raise. The compiler verifies this:

  ```nim  test = "nim c $1"
  proc p(what: bool) {.raises: [IOError, OSError].} =
    if what: raise newException(IOError, "IO")
    else: raise newException(OSError, "OS")
  ```

An empty `raises` list (`raises: []`) means that no exception may be raised:

  ```nim
  proc p(): bool {.raises: [].} =
    try:
      unsafeCall()
      result = true
    except CatchableError:
      result = false
  ```


A `raises` list can also be attached to a proc type. This affects type
compatibility:

  ```nim  test = "nim c $1"  status = 1
  type
    Callback = proc (s: string) {.raises: [IOError].}
  var
    c: Callback

  proc p(x: string) =
    raise newException(OSError, "OS")

  c = p # type error
  ```


For a routine `p`, the compiler uses inference rules to determine the set of
possibly raised exceptions; the algorithm operates on `p`'s call graph:

1. Every indirect call via some proc type `T` is assumed to
   raise `system.Exception` (the base type of the exception hierarchy) and
   thus any exception unless `T` has an explicit `raises` list.
   However, if the call is of the form `f(...)` where `f` is a parameter of
   the currently analyzed routine that is marked as `.effectsOf: f`, it is ignored.
   The call is optimistically assumed to have no effect.
   Rule 2 compensates for this case.
2. Every expression `e` of some proc type within a call that is passed to parameter
   marked as `.effectsOf` of proc `p` is assumed to be called indirectly and thus
   its raises list is added to `p`'s raises list.
3. Every call to a proc `q` which has an unknown body (due to a forward
   declaration) is assumed to
   raise `system.Exception` unless `q` has an explicit `raises` list.
   Procs that are `importc`'ed are assumed to have `.raises: []`, unless explicitly
   declared otherwise.
4. Every call to a method `m` is assumed to
   raise `system.Exception` unless `m` has an explicit `raises` list.
5. For every other call, the analysis can determine an exact `raises` list.
6. For determining a `raises` list, the `raise` and `try` statements
   of `p` are taken into consideration.


Exceptions inheriting from `system.Defect` are not tracked with
the `.raises: []` exception tracking mechanism. This is more consistent with the
built-in operations. The following code is valid:

  ```nim
  proc mydiv(a, b): int {.raises: [].} =
    a div b # can raise an DivByZeroDefect
  ```

And so is:

  ```nim
  proc mydiv(a, b): int {.raises: [].} =
    if b == 0: raise newException(DivByZeroDefect, "division by zero")
    else: result = a div b
  ```


The reason for this is that `DivByZeroDefect` inherits from `Defect` and
with `--panics:on`:option: Defects become unrecoverable errors.
(Since version 1.4 of the language.)


EffectsOf annotation
--------------------

Rules 1-2 of the exception tracking inference rules (see the previous section)
ensure the following works:

  ```nim
  proc weDontRaiseButMaybeTheCallback(callback: proc()) {.raises: [], effectsOf: callback.} =
    callback()

  proc doRaise() {.raises: [IOError].} =
    raise newException(IOError, "IO")

  proc use() {.raises: [].} =
    # doesn't compile! Can raise IOError!
    weDontRaiseButMaybeTheCallback(doRaise)
  ```

As can be seen from the example, a parameter of type `proc (...)` can be
annotated as `.effectsOf`. Such a parameter allows for effect polymorphism:
The proc `weDontRaiseButMaybeTheCallback` raises the exceptions
that `callback` raises.

So in many cases a callback does not cause the compiler to be overly
conservative in its effect analysis:

  ```nim  test = "nim c $1"  status = 1
  {.push warningAsError[Effect]: on.}

  import std/algorithm

  type
    MyInt = distinct int

  var toSort = @[MyInt 1, MyInt 2, MyInt 3]

  proc cmpN(a, b: MyInt): int =
    cmp(a.int, b.int)

  proc harmless {.raises: [].} =
    toSort.sort cmpN

  proc cmpE(a, b: MyInt): int {.raises: [Exception].} =
    cmp(a.int, b.int)

  proc harmful {.raises: [].} =
    # does not compile, `sort` can now raise Exception
    toSort.sort cmpE
  ```



Tag tracking
------------

Exception tracking is part of Nim's `effect system`:idx:. Raising an exception
is an *effect*. Other effects can also be defined. A user defined effect is a
means to *tag* a routine and to perform checks against this tag:

  ```nim  test = "nim c --warningAsError:Effect:on $1"  status = 1
  type IO = object ## input/output effect
  proc readLine(): string {.tags: [IO].} = discard

  proc no_effects_please() {.tags: [].} =
    # the compiler prevents this:
    let x = readLine()
  ```

A tag has to be a type name. A `tags` list - like a `raises` list - can
also be attached to a proc type. This affects type compatibility.

The inference for tag tracking is analogous to the inference for
exception tracking.

There is also a way which can be used to forbid certain effects:

  ```nim  test = "nim c --warningAsError:Effect:on $1"  status = 1
  type IO = object ## input/output effect
  proc readLine(): string {.tags: [IO].} = discard
  proc echoLine(): void = discard

  proc no_IO_please() {.forbids: [IO].} =
    # this is OK because it didn't define any tag:
    echoLine()
    # the compiler prevents this:
    let y = readLine()
  ```

The `forbids` pragma defines a list of illegal effects - if any statement
invokes any of those effects, the compilation will fail.
Procedure types with any disallowed effect are the subtypes of equal
procedure types without such lists:

  ```nim
  type MyEffect = object
  type ProcType1 = proc (i: int): void {.forbids: [MyEffect].}
  type ProcType2 = proc (i: int): void

  proc caller1(p: ProcType1): void = p(1)
  proc caller2(p: ProcType2): void = p(1)

  proc effectful(i: int): void {.tags: [MyEffect].} = echo $i
  proc effectless(i: int): void {.forbids: [MyEffect].} = echo $i

  proc toBeCalled1(i: int): void = effectful(i)
  proc toBeCalled2(i: int): void = effectless(i)

  ## this will fail because toBeCalled1 uses MyEffect which was forbidden by ProcType1:
  caller1(toBeCalled1)
  ## this is OK because both toBeCalled2 and ProcType1 have the same requirements:
  caller1(toBeCalled2)
  ## these are OK because ProcType2 doesn't have any effect requirement:
  caller2(toBeCalled1)
  caller2(toBeCalled2)
  ```

`ProcType2` is a subtype of `ProcType1`. Unlike with the `tags` pragma, the parent context - the
function which calls other functions with forbidden effects - doesn't inherit the forbidden list of effects.


Side effects
------------

The `noSideEffect` pragma is used to mark a proc/iterator that can have only
side effects through parameters. This means that the proc/iterator only changes locations that are
reachable from its parameters and the return value only depends on the
parameters. If none of its parameters have the type `var`, `ref`, `ptr`, `cstring`, or `proc`,
then no locations are modified.

In other words, a routine has no side effects if it does not access a threadlocal
or global variable and it does not call any routine that has a side effect.

It is a static error to mark a proc/iterator to have no side effect if the compiler cannot verify this.

As a special semantic rule, the built-in [debugEcho](
system.html#debugEcho,varargs[typed,]) pretends to be free of side effects
so that it can be used for debugging routines marked as `noSideEffect`.

`func` is syntactic sugar for a proc with no side effects:

  ```nim
  func `+` (x, y: int): int
  ```


To override the compiler's side effect analysis a `{.noSideEffect.}`
`cast` pragma block can be used:

  ```nim
  func f() =
    {.cast(noSideEffect).}:
      echo "test"
  ```

**Side effects are usually inferred. The inference for side effects is
analogous to the inference for exception tracking.**

When the compiler cannot infer side effects, as is the case for imported
functions, one can annotate them with the `sideEffect` pragma.

GC safety effect
----------------

We call a proc `p` `GC safe`:idx: when it doesn't access any global variable
that contains GC'ed memory (`string`, `seq`, `ref` or a closure) either
directly or indirectly through a call to a GC unsafe proc.

**The GC safety property is usually inferred. The inference for GC safety is
analogous to the inference for exception tracking.**

The `gcsafe`:idx: annotation can be used to mark a proc to be gcsafe,
otherwise this property is inferred by the compiler. Note that `noSideEffect`
implies `gcsafe`.

Routines that are imported from C are always assumed to be `gcsafe`.

To override the compiler's gcsafety analysis a `{.cast(gcsafe).}` pragma block can
be used:

  ```nim
  var
    someGlobal: string = "some string here"
    perThread {.threadvar.}: string

  proc setPerThread() =
    {.cast(gcsafe).}:
      deepCopy(perThread, someGlobal)
  ```


See also:

- [Shared heap memory management](mm.html).



Effects pragma
--------------

The `effects` pragma has been designed to assist the programmer with the
effects analysis. It is a statement that makes the compiler output all inferred
effects up to the `effects`'s position:

  ```nim
  proc p(what: bool) =
    if what:
      raise newException(IOError, "IO")
      {.effects.}
    else:
      raise newException(OSError, "OS")
  ```

The compiler produces a hint message that `IOError` can be raised. `OSError`
is not listed as it cannot be raised in the branch the `effects` pragma
appears in.


Generics
========

Generics are Nim's means to parametrize procs, iterators or types with
`type parameters`:idx:. Depending on the context, the brackets are used either to
introduce type parameters or to instantiate a generic proc, iterator, or type.


The following example shows how a generic binary tree can be modeled:

  ```nim  test = "nim c $1"
  type
    BinaryTree*[T] = ref object # BinaryTree is a generic type with
                                # generic parameter `T`
      le, ri: BinaryTree[T]     # left and right subtrees; may be nil
      data: T                   # the data stored in a node

  proc newNode*[T](data: T): BinaryTree[T] =
    # constructor for a node
    result = BinaryTree[T](le: nil, ri: nil, data: data)

  proc add*[T](root: var BinaryTree[T], n: BinaryTree[T]) =
    # insert a node into the tree
    if root == nil:
      root = n
    else:
      var it = root
      while it != nil:
        # compare the data items; uses the generic `cmp` proc
        # that works for any type that has a `==` and `<` operator
        var c = cmp(it.data, n.data)
        if c < 0:
          if it.le == nil:
            it.le = n
            return
          it = it.le
        else:
          if it.ri == nil:
            it.ri = n
            return
          it = it.ri

  proc add*[T](root: var BinaryTree[T], data: T) =
    # convenience proc:
    add(root, newNode(data))

  iterator preorder*[T](root: BinaryTree[T]): T =
    # Preorder traversal of a binary tree.
    # This uses an explicit stack (which is more efficient than
    # a recursive iterator factory).
    var stack: seq[BinaryTree[T]] = @[root]
    while stack.len > 0:
      var n = stack.pop()
      while n != nil:
        yield n.data
        add(stack, n.ri)  # push right subtree onto the stack
        n = n.le          # and follow the left pointer

  var
    root: BinaryTree[string] # instantiate a BinaryTree with `string`
  add(root, newNode("hello")) # instantiates `newNode` and `add`
  add(root, "world")          # instantiates the second `add` proc
  for str in preorder(root):
    stdout.writeLine(str)
  ```

The `T` is called a `generic type parameter`:idx: or
a `type variable`:idx:.


Generic Procs
---------------

Let's consider the anatomy of a generic `proc` to agree on defined terminology.

```nim
p[T: t](arg1: f): y
```

- `p`: Callee symbol
- `[...]`: Generic parameters
- `T: t`: Generic constraint
- `T`: Type variable
- `[T: t](arg1: f): y`: Formal signature
- `arg1: f`: Formal parameter
- `f`: Formal parameter type
- `y`: Formal return type

The use of the word "formal" here is to denote the symbols as they are defined by the programmer,
not as they may be at compile time contextually. Since generics may be instantiated and
types bound, we have more than one entity to think about when generics are involved.

The usage of a generic will resolve the formally defined expression into an instance of that
expression bound to only concrete types. This process is called "instantiation".

Brackets at the site of a generic's formal definition specify the "constraints" as in:

```nim
type Foo[T] = object
proc p[H;T: Foo[H]](param: T): H
```

A constraint definition may have more than one symbol defined by separating each definition by
a `;`. Notice how `T` is composed of `H` and the return  type of `p` is defined as `H`. When this
generic proc is instantiated `H` will be bound to a concrete type, thus making `T` concrete and
the return type of `p` will be bound to the same concrete type used to define `H`.

Brackets at the site of usage can be used to supply concrete types to instantiate the generic in the same
order that the symbols are defined in the constraint. Alternatively, type bindings may be inferred by the compiler
in some situations, allowing for cleaner code.


Is operator
-----------

The `is` operator is evaluated during semantic analysis to check for type
equivalence. It is therefore very useful for type specialization within generic
code:

  ```nim
  type
    Table[Key, Value] = object
      keys: seq[Key]
      values: seq[Value]
      when not (Key is string): # empty value for strings used for optimization
        deletedKeys: seq[bool]
  ```


Type classes
------------

A type class is a special pseudo-type that can be used to match against
types in the context of overload resolution or the `is` operator.
Nim supports the following built-in type classes:

==================   ===================================================
type class           matches
==================   ===================================================
`object`             any object type
`tuple`              any tuple type
`enum`               any enumeration
`proc`               any proc type
`iterator`           any iterator type
`ref`                any `ref` type
`ptr`                any `ptr` type
`var`                any `var` type
`distinct`           any distinct type
`array`              any array type
`set`                any set type
`seq`                any seq type
`auto`               any type
==================   ===================================================

Furthermore, every generic type automatically creates a type class of the same
name that will match any instantiation of the generic type.

Type classes can be combined using the standard boolean operators to form
more complex type classes:

  ```nim
  # create a type class that will match all tuple and object types
  type RecordType = (tuple or object)

  proc printFields[T: RecordType](rec: T) =
    for key, value in fieldPairs(rec):
      echo key, " = ", value
  ```

Type constraints on generic parameters can be grouped with `,` and propagation
stops with `;`, similarly to parameters for macros and templates:

  ```nim
  proc fn1[T; U, V: SomeFloat]() = discard # T is unconstrained
  template fn2(t; u, v: SomeFloat) = discard # t is unconstrained
  ```

Whilst the syntax of type classes appears to resemble that of ADTs/algebraic data
types in ML-like languages, it should be understood that type classes are static
constraints to be enforced at type instantiations. Type classes are not really
types in themselves but are instead a system of providing generic "checks" that
ultimately *resolve* to some singular type. Type classes do not allow for
runtime type dynamism, unlike object variants or methods.

As an example, the following would not compile:

  ```nim
  type TypeClass = int | string
  var foo: TypeClass = 2 # foo's type is resolved to an int here
  foo = "this will fail" # error here, because foo is an int
  ```

Nim allows for type classes and regular types to be specified
as `type constraints`:idx: of the generic type parameter:

  ```nim
  proc onlyIntOrString[T: int|string](x, y: T) = discard

  onlyIntOrString(450, 616) # valid
  onlyIntOrString(5.0, 0.0) # type mismatch
  onlyIntOrString("xy", 50) # invalid as 'T' cannot be both at the same time
  ```

`proc` and `iterator` type classes also accept a calling convention pragma
to restrict the calling convention of the matching `proc` or `iterator` type.

  ```nim
  proc onlyClosure[T: proc {.closure.}](x: T) = discard

  onlyClosure(proc() = echo "hello") # valid
  proc foo() {.nimcall.} = discard
  onlyClosure(foo) # type mismatch
  ```


Implicit generics
-----------------

A type class can be used directly as the parameter's type.

  ```nim
  # create a type class that will match all tuple and object types
  type RecordType = (tuple or object)

  proc printFields(rec: RecordType) =
    for key, value in fieldPairs(rec):
      echo key, " = ", value
  ```


Procedures utilizing type classes in such a manner are considered to be
`implicitly generic`:idx:. They will be instantiated once for each unique
combination of parameter types used within the program.

By default, during overload resolution, each named type class will bind to
exactly one concrete type. We call such type classes `bind once`:idx: types.
Here is an example taken directly from the system module to illustrate this:

  ```nim
  proc `==`*(x, y: tuple): bool =
    ## requires `x` and `y` to be of the same tuple type
    ## generic `==` operator for tuples that is lifted from the components
    ## of `x` and `y`.
    result = true
    for a, b in fields(x, y):
      if a != b: result = false
  ```

Alternatively, the `distinct` type modifier can be applied to the type class
to allow each parameter matching the type class to bind to a different type. Such
type classes are called `bind many`:idx: types.

Procs written with the implicitly generic style will often need to refer to the
type parameters of the matched generic type. They can be easily accessed using
the dot syntax:

  ```nim
  type Matrix[T, Rows, Columns] = object
    ...

  proc `[]`(m: Matrix, row, col: int): Matrix.T =
    m.data[col * high(Matrix.Columns) + row]
  ```


Here are more examples that illustrate implicit generics:

  ```nim
  proc p(t: Table; k: Table.Key): Table.Value

  # is roughly the same as:

  proc p[Key, Value](t: Table[Key, Value]; k: Key): Value
  ```


  ```nim
  proc p(a: Table, b: Table)

  # is roughly the same as:

  proc p[Key, Value](a, b: Table[Key, Value])
  ```


  ```nim
  proc p(a: Table, b: distinct Table)

  # is roughly the same as:

  proc p[Key, Value, KeyB, ValueB](a: Table[Key, Value], b: Table[KeyB, ValueB])
  ```


`typedesc` used as a parameter type also introduces an implicit
generic. `typedesc` has its own set of rules:

  ```nim
  proc p(a: typedesc)

  # is roughly the same as:

  proc p[T](a: typedesc[T])
  ```


`typedesc` is a "bind many" type class:

  ```nim
  proc p(a, b: typedesc)

  # is roughly the same as:

  proc p[T, T2](a: typedesc[T], b: typedesc[T2])
  ```


A parameter of type `typedesc` is itself usable as a type. If it is used
as a type, it's the underlying type. In other words, one level
of "typedesc"-ness is stripped off:

  ```nim
  proc p(a: typedesc; b: a) = discard

  # is roughly the same as:
  proc p[T](a: typedesc[T]; b: T) = discard

  # hence this is a valid call:
  p(int, 4)
  # as parameter 'a' requires a type, but 'b' requires a value.
  ```


Generic inference restrictions
------------------------------

The types `var T` and `typedesc[T]` cannot be inferred in a generic
instantiation. The following is not allowed:

  ```nim  test = "nim c $1"  status = 1
  proc g[T](f: proc(x: T); x: T) =
    f(x)

  proc c(y: int) = echo y
  proc v(y: var int) =
    y += 100
  var i: int

  # allowed: infers 'T' to be of type 'int'
  g(c, 42)

  # not valid: 'T' is not inferred to be of type 'var int'
  g(v, i)

  # also not allowed: explicit instantiation via 'var int'
  g[var int](v, i)
  ```



Symbol lookup in generics
-------------------------

### Open and Closed symbols

The symbol binding rules in generics are slightly subtle: There are "open" and
"closed" symbols. A "closed" symbol cannot be re-bound in the instantiation
context, an "open" symbol can. Per default, overloaded symbols are open
and every other symbol is closed.

Open symbols are looked up in two different contexts: Both the context
at definition and the context at instantiation are considered:

  ```nim  test = "nim c $1"
  type
    Index = distinct int

  proc `==` (a, b: Index): bool {.borrow.}

  var a = (0, 0.Index)
  var b = (0, 0.Index)

  echo a == b # works!
  ```

In the example, the [generic `==` for tuples](system.html#%3D%3D%2CT%2CT_2) (as defined in the system module)
uses the `==` operators of the tuple's components. However, the `==` for
the `Index` type is defined *after* the `==` for tuples; yet the example
compiles as the instantiation takes the currently defined symbols into account
too.

Mixin statement
---------------

A symbol can be forced to be open by a `mixin`:idx: declaration:

  ```nim  test = "nim c $1"
  proc create*[T](): ref T =
    # there is no overloaded 'init' here, so we need to state that it's an
    # open symbol explicitly:
    mixin init
    new result
    init result
  ```

`mixin` statements only make sense in templates and generics.


Bind statement
--------------

The `bind` statement is the counterpart to the `mixin` statement. It
can be used to explicitly declare identifiers that should be bound early (i.e.
the identifiers should be looked up in the scope of the template/generic
definition):

  ```nim
  # Module A
  var
    lastId = 0

  template genId*: untyped =
    bind lastId
    inc(lastId)
    lastId
  ```

  ```nim
  # Module B
  import A

  echo genId()
  ```

But a `bind` is rarely useful because symbol binding from the definition
scope is the default.

`bind` statements only make sense in templates and generics.


Delegating bind statements
--------------------------

The following example outlines a problem that can arise when generic
instantiations cross multiple different modules:

  ```nim
  # module A
  proc genericA*[T](x: T) =
    mixin init
    init(x)
  ```


  ```nim
  import C

  # module B
  proc genericB*[T](x: T) =
    # Without the `bind init` statement C's init proc is
    # not available when `genericB` is instantiated:
    bind init
    genericA(x)
  ```

  ```nim
  # module C
  type O = object
  proc init*(x: var O) = discard
  ```

  ```nim
  # module main
  import B, C

  genericB O()
  ```

In module B has an `init` proc from module C in its scope that is not
taken into account when `genericB` is instantiated which leads to the
instantiation of `genericA`. The solution is to `forward`:idx: these
symbols by a `bind` statement inside `genericB`.


Templates
=========

A template is a simple form of a macro: It is a simple substitution
mechanism that operates on Nim's abstract syntax trees. It is processed in
the semantic pass of the compiler.

The syntax to *invoke* a template is the same as calling a procedure.

Example:

  ```nim
  template `!=` (a, b: untyped): untyped =
    # this definition exists in the system module
    not (a == b)

  assert(5 != 6) # the compiler rewrites that to: assert(not (5 == 6))
  ```

The `!=`, `>`, `>=`, `in`, `notin`, `isnot` operators are in fact
templates:

| `a > b` is transformed into `b < a`.
| `a in b` is transformed into `contains(b, a)`.
| `notin` and `isnot` have the obvious meanings.

The "types" of templates can be the symbols `untyped`,
`typed` or `typedesc`. These are "meta types", they can only be used in certain
contexts. Regular types can be used too; this implies that `typed` expressions
are expected.


Typed vs untyped parameters
---------------------------

An `untyped` parameter means that symbol lookups and type resolution is not
performed before the expression is passed to the template. This means that
*undeclared* identifiers, for example, can be passed to the template:

  ```nim  test = "nim c $1"
  template declareInt(x: untyped) =
    var x: int

  declareInt(x) # valid
  x = 3
  ```


  ```nim  test = "nim c $1"  status = 1
  template declareInt(x: typed) =
    var x: int

  declareInt(x) # invalid, because x has not been declared and so it has no type
  ```

A template where every parameter is `untyped` is called an `immediate`:idx:
template. For historical reasons, templates can be explicitly annotated with
an `immediate` pragma and then these templates do not take part in
overloading resolution and the parameters' types are *ignored* by the
compiler. Explicit immediate templates are now deprecated.

**Note**: For historical reasons, `stmt` was an alias for `typed` and
`expr` was an alias for `untyped`, but they are removed.


Passing a code block to a template
----------------------------------

One can pass a block of statements as the last argument to a template
following the special `:` syntax:

  ```nim  test = "nim c $1"
  template withFile(f, fn, mode, actions: untyped): untyped =
    var f: File
    if open(f, fn, mode):
      try:
        actions
      finally:
        close(f)
    else:
      quit("cannot open: " & fn)

  withFile(txt, "ttempl3.txt", fmWrite):  # special colon
    txt.writeLine("line 1")
    txt.writeLine("line 2")
  ```

In the example, the two `writeLine` statements are bound to the `actions`
parameter.


Usually, to pass a block of code to a template, the parameter that accepts
the block needs to be of type `untyped`. Because symbol lookups are then
delayed until template instantiation time:

  ```nim  test = "nim c $1"  status = 1
  template t(body: typed) =
    proc p = echo "hey"
    block:
      body

  t:
    p()  # fails with 'undeclared identifier: p'
  ```

The above code fails with the error message that `p` is not declared.
The reason for this is that the `p()` body is type-checked before getting
passed to the `body` parameter and type checking in Nim implies symbol lookups.
The same code works with `untyped` as the passed body is not required to be
type-checked:

  ```nim  test = "nim c $1"
  template t(body: untyped) =
    proc p = echo "hey"
    block:
      body

  t:
    p()  # compiles
  ```


Varargs of untyped
------------------

In addition to the `untyped` meta-type that prevents type checking, there is
also `varargs[untyped]` so that not even the number of parameters is fixed:

  ```nim  test = "nim c $1"
  template hideIdentifiers(x: varargs[untyped]) = discard

  hideIdentifiers(undeclared1, undeclared2)
  ```

However, since a template cannot iterate over varargs, this feature is
generally much more useful for macros.


Symbol binding in templates
---------------------------

A template is a `hygienic`:idx: macro and so opens a new scope. Most symbols are
bound from the definition scope of the template:

  ```nim
  # Module A
  var
    lastId = 0

  template genId*: untyped =
    inc(lastId)
    lastId
  ```

  ```nim
  # Module B
  import A

  echo genId() # Works as 'lastId' has been bound in 'genId's defining scope
  ```

As in generics, symbol binding can be influenced via `mixin` or `bind`
statements.



Identifier construction
-----------------------

In templates, identifiers can be constructed with the backticks notation:

  ```nim  test = "nim c $1"
  template typedef(name: untyped, typ: typedesc) =
    type
      `T name`* {.inject.} = typ
      `P name`* {.inject.} = ref `T name`

  typedef(myint, int)
  var x: PMyInt
  ```

In the example, `name` is instantiated with `myint`, so \`T name\` becomes
`Tmyint`.


Lookup rules for template parameters
------------------------------------

A parameter `p` in a template is even substituted in the expression `x.p`.
Thus, template arguments can be used as field names and a global symbol can be
shadowed by the same argument name even when fully qualified:

  ```nim
  # module 'm'

  type
    Lev = enum
      levA, levB

  var abclev = levB

  template tstLev(abclev: Lev) =
    echo abclev, " ", m.abclev

  tstLev(levA)
  # produces: 'levA levA'
  ```

But the global symbol can properly be captured by a `bind` statement:

  ```nim
  # module 'm'

  type
    Lev = enum
      levA, levB

  var abclev = levB

  template tstLev(abclev: Lev) =
    bind m.abclev
    echo abclev, " ", m.abclev

  tstLev(levA)
  # produces: 'levA levB'
  ```


Hygiene in templates
--------------------

Per default, templates are `hygienic`:idx:\: Local identifiers declared in a
template cannot be accessed in the instantiation context:

  ```nim  test = "nim c $1"
  template newException*(exceptn: typedesc, message: string): untyped =
    var
      e: ref exceptn  # e is implicitly gensym'ed here
    new(e)
    e.msg = message
    e

  # so this works:
  let e = "message"
  raise newException(IoError, e)
  ```


Whether a symbol that is declared in a template is exposed to the instantiation
scope is controlled by the `inject`:idx: and `gensym`:idx: pragmas:
`gensym`'ed symbols are not exposed but `inject`'ed symbols are.

The default for symbols of entity `type`, `var`, `let` and `const`
is `gensym`. For `proc`, `iterator`, `converter`, `template`,
`macro`, the default is `inject`, but if a `gensym` symbol with the same name
is defined in the same syntax-level scope, it will be `gensym` by default.
This can be overridden by marking the routine as `inject`. 

If the name of the entity is passed as a template parameter, it is an `inject`'ed symbol:

  ```nim
  template withFile(f, fn, mode: untyped, actions: untyped): untyped =
    block:
      var f: File  # since 'f' is a template parameter, it's injected implicitly
      ...

  withFile(txt, "ttempl3.txt", fmWrite):
    txt.writeLine("line 1")
    txt.writeLine("line 2")
  ```


The `inject` and `gensym` pragmas are second class annotations; they have
no semantics outside a template definition and cannot be abstracted over:

  ```nim
  {.pragma myInject: inject.}

  template t() =
    var x {.myInject.}: int # does NOT work
  ```


To get rid of hygiene in templates, one can use the `dirty`:idx: pragma for
a template. `inject` and `gensym` have no effect in `dirty` templates.

`gensym`'ed symbols cannot be used as `field` in the `x.field` syntax.
Nor can they be used in the `ObjectConstruction(field: value)`
and `namedParameterCall(field = value)` syntactic constructs.

The reason for this is that code like

  ```nim  test = "nim c $1"
  type
    T = object
      f: int

  template tmp(x: T) =
    let f = 34
    echo x.f, T(f: 4)
  ```


should work as expected.

However, this means that the method call syntax is not available for
`gensym`'ed symbols:

  ```nim  test = "nim c $1"  status = 1
  template tmp(x) =
    type
      T {.gensym.} = int

    echo x.T # invalid: instead use:  'echo T(x)'.

  tmp(12)
  ```


Limitations of the method call syntax
-------------------------------------

The expression `x` in `x.f` needs to be semantically checked (that means
symbol lookup and type checking) before it can be decided that it needs to be
rewritten to `f(x)`. Therefore, the dot syntax has some limitations when it
is used to invoke templates/macros:

  ```nim  test = "nim c $1"  status = 1
  template declareVar(name: untyped) =
    const name {.inject.} = 45

  # Doesn't compile:
  unknownIdentifier.declareVar
  ```


It is also not possible to use fully qualified identifiers with module
symbol in method call syntax. The order in which the dot operator
binds to symbols prohibits this.

  ```nim  test = "nim c $1"  status = 1
  import std/sequtils

  var myItems = @[1,3,3,7]
  let N1 = count(myItems, 3) # OK
  let N2 = sequtils.count(myItems, 3) # fully qualified, OK
  let N3 = myItems.count(3) # OK
  let N4 = myItems.sequtils.count(3) # illegal, `myItems.sequtils` can't be resolved
  ```

This means that when for some reason a procedure needs a
disambiguation through the module name, the call needs to be
written in function call syntax.

Macros
======

A macro is a special function that is executed at compile time.
Normally, the input for a macro is an abstract syntax
tree (AST) of the code that is passed to it. The macro can then do
transformations on it and return the transformed AST. This can be used to
add custom language features and implement `domain-specific languages`:idx:.

Macro invocation is a case where semantic analysis does **not** entirely proceed
top to bottom and left to right. Instead, semantic analysis happens at least
twice:

* Semantic analysis recognizes and resolves the macro invocation.
* The compiler executes the macro body (which may invoke other procs).
* It replaces the AST of the macro invocation with the AST returned by the macro.
* It repeats semantic analysis of that region of the code.
* If the AST returned by the macro contains other macro invocations,
  this process iterates.

While macros enable advanced compile-time code transformations, they
cannot change Nim's syntax.

**Style note:** For code readability, it is best to use the least powerful
programming construct that remains expressive. So the "check list" is:

(1) Use an ordinary proc/iterator, if possible.
(2) Else: Use a generic proc/iterator, if possible.
(3) Else: Use a template, if possible.
(4) Else: Use a macro.

Debug example
-------------

The following example implements a powerful `debug` command that accepts a
variable number of arguments:

  ```nim  test = "nim c $1"
  # to work with Nim syntax trees, we need an API that is defined in the
  # `macros` module:
  import std/macros

  macro debug(args: varargs[untyped]): untyped =
    # `args` is a collection of `NimNode` values that each contain the
    # AST for an argument of the macro. A macro always has to
    # return a `NimNode`. A node of kind `nnkStmtList` is suitable for
    # this use case.
    result = nnkStmtList.newTree()
    # iterate over any argument that is passed to this macro:
    for n in args:
      # add a call to the statement list that writes the expression;
      # `toStrLit` converts an AST to its string representation:
      result.add newCall("write", newIdentNode("stdout"), newLit(n.repr))
      # add a call to the statement list that writes ": "
      result.add newCall("write", newIdentNode("stdout"), newLit(": "))
      # add a call to the statement list that writes the expressions value:
      result.add newCall("writeLine", newIdentNode("stdout"), n)

  var
    a: array[0..10, int]
    x = "some string"
  a[0] = 42
  a[1] = 45

  debug(a[0], a[1], x)
  ```

The macro call expands to:

  ```nim
  write(stdout, "a[0]")
  write(stdout, ": ")
  writeLine(stdout, a[0])

  write(stdout, "a[1]")
  write(stdout, ": ")
  writeLine(stdout, a[1])

  write(stdout, "x")
  write(stdout, ": ")
  writeLine(stdout, x)
  ```


Arguments that are passed to a `varargs` parameter are wrapped in an array
constructor expression. This is why `debug` iterates over all of `args`'s
children.


bindSym
-------

The above `debug` macro relies on the fact that `write`, `writeLine` and
`stdout` are declared in the system module and are thus visible in the
instantiating context. There is a way to use bound identifiers
(aka `symbols`:idx:) instead of using unbound identifiers. The `bindSym`
builtin can be used for that:

  ```nim  test = "nim c $1"
  import std/macros

  macro debug(n: varargs[typed]): untyped =
    result = newNimNode(nnkStmtList, n)
    for x in n:
      # we can bind symbols in scope via 'bindSym':
      add(result, newCall(bindSym"write", bindSym"stdout", toStrLit(x)))
      add(result, newCall(bindSym"write", bindSym"stdout", newStrLitNode(": ")))
      add(result, newCall(bindSym"writeLine", bindSym"stdout", x))

  var
    a: array[0..10, int]
    x = "some string"
  a[0] = 42
  a[1] = 45

  debug(a[0], a[1], x)
  ```

The macro call expands to:

  ```nim
  write(stdout, "a[0]")
  write(stdout, ": ")
  writeLine(stdout, a[0])

  write(stdout, "a[1]")
  write(stdout, ": ")
  writeLine(stdout, a[1])

  write(stdout, "x")
  write(stdout, ": ")
  writeLine(stdout, x)
  ```

In this version of `debug`, the symbols `write`, `writeLine` and `stdout`
are already bound and are not looked up again. As the example shows, `bindSym`
does work with overloaded symbols implicitly.

Note that the symbol names passed to `bindSym` have to be constant. The
experimental feature `dynamicBindSym` ([experimental manual](
manual_experimental.html#dynamic-arguments-for-bindsym))
allows this value to be computed dynamically.

Post-statement blocks
---------------------

Macros can receive `of`, `elif`, `else`, `except`, `finally` and `do`
blocks (including their different forms such as `do` with routine parameters)
as arguments if called in statement form.

  ```nim
  macro performWithUndo(task, undo: untyped) = ...

  performWithUndo do:
    # multiple-line block of code
    # to perform the task
  do:
    # code to undo it

  let num = 12
  # a single colon may be used if there is no initial block
  match (num mod 3, num mod 5):
  of (0, 0):
    echo "FizzBuzz"
  of (0, _):
    echo "Fizz"
  of (_, 0):
    echo "Buzz"
  else:
    echo num
  ```


For loop macro
--------------

A macro that takes as its only input parameter an expression of the special
type `system.ForLoopStmt` can rewrite the entirety of a `for` loop:

  ```nim  test = "nim c $1"
  import std/macros

  macro example(loop: ForLoopStmt) =
    result = newTree(nnkForStmt)    # Create a new For loop.
    result.add loop[^3]             # This is "item".
    result.add loop[^2][^1]         # This is "[1, 2, 3]".
    result.add newCall(bindSym"echo", loop[0])

  for item in example([1, 2, 3]): discard
  ```

Expands to:

  ```nim
  for item in items([1, 2, 3]):
    echo item
  ```

Another example:

  ```nim  test = "nim c $1"
  import std/macros

  macro enumerate(x: ForLoopStmt): untyped =
    expectKind x, nnkForStmt
    # check if the starting count is specified:
    var countStart = if x[^2].len == 2: newLit(0) else: x[^2][1]
    result = newStmtList()
    # we strip off the first for loop variable and use it as an integer counter:
    result.add newVarStmt(x[0], countStart)
    var body = x[^1]
    if body.kind != nnkStmtList:
      body = newTree(nnkStmtList, body)
    body.add newCall(bindSym"inc", x[0])
    var newFor = newTree(nnkForStmt)
    for i in 1..x.len-3:
      newFor.add x[i]
    # transform enumerate(X) to 'X'
    newFor.add x[^2][^1]
    newFor.add body
    result.add newFor
    # now wrap the whole macro in a block to create a new scope
    result = quote do:
      block: `result`

  for a, b in enumerate(items([1, 2, 3])):
    echo a, " ", b

  # without wrapping the macro in a block, we'd need to choose different
  # names for `a` and `b` here to avoid redefinition errors
  for a, b in enumerate(10, [1, 2, 3, 5]):
    echo a, " ", b
  ```


Case statement macros
---------------------

Macros named `` `case` `` can provide implementations of `case` statements
for certain types. The following is an example of such an implementation
for tuples, leveraging the existing equality operator for tuples
(as provided in `system.==`):

  ```nim  test = "nim c $1"
  import std/macros

  macro `case`(n: tuple): untyped =
    result = newTree(nnkIfStmt)
    let selector = n[0]
    for i in 1 ..< n.len:
      let it = n[i]
      case it.kind
      of nnkElse, nnkElifBranch, nnkElifExpr, nnkElseExpr:
        result.add it
      of nnkOfBranch:
        for j in 0..it.len-2:
          let cond = newCall("==", selector, it[j])
          result.add newTree(nnkElifBranch, cond, it[^1])
      else:
        error "custom 'case' for tuple cannot handle this node", it

  case ("foo", 78)
  of ("foo", 78): echo "yes"
  of ("bar", 88): echo "no"
  else: discard
  ```

`case` macros are subject to overload resolution. The type of the
`case` statement's selector expression is matched against the type
of the first argument of the `case` macro. Then the complete `case`
statement is passed in place of the argument and the macro is evaluated.

In other words, the macro needs to transform the full `case` statement
but only the statement's selector expression is used to determine which
macro to call.


Special Types
=============

static\[T]
----------

As their name suggests, static parameters must be constant expressions:

  ```nim
  proc precompiledRegex(pattern: static string): RegEx =
    var res {.global.} = re(pattern)
    return res

  precompiledRegex("/d+") # Replaces the call with a precompiled
                          # regex, stored in a global variable

  precompiledRegex(paramStr(1)) # Error, command-line options
                                # are not constant expressions
  ```


For the purposes of code generation, all static parameters are treated as
generic parameters - the proc will be compiled separately for each unique
supplied value (or combination of values).

Static parameters can also appear in the signatures of generic types:

  ```nim
  type
    Matrix[M,N: static int; T: Number] = array[0..(M*N - 1), T]
      # Note how `Number` is just a type constraint here, while
      # `static int` requires us to supply an int value

    AffineTransform2D[T] = Matrix[3, 3, T]
    AffineTransform3D[T] = Matrix[4, 4, T]

  var m1: AffineTransform3D[float]  # OK
  var m2: AffineTransform2D[string] # Error, `string` is not a `Number`
  ```

Please note that `static T` is just a syntactic convenience for the underlying
generic type `static[T]`. The type parameter can be omitted to obtain the type
class of all constant expressions. A more specific type class can be created by
instantiating `static` with another type class.

One can force an expression to be evaluated at compile time as a constant
expression by coercing it to a corresponding `static` type:

  ```nim
  import std/math

  echo static(fac(5)), " ", static[bool](16.isPowerOfTwo)
  ```

The compiler will report any failure to evaluate the expression or a
possible type mismatch error.

typedesc\[T]
------------

In many contexts, Nim treats the names of types as regular
values. These values exist only during the compilation phase, but since
all values must have a type, `typedesc` is considered their special type.

`typedesc` acts as a generic type. For instance, the type of the symbol
`int` is `typedesc[int]`. Just like with regular generic types, when the
generic parameter is omitted, `typedesc` denotes the type class of all types.
As a syntactic convenience, one can also use `typedesc` as a modifier.

Procs featuring `typedesc` parameters are considered implicitly generic.
They will be instantiated for each unique combination of supplied types,
and within the body of the proc, the name of each parameter will refer to
the bound concrete type:

  ```nim
  proc new(T: typedesc): ref T =
    echo "allocating ", T.name
    new(result)

  var n = Node.new
  var tree = new(BinaryTree[int])
  ```

When multiple type parameters are present, they will bind freely to different
types. To force a bind-once behavior, one can use an explicit generic parameter:

  ```nim
  proc acceptOnlyTypePairs[T, U](A, B: typedesc[T]; C, D: typedesc[U])
  ```

Once bound, type parameters can appear in the rest of the proc signature:

  ```nim  test = "nim c $1"
  template declareVariableWithType(T: typedesc, value: T) =
    var x: T = value

  declareVariableWithType int, 42
  ```


Overload resolution can be further influenced by constraining the set
of types that will match the type parameter. This works in practice by
attaching attributes to types via templates. The constraint can be a
concrete type or a type class.

  ```nim  test = "nim c $1"
  template maxval(T: typedesc[int]): int = high(int)
  template maxval(T: typedesc[float]): float = Inf

  var i = int.maxval
  var f = float.maxval
  when false:
    var s = string.maxval # error, maxval is not implemented for string

  template isNumber(t: typedesc[object]): string = "Don't think so."
  template isNumber(t: typedesc[SomeInteger]): string = "Yes!"
  template isNumber(t: typedesc[SomeFloat]): string = "Maybe, could be NaN."

  echo "is int a number? ", isNumber(int)
  echo "is float a number? ", isNumber(float)
  echo "is RootObj a number? ", isNumber(RootObj)
  ```

Passing `typedesc` is almost identical, just with the difference that
the macro is not instantiated generically. The type expression is
simply passed as a `NimNode` to the macro, like everything else.

  ```nim
  import std/macros

  macro forwardType(arg: typedesc): typedesc =
    # `arg` is of type `NimNode`
    let tmp: NimNode = arg
    result = tmp

  var tmp: forwardType(int)
  ```

typeof operator
---------------

**Note**: `typeof(x)` can for historical reasons also be written as
`type(x)` but `type(x)` is discouraged.

One can obtain the type of a given expression by constructing a `typeof`
value from it (in many other languages this is known as the `typeof`:idx:
operator):

  ```nim
  var x = 0
  var y: typeof(x) # y has type int
  ```


If `typeof` is used to determine the result type of a proc/iterator/converter
call `c(X)` (where `X` stands for a possibly empty list of arguments), the
interpretation, where `c` is an iterator, is preferred over the
other interpretations, but this behavior can be changed by
passing `typeOfProc` as the second argument to `typeof`:

  ```nim  test = "nim c $1"
  iterator split(s: string): string = discard
  proc split(s: string): seq[string] = discard

  # since an iterator is the preferred interpretation, this has the type `string`:
  assert typeof("a b c".split) is string

  assert typeof("a b c".split, typeOfProc) is seq[string]
  ```



Modules
=======
Nim supports splitting a program into pieces by a module concept.
Each module needs to be in its own file and has its own `namespace`:idx:.
Modules enable `information hiding`:idx: and `separate compilation`:idx:.
A module may gain access to the symbols of another module by the `import`:idx:
statement. `Recursive module dependencies`:idx: are allowed, but are slightly
subtle. Only top-level symbols that are marked with an asterisk (`*`) are
exported. A valid module name can only be a valid Nim identifier (and thus its
filename is ``identifier.nim``).

The algorithm for compiling modules is:

- Compile the whole module as usual, following import statements recursively.

- If there is a cycle, only import the already parsed symbols (that are
  exported); if an unknown identifier occurs then abort.

This is best illustrated by an example:

  ```nim
  # Module A
  type
    T1* = int  # Module A exports the type `T1`
  import B     # the compiler starts parsing B

  proc main() =
    var i = p(3) # works because B has been parsed completely here

  main()
  ```


  ```nim
  # Module B
  import A  # A is not parsed here! Only the already known symbols
            # of A are imported.

  proc p*(x: A.T1): A.T1 =
    # this works because the compiler has already
    # added T1 to A's interface symbol table
    result = x + 1
  ```


Import statement
----------------

After the `import` keyword, a list of module names can follow or a single
module name followed by an `except` list to prevent some symbols from being
imported:

  ```nim  test = "nim c $1"  status = 1
  import std/strutils except `%`, toUpperAscii

  # doesn't work then:
  echo "$1" % "abc".toUpperAscii
  ```


It is not checked that the `except` list is really exported from the module.
This feature allows us to compile against different versions of the module,
even when one version does not export some of these identifiers.

The `import` statement is only allowed at the top level.

String literals can be used for import/include statements.
The compiler performs [path substitution](nimc.html#compiler-usage-commandminusline-switches) when used.

Include statement
-----------------

The `include` statement does something fundamentally different than
importing a module: it merely includes the contents of a file. The `include`
statement is useful to split up a large module into several files:

  ```nim
  include fileA, fileB, fileC
  ```

The `include` statement can be used outside the top level, as such:

  ```nim
  # Module A
  echo "Hello World!"
  ```

  ```nim
  # Module B
  proc main() =
    include A

  main() # => Hello World!
  ```


Module names in imports
-----------------------

A module alias can be introduced via the `as` keyword, after which the original module name
is inaccessible:

  ```nim
  import std/strutils as su, std/sequtils as qu

  echo su.format("$1", "lalelu")
  ```

The notations `path/to/module` or `"path/to/module"` can be used to refer to a module
in subdirectories:

  ```nim
  import lib/pure/os, "lib/pure/times"
  ```

Note that the module name is still `strutils` and not `lib/pure/strutils`,
thus one **cannot** do:

  ```nim
  import lib/pure/strutils
  echo lib/pure/strutils.toUpperAscii("abc")
  ```

Likewise, the following does not make sense as the name is `strutils` already:

  ```nim
  import lib/pure/strutils as strutils
  ```


Collective imports from a directory
-----------------------------------

The syntax `import dir / [moduleA, moduleB]` can be used to import multiple modules
from the same directory.

Path names are syntactically either Nim identifiers or string literals. If the path
name is not a valid Nim identifier it needs to be a string literal:

  ```nim
  import "gfx/3d/somemodule" # in quotes because '3d' is not a valid Nim identifier
  ```


Pseudo import/include paths
---------------------------

A directory can also be a so-called "pseudo directory". They can be used to
avoid ambiguity when there are multiple modules with the same path.

There are two pseudo directories:

1. `std`: The `std` pseudo directory is the abstract location of Nim's standard
   library. For example, the syntax `import std / strutils` is used to unambiguously
   refer to the standard library's `strutils` module.
2. `pkg`: The `pkg` pseudo directory is used to unambiguously refer to a Nimble
   package. However, for technical details that lie outside the scope of this document,
   its semantics are: *Use the search path to look for module name but ignore the standard
   library locations*. In other words, it is the opposite of `std`.

It is recommended and preferred but not currently enforced that all stdlib module imports include the std/ "pseudo directory" as part of the import name.

From import statement
---------------------

After the `from` keyword, a module name followed by
an `import` to list the symbols one likes to use without explicit
full qualification:

  ```nim  test = "nim c $1"
  from std/strutils import `%`

  echo "$1" % "abc"
  # always possible: full qualification:
  echo strutils.replace("abc", "a", "z")
  ```

It's also possible to use `from module import nil` if one wants to import
the module but wants to enforce fully qualified access to every symbol
in `module`.


Export statement
----------------

An `export` statement can be used for symbol forwarding so that client
modules don't need to import a module's dependencies:

  ```nim
  # module B
  type MyObject* = object
  ```

  ```nim
  # module A
  import B
  export B.MyObject

  proc `$`*(x: MyObject): string = "my object"
  ```


  ```nim
  # module C
  import A

  # B.MyObject has been imported implicitly here:
  var x: MyObject
  echo $x
  ```

When the exported symbol is another module, all of its definitions will
be forwarded. One can use an `except` list to exclude some of the symbols.

Notice that when exporting, one needs to specify only the module name:

  ```nim
  import foo/bar/baz
  export baz
  ```



Scope rules
-----------
Identifiers are valid from the point of their declaration until the end of
the block in which the declaration occurred. The range where the identifier
is known is the scope of the identifier. The exact scope of an
identifier depends on the way it was declared.

### Block scope

The *scope* of a variable declared in the declaration part of a block
is valid from the point of declaration until the end of the block. If a
block contains a second block, in which the identifier is redeclared,
then inside this block, the second declaration will be valid. Upon
leaving the inner block, the first declaration is valid again. An
identifier cannot be redefined in the same block, except if valid for
procedure or iterator overloading purposes.


### Tuple or object scope

The field identifiers inside a tuple or object definition are valid in the
following places:

* To the end of the tuple/object definition.
* Field designators of a variable of the given tuple/object type.
* In all descendant types of the object type.

### Module scope

All identifiers of a module are valid from the point of declaration until
the end of the module. Identifiers from indirectly dependent modules are *not*
available. The `system`:idx: module is automatically imported in every module.

If a module imports the same identifier from two different modules, the
identifier is considered ambiguous, which can be resolved in the following ways:

* Qualifying the identifier as `module.identifier` resolves ambiguity
  between modules. (See below for the case that the module name itself
  is ambiguous.)
* Calling the identifier as a routine makes overload resolution take place,
  which resolves ambiguity in the case that one overload matches stronger
  than the others.
* Using the identifier in a context where the compiler can infer the type
  of the identifier resolves ambiguity in the case that one definition
  matches the type stronger than the others.

  ```nim
  # Module A
  var x*: string
  proc foo*(a: string) =
    echo "A: ", a
  ```

  ```nim
  # Module B
  var x*: int
  proc foo*(b: int) =
    echo "B: ", b
  ```

  ```nim
  # Module C
  import A, B

  foo("abc") # A: abc
  foo(123) # B: 123
  let inferred: proc (x: string) = foo
  foo("def") # A: def

  write(stdout, x) # error: x is ambiguous
  write(stdout, A.x) # no error: qualifier used
  
  proc bar(a: int): int = a + 1
  assert bar(x) == x + 1 # no error: only A.x of type int matches

  var x = 4
  write(stdout, x) # not ambiguous: uses the module C's x
  ```
Modules can share their name, however, when trying to qualify an identifier with the module name the compiler will fail with ambiguous identifier error. One can qualify the identifier by aliasing the module.


```nim
# Module A/C
proc fb* = echo "fizz"
```


```nim
# Module B/C
proc fb* = echo "buzz"
```


```nim
import A/C
import B/C

C.fb() # Error: ambiguous identifier: 'C'
```


```nim
import A/C as fizz
import B/C

fizz.fb() # Works
```


Packages
--------
A collection of modules in a file tree with an ``identifier.nimble`` file in the
root of the tree is called a Nimble package. A valid package name can only be a
valid Nim identifier and thus its filename is ``identifier.nimble`` where
``identifier`` is the desired package name. A module without a ``.nimble`` file
is assigned the package identifier: `unknown`.

The distinction between packages allows diagnostic compiler messages to be
scoped to the current project's package vs foreign packages.



Compiler Messages
=================

The Nim compiler emits different kinds of messages: `hint`:idx:,
`warning`:idx:, and `error`:idx: messages. An *error* message is emitted if
the compiler encounters any static error.



Pragmas
=======

Pragmas are Nim's method to give the compiler additional information /
commands without introducing a massive number of new keywords. Pragmas are
processed on the fly during semantic checking. Pragmas are enclosed in the
special `{.` and `.}` curly brackets. Pragmas are also often used as a
first implementation to play with a language feature before a nicer syntax
to access the feature becomes available.


deprecated pragma
-----------------

The deprecated pragma is used to mark a symbol as deprecated:

  ```nim
  proc p() {.deprecated.}
  var x {.deprecated.}: char
  ```

This pragma can also take in an optional warning string to relay to developers.

  ```nim
  proc thing(x: bool) {.deprecated: "use thong instead".}
  ```



compileTime pragma
------------------
The `compileTime` pragma is used to mark a proc or variable to be used only
during compile-time execution. No code will be generated for it. Compile-time
procs are useful as helpers for macros. Since version 0.12.0 of the language, a
proc that uses `system.NimNode` within its parameter types is implicitly
declared `compileTime`:

  ```nim
  proc astHelper(n: NimNode): NimNode =
    result = n
  ```

Is the same as:

  ```nim
  proc astHelper(n: NimNode): NimNode {.compileTime.} =
    result = n
  ```

`compileTime` variables are available at runtime too. This simplifies certain
idioms where variables are filled at compile-time (for example, lookup tables)
but accessed at runtime:

  ```nim  test = "nim c -r $1"
  import std/macros

  var nameToProc {.compileTime.}: seq[(string, proc (): string {.nimcall.})]

  macro registerProc(p: untyped): untyped =
    result = newTree(nnkStmtList, p)

    let procName = p[0]
    let procNameAsStr = $p[0]
    result.add quote do:
      nameToProc.add((`procNameAsStr`, `procName`))

  proc foo: string {.registerProc.} = "foo"
  proc bar: string {.registerProc.} = "bar"
  proc baz: string {.registerProc.} = "baz"

  doAssert nameToProc[2][1]() == "baz"
  ```


noreturn pragma
---------------
The `noreturn` pragma is used to mark a proc that never returns.


acyclic pragma
--------------
The `acyclic` pragma can be used for object types to mark them as acyclic
even though they seem to be cyclic. This is an **optimization** for the garbage
collector to not consider objects of this type as part of a cycle:

  ```nim
  type
    Node = ref NodeObj
    NodeObj {.acyclic.} = object
      left, right: Node
      data: string
  ```

Or if we directly use a ref object:

  ```nim
  type
    Node {.acyclic.} = ref object
      left, right: Node
      data: string
  ```

In the example, a tree structure is declared with the `Node` type. Note that
the type definition is recursive and the GC has to assume that objects of
this type may form a cyclic graph. The `acyclic` pragma passes the
information that this cannot happen to the GC. If the programmer uses the
`acyclic` pragma for data types that are in reality cyclic, this may result
in memory leaks, but memory safety is preserved.



final pragma
------------
The `final` pragma can be used for an object type to specify that it
cannot be inherited from. Note that inheritance is only available for
objects that inherit from an existing object (via the `object of SuperType`
syntax) or that have been marked as `inheritable`.


shallow pragma
--------------
The `shallow` pragma affects the semantics of a type: The compiler is
allowed to make a shallow copy. This can cause serious semantic issues and
break memory safety! However, it can speed up assignments considerably,
because the semantics of Nim require deep copying of sequences and strings.
This can be expensive, especially if sequences are used to build a tree
structure:

  ```nim
  type
    NodeKind = enum nkLeaf, nkInner
    Node {.shallow.} = object
      case kind: NodeKind
      of nkLeaf:
        strVal: string
      of nkInner:
        children: seq[Node]
  ```


pure pragma
-----------
An object type can be marked with the `pure` pragma so that its type field
which is used for runtime type identification is omitted. This used to be
necessary for binary compatibility with other compiled languages.

An enum type can be marked as `pure`. Then access of its fields always
requires full qualification.


asmNoStackFrame pragma
----------------------
A proc can be marked with the `asmNoStackFrame` pragma to tell the compiler
it should not generate a stack frame for the proc. There are also no exit
statements like `return result;` generated and the generated C function is
declared as `__declspec(naked)`:c: or `__attribute__((naked))`:c: (depending on
the used C compiler).

**Note**: This pragma should only be used by procs which consist solely of
assembler statements.

error pragma
------------
The `error` pragma is used to make the compiler output an error message
with the given content. The compilation does not necessarily abort after an error
though.

The `error` pragma can also be used to
annotate a symbol (like an iterator or proc). The *usage* of the symbol then
triggers a static error. This is especially useful to rule out that some
operation is valid due to overloading and type conversions:

  ```nim
  ## check that underlying int values are compared and not the pointers:
  proc `==`(x, y: ptr int): bool {.error.}
  ```


fatal pragma
------------
The `fatal` pragma is used to make the compiler output an error message
with the given content. In contrast to the `error` pragma, the compilation
is guaranteed to be aborted by this pragma. Example:

  ```nim
  when not defined(objc):
    {.fatal: "Compile this program with the objc command!".}
  ```

warning pragma
--------------
The `warning` pragma is used to make the compiler output a warning message
with the given content. Compilation continues after the warning.

hint pragma
-----------
The `hint` pragma is used to make the compiler output a hint message with
the given content. Compilation continues after the hint.

line pragma
-----------
The `line` pragma can be used to affect line information of the annotated
statement, as seen in stack backtraces:

  ```nim
  template myassert*(cond: untyped, msg = "") =
    if not cond:
      # change run-time line information of the 'raise' statement:
      {.line: instantiationInfo().}:
        raise newException(AssertionDefect, msg)
  ```

If the `line` pragma is used with a parameter, the parameter needs to be a
`tuple[filename: string, line: int]`. If it is used without a parameter,
`system.instantiationInfo()` is used.


linearScanEnd pragma
--------------------
The `linearScanEnd` pragma can be used to tell the compiler how to
compile a Nim `case`:idx: statement. Syntactically it has to be used as a
statement:

  ```nim
  case myInt
  of 0:
    echo "most common case"
  of 1:
    {.linearScanEnd.}
    echo "second most common case"
  of 2: echo "unlikely: use branch table"
  else: echo "unlikely too: use branch table for ", myInt
  ```

In the example, the case branches `0` and `1` are much more common than
the other cases. Therefore, the generated assembler code should test for these
values first so that the CPU's branch predictor has a good chance to succeed
(avoiding an expensive CPU pipeline stall). The other cases might be put into a
jump table for O(1) overhead but at the cost of a (very likely) pipeline
stall.

The `linearScanEnd` pragma should be put into the last branch that should be
tested against via linear scanning. If put into the last branch of the
whole `case` statement, the whole `case` statement uses linear scanning.


computedGoto pragma
-------------------
The `computedGoto` pragma can be used to tell the compiler how to
compile a Nim `case`:idx: in a `while true` statement.
Syntactically it has to be used as a statement inside the loop:

  ```nim
  type
    MyEnum = enum
      enumA, enumB, enumC, enumD, enumE

  proc vm() =
    var instructions: array[0..100, MyEnum]
    instructions[2] = enumC
    instructions[3] = enumD
    instructions[4] = enumA
    instructions[5] = enumD
    instructions[6] = enumC
    instructions[7] = enumA
    instructions[8] = enumB

    instructions[12] = enumE
    var pc = 0
    while true:
      {.computedGoto.}
      let instr = instructions[pc]
      case instr
      of enumA:
        echo "yeah A"
      of enumC, enumD:
        echo "yeah CD"
      of enumB:
        echo "yeah B"
      of enumE:
        break
      inc(pc)

  vm()
  ```

As the example shows, `computedGoto` is mostly useful for interpreters. If
the underlying backend (C compiler) does not support the computed goto
extension the pragma is simply ignored.


immediate pragma
----------------

The immediate pragma is obsolete. See [Typed vs untyped parameters].

redefine pragma
---------------

Redefinition of template symbols with the same signature is allowed.
This can be made explicit with the `redefine` pragma:

```nim
template foo: int = 1
echo foo() # 1
template foo: int {.redefine.} = 2
echo foo() # 2
# warning: implicit redefinition of template
template foo: int = 3
```

This is mostly intended for macro generated code.

compilation option pragmas
--------------------------
The listed pragmas here can be used to override the code generation options
for a proc/method/converter.

The implementation currently provides the following possible options (various
others may be added later).

===============  ===============  ============================================
pragma           allowed values   description
===============  ===============  ============================================
checks           on|off           Turns the code generation for all runtime
                                  checks on or off.
boundChecks      on|off           Turns the code generation for array bound
                                  checks on or off.
overflowChecks   on|off           Turns the code generation for over- or
                                  underflow checks on or off.
nilChecks        on|off           Turns the code generation for nil pointer
                                  checks on or off.
assertions       on|off           Turns the code generation for assertions
                                  on or off.
warnings         on|off           Turns the warning messages of the compiler
                                  on or off.
hints            on|off           Turns the hint messages of the compiler
                                  on or off.
optimization     none|speed|size  Optimize the code for speed or size, or
                                  disable optimization.
patterns         on|off           Turns the term rewriting templates/macros
                                  on or off.
callconv         cdecl|...        Specifies the default calling convention for
                                  all procedures (and procedure types) that
                                  follow.
===============  ===============  ============================================

Example:

  ```nim
  {.checks: off, optimization: speed.}
  # compile without runtime checks and optimize for speed
  ```


push and pop pragmas
--------------------
The `push/pop`:idx: pragmas are very similar to the option directive,
but are used to override the settings temporarily. Example:

  ```nim
  {.push checks: off.}
  # compile this section without runtime checks as it is
  # speed critical
  # ... some code ...
  {.pop.} # restore old settings
  ```

`push/pop`:idx: can switch on/off some standard library pragmas, example:

  ```nim
  {.push inline.}
  proc thisIsInlined(): int = 42
  func willBeInlined(): float = 42.0
  {.pop.}
  proc notInlined(): int = 9

  {.push discardable, boundChecks: off, compileTime, noSideEffect, experimental.}
  template example(): string = "https://nim-lang.org"
  {.pop.}

  {.push deprecated, used, stackTrace: off.}
  proc sample(): bool = true
  {.pop.}
  ```

For third party pragmas, it depends on its implementation but uses the same syntax.


register pragma
---------------
The `register` pragma is for variables only. It declares the variable as
`register`, giving the compiler a hint that the variable should be placed
in a hardware register for faster access. C compilers usually ignore this
though and for good reasons: Often they do a better job without it anyway.

However, in highly specific cases (a dispatch loop of a bytecode interpreter
for example) it may provide benefits.


global pragma
-------------
The `global` pragma can be applied to a variable within a proc to instruct
the compiler to store it in a global location and initialize it once at program
startup.

  ```nim
  proc isHexNumber(s: string): bool =
    var pattern {.global.} = re"[0-9a-fA-F]+"
    result = s.match(pattern)
  ```

When used within a generic proc, a separate unique global variable will be
created for each instantiation of the proc. The order of initialization of
the created global variables within a module is not defined, but all of them
will be initialized after any top-level variables in their originating module
and before any variable in a module that imports it.

Disabling certain messages
--------------------------
Nim generates some warnings and hints that may annoy the
user. A mechanism for disabling certain messages is provided: Each hint
and warning message is associated with a symbol. This is the message's
identifier, which can be used to enable or disable the message by putting it
in brackets following the pragma:

  ```Nim
  {.hint[XDeclaredButNotUsed]: off.} # Turn off the hint about declared but not used symbols.
  ```

This is often better than disabling all warnings at once.


used pragma
-----------

Nim produces a warning for symbols that are not exported and not used either.
The `used` pragma can be attached to a symbol to suppress this warning. This
is particularly useful when the symbol was generated by a macro:

  ```nim
  template implementArithOps(T) =
    proc echoAdd(a, b: T) {.used.} =
      echo a + b
    proc echoSub(a, b: T) {.used.} =
      echo a - b

  # no warning produced for the unused 'echoSub'
  implementArithOps(int)
  echoAdd 3, 5
  ```

`used` can also be used as a top-level statement to mark a module as "used".
This prevents the "Unused import" warning:

  ```nim
  # module: debughelper.nim
  when defined(nimHasUsed):
    # 'import debughelper' is so useful for debugging
    # that Nim shouldn't produce a warning for that import,
    # even if currently unused:
    {.used.}
  ```


experimental pragma
-------------------

The `experimental` pragma enables experimental language features. Depending
on the concrete feature, this means that the feature is either considered
too unstable for an otherwise stable release or that the future of the feature
is uncertain (it may be removed at any time). See the
[experimental manual](manual_experimental.html) for more details.

Example:

  ```nim
  import std/threadpool
  {.experimental: "parallel".}

  proc threadedEcho(s: string, i: int) =
    echo(s, " ", $i)

  proc useParallel() =
    parallel:
      for i in 0..4:
        spawn threadedEcho("echo in parallel", i)

  useParallel()
  ```


As a top-level statement, the experimental pragma enables a feature for the
rest of the module it's enabled in. This is problematic for macro and generic
instantiations that cross a module scope. Currently, these usages have to be
put into a `.push/pop` environment:

  ```nim
  # client.nim
  proc useParallel*[T](unused: T) =
    # use a generic T here to show the problem.
    {.push experimental: "parallel".}
    parallel:
      for i in 0..4:
        echo "echo in parallel"

    {.pop.}
  ```


  ```nim
  import client
  useParallel(1)
  ```


Implementation Specific Pragmas
===============================

This section describes additional pragmas that the current Nim implementation
supports but which should not be seen as part of the language specification.

Bitsize pragma
--------------

The `bitsize` pragma is for object field members. It declares the field as
a bitfield in C/C++.

  ```Nim
  type
    mybitfield = object
      flag {.bitsize:1.}: cuint
  ```

generates:

  ```C
  struct mybitfield {
    unsigned int flag:1;
  };
  ```


size pragma
-----------
Nim automatically determines the size of an enum.
But when wrapping a C enum type, it needs to be of a specific size.
The `size pragma` allows specifying the size of the enum type.

  ```Nim
  type
    EventType* {.size: sizeof(uint32).} = enum
      QuitEvent,
      AppTerminating,
      AppLowMemory

  doAssert sizeof(EventType) == sizeof(uint32)
  ```

When used for enum types, the `size pragma` accepts only the values 1, 2, 4 or 8.

The `size pragma` can also specify the size of an `importc` incomplete object type
so that one can get the size of it at compile time even if it was declared without fields.

  ```Nim
    type
      AtomicFlag* {.importc: "atomic_flag", header: "<stdatomic.h>", size: 1.} = object

    static:
      # if AtomicFlag didn't have the size pragma, this code would result in a compile time error.
      echo sizeof(AtomicFlag)
  ```


Align pragma
------------

The `align`:idx: pragma is for variables and object field members. It
modifies the alignment requirement of the entity being declared. The
argument must be a constant power of 2. Valid non-zero
alignments that are weaker than other align pragmas on the same
declaration are ignored. Alignments that are weaker than the
alignment requirement of the type are ignored.

  ```Nim
  type
    sseType = object
      sseData {.align(16).}: array[4, float32]

    # every object will be aligned to 128-byte boundary
    Data = object
      x: char
      cacheline {.align(128).}: array[128, char] # over-aligned array of char,

  proc main() =
    echo "sizeof(Data) = ", sizeof(Data), " (1 byte + 127 bytes padding + 128-byte array)"
    # output: sizeof(Data) = 256 (1 byte + 127 bytes padding + 128-byte array)
    echo "alignment of sseType is ", alignof(sseType)
    # output: alignment of sseType is 16
    var d {.align(2048).}: Data # this instance of data is aligned even stricter

  main()
  ```

This pragma has no effect on the JS backend.


Noalias pragma
--------------

Since version 1.4 of the Nim compiler, there is a `.noalias` annotation for variables
and parameters. It is mapped directly to C/C++'s `restrict`:c: keyword and means that
the underlying pointer is pointing to a unique location in memory, no other aliases to
this location exist. It is *unchecked* that this alias restriction is followed. If the
restriction is violated, the backend optimizer is free to miscompile the code.
This is an **unsafe** language feature.

Ideally in later versions of the language, the restriction will be enforced at
compile time. (This is also why the name `noalias` was chosen instead of a more
verbose name like `unsafeAssumeNoAlias`.)


Volatile pragma
---------------
The `volatile` pragma is for variables only. It declares the variable as
`volatile`:c:, whatever that means in C/C++ (its semantics are not well-defined
in C/C++).

**Note**: This pragma will not exist for the LLVM backend.


nodecl pragma
-------------
The `nodecl` pragma can be applied to almost any symbol (variable, proc,
type, etc.) and is sometimes useful for interoperability with C:
It tells Nim that it should not generate a declaration for the symbol in
the C code. For example:

  ```Nim
  var
    EACCES {.importc, nodecl.}: cint # pretend EACCES was a variable, as
                                     # Nim does not know its value
  ```

However, the `header` pragma is often the better alternative.

**Note**: This will not work for the LLVM backend.


Header pragma
-------------
The `header` pragma is very similar to the `nodecl` pragma: It can be
applied to almost any symbol and specifies that it should not be declared
and instead, the generated code should contain an `#include`:c:\:

  ```Nim
  type
    PFile {.importc: "FILE*", header: "<stdio.h>".} = distinct pointer
      # import C's FILE* type; Nim will treat it as a new pointer type
  ```

The `header` pragma always expects a string constant. The string constant
contains the header file: As usual for C, a system header file is enclosed
in angle brackets: `<>`:c:. If no angle brackets are given, Nim
encloses the header file in `""`:c: in the generated C code.

**Note**: This will not work for the LLVM backend.


IncompleteStruct pragma
-----------------------
The `incompleteStruct` pragma tells the compiler to not use the
underlying C `struct`:c: in a `sizeof` expression:

  ```Nim
  type
    DIR* {.importc: "DIR", header: "<dirent.h>",
           pure, incompleteStruct.} = object
  ```


CompleteStruct pragma
---------------------
The `completeStruct` pragma is a contract indicating that an `importc` type
declaration contains all fields of the corresponding C type, allowing
`sizeof`, `alignof`, and `offsetof` to be computed at compile-time.

By default, `importc` types are assumed to be incomplete (their size is
unknown at compile-time). Use `completeStruct` when you need compile-time
size information and can guarantee the Nim definition matches the C layout:

  ```Nim
  type
    InotifyEvent {.importc: "struct inotify_event", header: "<sys/inotify.h>",
                   completeStruct.} = object
      wd: cint
      mask: uint32
      cookie: uint32
      len: uint32
      # All fields must match the C struct exactly
  ```

If the Nim fields don't match the C struct, a static assertion will fail
during C code generation.

Without `completeStruct`, attempting to use `sizeof` on an `importc` type
at compile-time will error with "'sizeof' requires '.importc' types to be
'.completeStruct'".


Compile pragma
--------------
The `compile` pragma can be used to compile and link a C/C++ source file
with the project:

This pragma can take three forms. The first is a simple file input:
  ```Nim
  {.compile: "myfile.cpp".}
  ```

The second form is a tuple where the second arg is the output name strutils formatter:
  ```Nim
  {.compile: ("file.c", "$1.o").}
  ```

**Note**: Nim computes a SHA1 checksum and only recompiles the file if it
has changed. One can use the `-f`:option: command-line option to force
the recompilation of the file.

Since 1.4 the `compile` pragma is also available with this syntax:

  ```Nim
  {.compile("myfile.cpp", "--custom flags here").}
  ```

As can be seen in the example, this new variant allows for custom flags
that are passed to the C compiler when the file is recompiled.


Link pragma
-----------
The `link` pragma can be used to link an additional file with the project:

  ```Nim
  {.link: "myfile.o".}
  ```


passc pragma
------------
The `passc` pragma can be used to pass additional parameters to the C
compiler like one would use the command-line switch `--passc`:option:\:

  ```Nim
  {.passc: "-Wall -Werror".}
  ```

Note that one can use `gorge` from the [system module](system.html) to
embed parameters from an external command that will be executed
during semantic analysis:

  ```Nim
  {.passc: gorge("pkg-config --cflags sdl").}
  ```


localPassC pragma
-----------------
The `localPassC` pragma can be used to pass additional parameters to the C
compiler, but only for the C/C++ file that is produced from the Nim module
the pragma resides in:

  ```Nim
  # Module A.nim
  # Produces: A.nim.cpp
  {.localPassC: "-Wall -Werror".} # Passed when compiling A.nim.cpp
  ```


passl pragma
------------
The `passl` pragma can be used to pass additional parameters to the linker
like one would be using the command-line switch `--passl`:option:\:

  ```Nim
  {.passl: "-lSDLmain -lSDL".}
  ```

Note that one can use `gorge` from the [system module](system.html) to
embed parameters from an external command that will be executed
during semantic analysis:

  ```Nim
  {.passl: gorge("pkg-config --libs sdl").}
  ```


Emit pragma
-----------
The `emit` pragma can be used to directly affect the output of the
compiler's code generator. The code is then unportable to other code
generators/backends. Its usage is highly discouraged! However, it can be
extremely useful for interfacing with `C++`:idx: or `Objective C`:idx: code.

Example:

  ```Nim
  {.emit: """
  static int cvariable = 420;
  """.}

  {.push stackTrace:off.}
  proc embedsC() =
    var nimVar = 89
    # access Nim symbols within an emit section outside of string literals:
    {.emit: ["""fprintf(stdout, "%d\n", cvariable + (int)""", nimVar, ");"].}
  {.pop.}

  embedsC()
  ```

``nimbase.h`` defines `NIM_EXTERNC`:c: C macro that can be used for
`extern "C"`:cpp: code to work with both `nim c`:cmd: and `nim cpp`:cmd:, e.g.:

  ```Nim
  proc foobar() {.importc:"$1".}
  {.emit: """
  #include <stdio.h>
  NIM_EXTERNC
  void fun(){}
  """.}
  ```

.. note:: For backward compatibility, if the argument to the `emit` statement
  is a single string literal, Nim symbols can be referred to via backticks.
  This usage is however deprecated.

For a top-level emit statement, the section where in the generated C/C++ file
the code should be emitted can be influenced via the prefixes
`/*TYPESECTION*/`:c: or `/*VARSECTION*/`:c: or `/*INCLUDESECTION*/`:c:\:

  ```Nim
  {.emit: """/*TYPESECTION*/
  struct Vector3 {
  public:
    Vector3(): x(5) {}
    Vector3(float x_): x(x_) {}
    float x;
  };
  """.}

  type Vector3 {.importcpp: "Vector3", nodecl} = object
    x: cfloat

  proc constructVector3(a: cfloat): Vector3 {.importcpp: "Vector3(@)", nodecl}
  ```


ImportCpp pragma
----------------

**Note**: [c2nim](https://github.com/nim-lang/c2nim/blob/master/doc/c2nim.rst)
can parse a large subset of C++ and knows
about the `importcpp` pragma pattern language. It is not necessary
to know all the details described here.


Similar to the [importc pragma] for C, the
`importcpp` pragma can be used to import `C++`:idx: methods or C++ symbols
in general. The generated code then uses the C++ method calling
syntax: `obj->method(arg)`:cpp:. In combination with the `header` and `emit`
pragmas this allows *sloppy* interfacing with libraries written in C++:

  ```Nim
  # Horrible example of how to interface with a C++ engine ... ;-)

  {.link: "/usr/lib/libIrrlicht.so".}

  {.emit: """
  using namespace irr;
  using namespace core;
  using namespace scene;
  using namespace video;
  using namespace io;
  using namespace gui;
  """.}

  const
    irr = "<irrlicht/irrlicht.h>"

  type
    IrrlichtDeviceObj {.header: irr,
                        importcpp: "IrrlichtDevice".} = object
    IrrlichtDevice = ptr IrrlichtDeviceObj

  proc createDevice(): IrrlichtDevice {.
    header: irr, importcpp: "createDevice(@)".}
  proc run(device: IrrlichtDevice): bool {.
    header: irr, importcpp: "#.run(@)".}
  ```

The compiler needs to be told to generate C++ (command `cpp`:option:) for
this to work. The conditional symbol `cpp` is defined when the compiler
emits C++ code.

### Namespaces

The *sloppy interfacing* example uses `.emit` to produce `using namespace`:cpp:
declarations. It is usually much better to instead refer to the imported name
via the `namespace::identifier`:cpp: notation:

  ```nim
  type
    IrrlichtDeviceObj {.header: irr,
                        importcpp: "irr::IrrlichtDevice".} = object
  ```


### Importcpp for enums

When `importcpp` is applied to an enum type the numerical enum values are
annotated with the C++ enum type, like in this example:
`((TheCppEnum)(3))`:cpp:.
(This turned out to be the simplest way to implement it.)


### Importcpp for procs

Note that the `importcpp` variant for procs uses a somewhat cryptic pattern
language for maximum flexibility:

- A hash ``#`` symbol is replaced by the first or next argument.
- A dot following the hash ``#.`` indicates that the call should use C++'s dot
  or arrow notation.
- An at symbol ``@`` is replaced by the remaining arguments,
  separated by commas.

For example:

  ```nim
  proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "#.CppMethod(@)".}
  var x: ptr CppObj
  cppMethod(x[], 1, 2, 3)
  ```

Produces:

  ```C
  x->CppMethod(1, 2, 3)
  ```

As a special rule to keep backward compatibility with older versions of the
`importcpp` pragma, if there is no special pattern
character (any of ``# ' @``) at all, C++'s
dot or arrow notation is assumed, so the above example can also be written as:

  ```nim
  proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "CppMethod".}
  ```

Note that the pattern language naturally also covers C++'s operator overloading
capabilities:

  ```nim
  proc vectorAddition(a, b: Vec3): Vec3 {.importcpp: "# + #".}
  proc dictLookup(a: Dict, k: Key): Value {.importcpp: "#[#]".}
  ```


- An apostrophe ``'`` followed by an integer ``i`` in the range 0..9
  is replaced by the i'th parameter *type*. The 0th position is the result
  type. This can be used to pass types to C++ function templates. Between
  the ``'`` and the digit, an asterisk can be used to get to the base type
  of the type. (So it "takes away a star" from the type; `T*`:c: becomes `T`.)
  Two stars can be used to get to the element type of the element type etc.

For example:

  ```nim
  type Input {.importcpp: "System::Input".} = object
  proc getSubsystem*[T](): ptr T {.importcpp: "SystemManager::getSubsystem<'*0>()", nodecl.}

  let x: ptr Input = getSubsystem[Input]()
  ```

Produces:

  ```C
  x = SystemManager::getSubsystem<System::Input>()
  ```


- ``#@`` is a special case to support a `cnew` operation. It is required so
  that the call expression is inlined directly, without going through a
  temporary location. This is only required to circumvent a limitation of the
  current code generator.

For example C++'s `new`:cpp: operator can be "imported" like this:

  ```nim
  proc cnew*[T](x: T): ptr T {.importcpp: "(new '*0#@)", nodecl.}

  # constructor of 'Foo':
  proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)".}

  let x = cnew constructFoo(3, 4)
  ```

Produces:

  ```C
  x = new Foo(3, 4)
  ```

However, depending on the use case `new Foo`:cpp: can also be wrapped like this
instead:

  ```nim
  proc newFoo(a, b: cint): ptr Foo {.importcpp: "new Foo(@)".}

  let x = newFoo(3, 4)
  ```


### Wrapping constructors

Sometimes a C++ class has a private copy constructor and so code like
`Class c = Class(1,2);`:cpp: must not be generated but instead
`Class c(1,2);`:cpp:.
For this purpose the Nim proc that wraps a C++ constructor needs to be
annotated with the `constructor`:idx: pragma. This pragma also helps to generate
faster C++ code since construction then doesn't invoke the copy constructor:

  ```nim
  # a better constructor of 'Foo':
  proc constructFoo(a, b: cint): Foo {.importcpp: "Foo(@)", constructor.}
  ```


### Wrapping destructors

Since Nim generates C++ directly, any destructor is called implicitly by the
C++ compiler at the scope exits. This means that often one can get away with
not wrapping the destructor at all! However, when it needs to be invoked
explicitly, it needs to be wrapped. The pattern language provides
everything that is required:

  ```nim
  proc destroyFoo(this: var Foo) {.importcpp: "#.~Foo()".}
  ```


### Importcpp for objects

Generic `importcpp`'ed objects are mapped to C++ templates. This means that
one can import C++'s templates rather easily without the need for a pattern
language for object types:

  ```nim  test = "nim cpp $1"
  type
    StdMap[K, V] {.importcpp: "std::map", header: "<map>".} = object
  proc `[]=`[K, V](this: var StdMap[K, V]; key: K; val: V) {.
    importcpp: "#[#] = #", header: "<map>".}

  var x: StdMap[cint, cdouble]
  x[6] = 91.4
  ```


Produces:

  ```C
  std::map<int, double> x;
  x[6] = 91.4;
  ```


- If more precise control is needed, the apostrophe `'` can be used in the
  supplied pattern to denote the concrete type parameters of the generic type.
  See the usage of the apostrophe operator in proc patterns for more details.

    ```nim
    type
      VectorIterator[T] {.importcpp: "std::vector<'0>::iterator".} = object

    var x: VectorIterator[cint]
    ```

  Produces:

    ```C

    std::vector<int>::iterator x;
    ```


ImportJs pragma
---------------

Similar to the [importcpp pragma] for C++,
the `importjs` pragma can be used to import Javascript methods or
symbols in general. The generated code then uses the Javascript method
calling syntax: ``obj.method(arg)``.


ImportObjC pragma
-----------------
Similar to the [importc pragma] for C, the `importobjc` pragma can
be used to import `Objective C`:idx: methods. The generated code then uses the
Objective C method calling syntax: ``[obj method param1: arg]``.
In addition with the `header` and `emit` pragmas this
allows *sloppy* interfacing with libraries written in Objective C:

  ```Nim
  # horrible example of how to interface with GNUStep ...

  {.passl: "-lobjc".}
  {.emit: """
  #include <objc/Object.h>
  @interface Greeter:Object
  {
  }

  - (void)greet:(long)x y:(long)dummy;
  @end

  #include <stdio.h>
  @implementation Greeter

  - (void)greet:(long)x y:(long)dummy
  {
    printf("Hello, World!\n");
  }
  @end

  #include <stdlib.h>
  """.}

  type
    Id {.importc: "id", header: "<objc/Object.h>", final.} = distinct int

  proc newGreeter: Id {.importobjc: "Greeter new", nodecl.}
  proc greet(self: Id, x, y: int) {.importobjc: "greet", nodecl.}
  proc free(self: Id) {.importobjc: "free", nodecl.}

  var g = newGreeter()
  g.greet(12, 34)
  g.free()
  ```

The compiler needs to be told to generate Objective C (command `objc`:option:) for
this to work. The conditional symbol ``objc`` is defined when the compiler
emits Objective C code.


CodegenDecl pragma
------------------

The `codegenDecl` pragma can be used to directly influence Nim's code
generator. It receives a format string that determines how the variable,
proc or object type is declared in the generated code.

For variables, $1 in the format string represents the type of the variable,
$2 is the name of the variable, and each appearance of $# represents $1/$2
respectively according to its position.

The following Nim code:

  ```nim
  var
    a {.codegenDecl: "$# progmem $#".}: int
  ```

will generate this C code:

  ```c
  int progmem a
  ```

For procedures, $1 is the return type of the procedure, $2 is the name of
the procedure, $3 is the parameter list, and each appearance of $# represents
$1/$2/$3 respectively according to its position.

The following nim code:

  ```nim
  proc myinterrupt() {.codegenDecl: "__interrupt $# $#$#".} =
    echo "realistic interrupt handler"
  ```

will generate this code:

  ```c
  __interrupt void myinterrupt()
  ```

For object types, the $1 represents the name of the object type, $2 is the list of
fields and $3 is the base type.

```nim

const strTemplate = """
  struct $1 {
    $2
  };
"""
type Foo {.codegenDecl:strTemplate.} = object
  a, b: int
```

will generate this code:


```c
struct Foo {
  NI a;
  NI b;
};
```

`cppNonPod` pragma
------------------

The `cppNonPod` pragma should be used for non-POD `importcpp` types so that they
work properly (in particular regarding constructor and destructor) for
`threadvar` variables. This requires `--tlsEmulation:off`:option:.

  ```nim
  type Foo {.cppNonPod, importcpp, header: "funs.h".} = object
    x: cint
  proc main()=
    var a {.threadvar.}: Foo
  ```


compile-time define pragmas
---------------------------

The pragmas listed here can be used to optionally accept values from
the `-d/--define`:option: option at compile time.

The implementation currently provides the following possible options (various
others may be added later).

=================  ============================================
pragma             description
=================  ============================================
`intdefine`:idx:   Reads in a build-time define as an integer
`strdefine`:idx:   Reads in a build-time define as a string
`booldefine`:idx:  Reads in a build-time define as a bool
=================  ============================================

  ```nim
  const FooBar {.intdefine.}: int = 5
  echo FooBar
  ```

  ```cmd
  nim c -d:FooBar=42 foobar.nim
  ```

In the above example, providing the `-d`:option: flag causes the symbol
`FooBar` to be overwritten at compile-time, printing out 42. If the
`-d:FooBar=42`:option: were to be omitted, the default value of 5 would be
used. To see if a value was provided, `defined(FooBar)` can be used.

The syntax `-d:flag`:option: is actually just a shortcut for
`-d:flag=true`:option:.

These pragmas also accept an optional string argument for qualified
define names.

  ```nim
  const FooBar {.intdefine: "package.FooBar".}: int = 5
  echo FooBar
  ```

  ```cmd
  nim c -d:package.FooBar=42 foobar.nim
  ```

This helps disambiguate define names in different packages.

See also the [generic `define` pragma](manual_experimental.html#generic-nimdefine-pragma)
for a version of these pragmas that detects the type of the define based on
the constant value.

User-defined pragmas
====================


pragma pragma
-------------

The `pragma` pragma can be used to declare user-defined pragmas. This is
useful because Nim's templates and macros do not affect pragmas.
User-defined pragmas are in a different module-wide scope than all other symbols.
They cannot be imported from a module.

Example:

  ```nim
  when appType == "lib":
    {.pragma: rtl, exportc, dynlib, cdecl.}
  else:
    {.pragma: rtl, importc, dynlib: "client.dll", cdecl.}

  proc p*(a, b: int): int {.rtl.} =
    result = a + b
  ```

In the example, a new pragma named `rtl` is introduced that either imports
a symbol from a dynamic library or exports the symbol for dynamic library
generation.


Custom annotations
------------------
It is possible to define custom typed pragmas. Custom pragmas do not affect
code generation directly, but their presence can be detected by macros.
Custom pragmas are defined using templates annotated with pragma `pragma`:

  ```nim
  template dbTable(name: string, table_space: string = "") {.pragma.}
  template dbKey(name: string = "", primary_key: bool = false) {.pragma.}
  template dbForeignKey(t: typedesc) {.pragma.}
  template dbIgnore {.pragma.}
  ```


Consider this stylized example of a possible Object Relation Mapping (ORM)
implementation:

  ```nim
  const tblspace {.strdefine.} = "dev" # switch for dev, test and prod environments

  type
    User {.dbTable("users", tblspace).} = object
      id {.dbKey(primary_key = true).}: int
      name {.dbKey"full_name".}: string
      is_cached {.dbIgnore.}: bool
      age: int

    UserProfile {.dbTable("profiles", tblspace).} = object
      id {.dbKey(primary_key = true).}: int
      user_id {.dbForeignKey: User.}: int
      read_access: bool
      write_access: bool
      admin_access: bool
  ```

In this example, custom pragmas are used to describe how Nim objects are
mapped to the schema of the relational database. Custom pragmas can have
zero or more arguments. In order to pass multiple arguments use one of
template call syntaxes. All arguments are typed and follow standard
overload resolution rules for templates. Therefore, it is possible to have
default values for arguments, pass by name, varargs, etc.

Custom pragmas can be used in all locations where ordinary pragmas can be
specified. It is possible to annotate procs, templates, type and variable
definitions, statements, etc.

The macros module includes helpers which can be used to simplify custom pragma
access `hasCustomPragma`, `getCustomPragmaVal`. Please consult the
[macros](macros.html) module documentation for details. These macros are not
magic, everything they do can also be achieved by walking the AST of the object
representation.

More examples with custom pragmas:

- Better serialization/deserialization control:

    ```nim
    type MyObj = object
      a {.dontSerialize.}: int
      b {.defaultDeserialize: 5.}: int
      c {.serializationKey: "_c".}: string
    ```

- Adopting type for gui inspector in a game engine:

    ```nim
    type MyComponent = object
      position {.editable, animatable.}: Vector3
      alpha {.editRange: [0.0..1.0], animatable.}: float32
    ```


Macro pragmas
-------------

Macros and templates can sometimes be called with the pragma syntax. Cases
where this is possible include when attached to routine (procs, iterators, etc.)
declarations or routine type expressions. The compiler will perform the
following simple syntactic transformations:

  ```nim
  template command(name: string, def: untyped) = discard

  proc p() {.command("print").} = discard
  ```

This is translated to:

  ```nim
  command("print"):
    proc p() = discard
  ```

------

  ```nim
  type
    AsyncEventHandler = proc (x: Event) {.async.}
  ```

This is translated to:

  ```nim
  type
    AsyncEventHandler = async(proc (x: Event))
  ```

------

When multiple macro pragmas are applied to the same definition, the first one
from left to right will be evaluated. This macro can then choose to keep
the remaining macro pragmas in its output, and those will be evaluated in
the same way.

There are a few more applications of macro pragmas, such as in type,
variable and constant declarations, but this behavior is considered to be
experimental and is documented in the [experimental manual](
manual_experimental.html#extended-macro-pragmas) instead.


Foreign function interface
==========================

Nim's `FFI`:idx: (foreign function interface) is extensive and only the
parts that scale to other future backends (like the LLVM/JavaScript backends)
are documented here.


Importc pragma
--------------
The `importc` pragma provides a means to import a proc or a variable
from C. The optional argument is a string containing the C identifier. If
the argument is missing, the C name is the Nim identifier *exactly as
spelled*:

  ```nim
  proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.}
  ```

When `importc` is applied to a `let` statement it can omit its value which
will then be expected to come from C. This can be used to import a C `const`:c:\:

  ```nim
  {.emit: "const int cconst = 42;".}

  let cconst {.importc, nodecl.}: cint

  assert cconst == 42
  ```

Note that this pragma has been abused in the past to also work in the
JS backend for JS objects and functions. Other backends do provide
the same feature under the same name. Also, when the target language
is not set to C, other pragmas are available:

 * [importcpp][importcpp pragma]
 * [importobjc][importobjc pragma]
 * [importjs][importjs pragma]

The string literal passed to `importc` can be a format string:

  ```Nim
  proc p(s: cstring) {.importc: "prefix$1".}
  ```

In the example, the external name of `p` is set to `prefixp`. Only ``$1``
is available and a literal dollar sign must be written as ``$$``.


Exportc pragma
--------------
The `exportc` pragma provides a means to export a type, a variable, or a
procedure to C. Enums and constants can't be exported. The optional argument
is a string containing the C identifier. If the argument is missing, the C
name is the Nim identifier *exactly as spelled*:

  ```Nim
  proc callme(formatstr: cstring) {.exportc: "callMe", varargs.}
  ```

Note that this pragma is somewhat of a misnomer: Other backends do provide
the same feature under the same name.

The string literal passed to `exportc` can be a format string:

  ```Nim
  proc p(s: string) {.exportc: "prefix$1".} =
    echo s
  ```

In the example, the external name of `p` is set to `prefixp`. Only ``$1``
is available and a literal dollar sign must be written as ``$$``.

If the symbol should also be exported to a dynamic library, the `dynlib`
pragma should be used in addition to the `exportc` pragma. See
[Dynlib pragma for export].


Exportcpp pragma
----------------
The `exportcpp` pragma works like the `exportc` pragma but it requires the `cpp` backend.
When compiled with the `cpp` backend, the `exportc` pragma adds `export "C"` to
the declaration in the generated code so that it can be called from both C and
C++ code. `exportcpp` pragma doesn't add `export "C"`.


Extern pragma
-------------
Like `exportc` or `importc`, the `extern` pragma affects name
mangling. The string literal passed to `extern` can be a format string:

  ```Nim
  proc p(s: string) {.extern: "prefix$1".} =
    echo s
  ```

In the example, the external name of `p` is set to `prefixp`. Only ``$1``
is available and a literal dollar sign must be written as ``$$``.


Bycopy pragma
-------------

The `bycopy` pragma can be applied to an object or tuple type or a proc param. It instructs the compiler to pass the type by value to procs:

  ```nim
  type
    Vector {.bycopy.} = object
      x, y, z: float
  ```

The Nim compiler automatically determines whether a parameter is passed by value or
by reference based on the parameter type's size. If a parameter must be passed by value
or by reference, (such as when interfacing with a C library) use the bycopy or byref pragmas.
Notice params marked as `byref` takes precedence over types marked as `bycopy`.

Byref pragma
------------

The `byref` pragma can be applied to an object or tuple type or a proc param.
When applied to a type it instructs the compiler to pass the type by reference
(hidden pointer) to procs. When applied to a param it will take precedence, even
if the the type was marked as `bycopy`. When an `importc` type has a `byref` pragma or
parameters are marked as `byref` in an `importc` proc, these params translate to pointers.
When an `importcpp` type has a `byref` pragma, these params translate to
C++ references `&`.

  ```Nim
  {.emit: """/*TYPESECTION*/
  typedef struct {
    int x;
  } CStruct;
  """.}

  {.emit: """
  #ifdef __cplusplus
  extern "C"
  #endif
  int takesCStruct(CStruct* x) {
    return x->x;
  }
  """.}

  type
    CStruct {.importc, byref.} = object
      x: cint

  proc takesCStruct(x: CStruct): cint {.importc.}
  ```

  or


  ```Nim
  type
    CStruct {.importc.} = object
      x: cint

  proc takesCStruct(x {.byref.}: CStruct): cint {.importc.}
  ```

  ```Nim
  {.emit: """/*TYPESECTION*/
  struct CppStruct {
    int x;

    int takesCppStruct(CppStruct& y) {
      return x + y.x;
    }
  };
  """.}

  type
    CppStruct {.importcpp, byref.} = object
      x: cint

  proc takesCppStruct(x, y: CppStruct): cint {.importcpp.}
  ```

Varargs pragma
--------------
The `varargs` pragma can be applied to procedures only (and procedure
types). It tells Nim that the proc can take a variable number of parameters
after the last specified parameter. Nim string values will be converted to C
strings automatically:

  ```Nim
  proc printf(formatstr: cstring) {.header: "<stdio.h>", varargs.}

  printf("hallo %s", "world") # "world" will be passed as C string
  ```


Union pragma
------------
The `union` pragma can be applied to any `object` type. It means all
of an object's fields are overlaid in memory. This produces a `union`:c:
instead of a `struct`:c: in the generated C/C++ code. The object declaration
then must not use inheritance or any GC'ed memory but this is currently not
checked.

**Future directions**: GC'ed memory should be allowed in unions and the GC
should scan unions conservatively.


Packed pragma
-------------
The `packed` pragma can be applied to any `object` type. It ensures
that the fields of an object are packed back-to-back in memory. It is useful
to store packets or messages from/to network or hardware drivers, and for
interoperability with C. Combining packed pragma with inheritance is not
defined, and it should not be used with GC'ed memory (ref's).

**Future directions**: Using GC'ed memory in packed pragma will result in
a static error. Usage with inheritance should be defined and documented.


Dynlib pragma for import
------------------------
With the `dynlib` pragma, a procedure or a variable can be imported from
a dynamic library (``.dll`` files for Windows, ``lib*.so`` files for UNIX).
The non-optional argument has to be the name of the dynamic library:

  ```Nim
  proc gtk_image_new(): PGtkWidget
    {.cdecl, dynlib: "libgtk-x11-2.0.so", importc.}
  ```

In general, importing a dynamic library does not require any special linker
options or linking with import libraries. This also implies that no *devel*
packages need to be installed.

The `dynlib` import mechanism supports a versioning scheme:

  ```nim
  proc Tcl_Eval(interp: pTcl_Interp, script: cstring): int {.cdecl,
    importc, dynlib: "libtcl(|8.5|8.4|8.3).so.(1|0)".}
  ```

At runtime, the dynamic library is searched for (in this order):

    libtcl.so.1
    libtcl.so.0
    libtcl8.5.so.1
    libtcl8.5.so.0
    libtcl8.4.so.1
    libtcl8.4.so.0
    libtcl8.3.so.1
    libtcl8.3.so.0

The `dynlib` pragma supports not only constant strings as an argument but also
string expressions in general:

  ```nim
  import std/os

  proc getDllName: string =
    result = "mylib.dll"
    if fileExists(result): return
    result = "mylib2.dll"
    if fileExists(result): return
    quit("could not load dynamic library")

  proc myImport(s: cstring) {.cdecl, importc, dynlib: getDllName().}
  ```

**Note**: Patterns like ``libtcl(|8.5|8.4).so`` are only supported in constant
strings, because they are precompiled.

**Note**: Passing variables to the `dynlib` pragma will fail at runtime
because of order of initialization problems.

**Note**: A `dynlib` import can be overridden with
the `--dynlibOverride:name`:option: command-line option. The
[Compiler User Guide](nimc.html) contains further information.


Dynlib pragma for export
------------------------

With the `dynlib` pragma, a procedure can also be exported to
a dynamic library. The pragma then has no argument and has to be used in
conjunction with the `exportc` pragma:

  ```Nim
  proc exportme(): int {.cdecl, exportc, dynlib.}
  ```

This is only useful if the program is compiled as a dynamic library via the
`--app:lib`:option: command-line option.


Threads
=======

The `--threads:on`:option: command-line switch is enabled by default. The [typedthreads module](typedthreads.html) module then contains several threading primitives. See [spawn](manual_experimental.html#parallel-amp-spawn) for
further details.

The only ways to create a thread is via `spawn` or `createThread`.


Thread pragma
-------------

A proc that is executed as a new thread of execution should be marked by the
`thread` pragma for reasons of readability. The compiler checks for
violations of the `no heap sharing restriction`:idx:\: This restriction implies
that it is invalid to construct a data structure that consists of memory
allocated from different (thread-local) heaps.

A thread proc can be passed to `createThread` or `spawn`.



Threadvar pragma
----------------

A variable can be marked with the `threadvar` pragma, which makes it a
`thread-local`:idx: variable; Additionally, this implies all the effects
of the `global` pragma.

  ```nim
  var checkpoints* {.threadvar.}: seq[string]
  ```

Due to implementation restrictions, thread-local variables cannot be
initialized within the `var` section. (Every thread-local variable needs to
be replicated at thread creation.)


Threads and exceptions
----------------------

The interaction between threads and exceptions is simple: A *handled* exception
in one thread cannot affect any other thread. However, an *unhandled* exception
in one thread terminates the whole *process*.


Guards and locks
================

Nim provides common low level concurrency mechanisms like locks, atomic
intrinsics or condition variables.

Nim significantly improves on the safety of these features via additional
pragmas:

1) A `guard`:idx: annotation is introduced to prevent data races.
2) Every access of a guarded memory location needs to happen in an
   appropriate `locks`:idx: statement.


Guards and locks sections
-------------------------

### Protecting global variables

Object fields and global variables can be annotated via a `guard` pragma:

  ```nim
  import std/locks

  var glock: Lock
  var gdata {.guard: glock.}: int
  ```

The compiler then ensures that every access of `gdata` is within a `locks`
section:

  ```nim
  proc invalid =
    # invalid: unguarded access:
    echo gdata

  proc valid =
    # valid access:
    {.locks: [glock].}:
      echo gdata
  ```

Top level accesses to `gdata` are always allowed so that it can be initialized
conveniently. It is *assumed* (but not enforced) that every top level statement
is executed before any concurrent action happens.

The `locks` section deliberately looks ugly because it has no runtime
semantics and should not be used directly! It should only be used in templates
that also implement some form of locking at runtime:

  ```nim
  template lock(a: Lock; body: untyped) =
    pthread_mutex_lock(a)
    {.locks: [a].}:
      try:
        body
      finally:
        pthread_mutex_unlock(a)
  ```


The guard does not need to be of any particular type. It is flexible enough to
model low level lockfree mechanisms:

  ```nim
  var dummyLock {.compileTime.}: int
  var atomicCounter {.guard: dummyLock.}: int

  template atomicRead(x): untyped =
    {.locks: [dummyLock].}:
      memoryReadBarrier()
      x

  echo atomicRead(atomicCounter)
  ```


The `locks` pragma takes a list of lock expressions `locks: [a, b, ...]`
in order to support *multi lock* statements.


### Protecting general locations

The `guard` annotation can also be used to protect fields within an object.
The guard then needs to be another field within the same object or a
global variable.

Since objects can reside on the heap or on the stack, this greatly enhances
the expressiveness of the language:

  ```nim
  import std/locks

  type
    ProtectedCounter = object
      v {.guard: L.}: int
      L: Lock

  proc incCounters(counters: var openArray[ProtectedCounter]) =
    for i in 0..counters.high:
      lock counters[i].L:
        inc counters[i].v
  ```

The access to field `x.v` is allowed since its guard `x.L`  is active.
After template expansion, this amounts to:

  ```nim
  proc incCounters(counters: var openArray[ProtectedCounter]) =
    for i in 0..counters.high:
      pthread_mutex_lock(counters[i].L)
      {.locks: [counters[i].L].}:
        try:
          inc counters[i].v
        finally:
          pthread_mutex_unlock(counters[i].L)
  ```

There is an analysis that checks that `counters[i].L` is the lock that
corresponds to the protected location `counters[i].v`. This analysis is called
`path analysis`:idx: because it deals with paths to locations
like `obj.field[i].fieldB[j]`.

The path analysis is **currently unsound**, but that doesn't make it useless.
Two paths are considered equivalent if they are syntactically the same.

This means the following compiles (for now) even though it really should not:

  ```nim
  {.locks: [a[i].L].}:
    inc i
    access a[i].v
  ```
