Nim Manual

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Authors:Andreas Rumpf, Zahary Karadjov

"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.

Note: Assignments, moves, and destruction are specified in the destructors 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 and the DocGen Tools Guide.

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.


Nim code specifies a computation that acts on a memory consisting of components called locations. A variable is basically a name for a location. Each variable and location is of a certain type. The variable's type is called static type, the location's type is called dynamic type. 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 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 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. An l-value can denote either a location or the value the location contains, depending on the context.

A Nim program consists of one or more text source files containing Nim code. It is processed by a Nim compiler into an executable. 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. 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 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 (AST). Then, before executing the code or compiling it into the executable, it transforms the AST through semantic analysis. 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. Errors described in this manual are static errors when not otherwise specified.

A panic 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. 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.

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

The current implementation allows switching between these different behaviors via --panics:on|off. 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 is that it produces smaller binary code and the compiler has more freedom to optimize the code.

An unchecked runtime error 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 language features are used and if no runtime checks are disabled.

A constant expression 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


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.


Nim's standard grammar describes an indentation sensitive 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 start anywhere outside a string or character literal with the hash character #. Comments consist of a concatenation of comment pieces. 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:

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

Documentation comments 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:

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

Multiline comments support nesting:

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

Multiline documentation comments also exist and support nesting too:

proc foo =
  ##[Long documentation comment

You can also use the discard statement together with triple quoted string literals to create multiline comments:

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 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:

addr and as asm
bind block break
case cast concept const continue converter
defer discard distinct div do
elif else end enum except export
finally for from func
if import in include interface is isnot iterator
macro method mixin mod
nil not notin
object of or out
proc ptr
raise ref return
shl shr static
template try tuple type
when while

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:

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 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 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.


var `var` = "Hello Stropping"

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:

Escape sequenceMeaning
\pplatform specific newline: CRLF on Windows, LF on Unix
\r, \ccarriage return
\n, \lline feed (often called newline)
\fform feed
\vvertical tabulator
\"quotation mark
\ '0'..'9'+character with decimal value d; all decimal digits directly following are used for the character
\eescape [ESC]
\x HHcharacter with hex value HH; exactly two hex digits are allowed
\u HHHHunicode codepoint with hex value HHHH; exactly four hex digits are allowed
\u {H+}unicode codepoint; 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:

""""long string within quotes""""


"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:

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:




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


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 (\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 for character literals:

Escape sequenceMeaning
\r, \ccarriage return
\n, \lline feed
\fform feed
\vvertical tabulator
\"quotation mark
\ '0'..'9'+character with decimal value d; all decimal digits directly following are used for the character
\eescape [ESC]
\x HHcharacter with hex value HH; 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.

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 )*

        | 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']
            | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT32_SUFFIX
FLOAT64_SUFFIX = ( ('f' | 'F') '64' ) | 'd' | 'D'
            | (FLOAT_LIT | DEC_LIT | OCT_LIT | BIN_LIT) ['\''] FLOAT64_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:

echo -1
echo [-1]
echo 3,-1


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

echo x-1
echo (int)-1
echo [a]-1

The suffix starting with an apostrophe (''') is called a type suffix. 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 SuffixResulting type of literal

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:

import 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:

import 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.


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.

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

*: is as a special case treated as the two tokens * and : (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 operator .. takes precedence over other tokens that contain a dot: {..} are the three tokens {, .., } and not the two tokens {., .}.


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.


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

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)


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 operator which binds stronger than a primarySuffix: is parsed as (@x).abc whereas $ is parsed as $(

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

Operators ending in either ->, ~> or => are called arrow like, 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 levelOperatorsFirst characterTerminal symbol
10 (highest)$ ^OP10
9* / div mod shl shr %* % \ /OP9
8+ -+ - ~ |OP8
5== <= < >= > != in notin is isnot not of as from= < > !OP5
3or xorOP3
2@ : ?OP2
1assignment 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):

echo $foo
# is parsed as

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

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

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).


The grammar's start symbol is module.

# This file is generated by compiler/parser.nim.
module = complexOrSimpleStmt ^* (';' / IND{=})
comma = ',' COMMENT?
semicolon = ';' COMMENT?
colon = ':' COMMENT?
colcom = ':' COMMENT?
operator =  OP0 | OP1 | OP2 | OP3 | OP4 | OP5 | OP6 | OP7 | OP8 | OP9
         | 'or' | 'xor' | 'and'
         | 'is' | 'isnot' | 'in' | 'notin' | 'of' | 'as' | 'from'
         | 'div' | 'mod' | 'shl' | 'shr' | 'not' | '..'
prefixOperator = operator
optInd = COMMENT? IND?
optPar = (IND{>} | IND{=})?
simpleExpr = arrowExpr (OP0 optInd arrowExpr)* pragma?
arrowExpr = assignExpr (OP1 optInd assignExpr)*
assignExpr = orExpr (OP2 optInd orExpr)*
orExpr = andExpr (OP3 optInd andExpr)*
andExpr = cmpExpr (OP4 optInd cmpExpr)*
cmpExpr = sliceExpr (OP5 optInd sliceExpr)*
sliceExpr = ampExpr (OP6 optInd ampExpr)*
ampExpr = plusExpr (OP7 optInd plusExpr)*
plusExpr = mulExpr (OP8 optInd mulExpr)*
mulExpr = dollarExpr (OP9 optInd dollarExpr)*
dollarExpr = primary (OP10 optInd primary)*
operatorB = OP0 | OP1 | OP2 | OP3 | OP4 | OP5 | OP6 | OP7 | OP8 | OP9 |
            'div' | 'mod' | 'shl' | 'shr' | 'in' | 'notin' |
            'is' | 'isnot' | 'not' | 'of' | 'as' | 'from' | '..' | 'and' | 'or' | 'xor'
symbol = '`' (KEYW|IDENT|literal|(operator|'('|')'|'['|']'|'{'|'}'|'=')+)+ '`'
       | IDENT | 'addr' | 'type' | 'static'
symbolOrKeyword = symbol | KEYW
exprColonEqExpr = expr ((':'|'=') expr
                       / doBlock extraPostExprBlock*)?
exprEqExpr = expr ('=' expr
                  / doBlock extraPostExprBlock*)?
exprList = expr ^+ comma
optionalExprList = expr ^* comma
exprColonEqExprList = exprColonEqExpr (comma exprColonEqExpr)* (comma)?
qualifiedIdent = symbol ('.' optInd symbolOrKeyword)?
setOrTableConstr = '{' ((exprColonEqExpr comma)* | ':' ) '}'
castExpr = 'cast' ('[' optInd typeDesc optPar ']' '(' optInd expr optPar ')') /
parKeyw = 'discard' | 'include' | 'if' | 'while' | 'case' | 'try'
        | 'finally' | 'except' | 'for' | 'block' | 'const' | 'let'
        | 'when' | 'var' | 'mixin'
par = '(' optInd
          ( &parKeyw (ifExpr / complexOrSimpleStmt) ^+ ';'
          | ';' (ifExpr / complexOrSimpleStmt) ^+ ';'
          | pragmaStmt
          | simpleExpr ( (doBlock extraPostExprBlock*)
                       | ('=' expr (';' (ifExpr / complexOrSimpleStmt) ^+ ';' )? )
                       | (':' expr (',' exprColonEqExpr     ^+ ',' )? ) ) )
          optPar ')'
literal = | INT_LIT | INT8_LIT | INT16_LIT | INT32_LIT | INT64_LIT
          | UINT_LIT | UINT8_LIT | UINT16_LIT | UINT32_LIT | UINT64_LIT
          | FLOAT_LIT | FLOAT32_LIT | FLOAT64_LIT
          | NIL
identOrLiteral = generalizedLit | symbol | literal
               | par | arrayConstr | setOrTableConstr | tupleConstr
               | castExpr
tupleConstr = '(' optInd (exprColonEqExpr comma?)* optPar ')'
arrayConstr = '[' optInd (exprColonEqExpr comma?)* optPar ']'
primarySuffix = '(' (exprColonEqExpr comma?)* ')'
      | '.' optInd symbolOrKeyword ('[:' exprList ']' ( '(' exprColonEqExpr ')' )?)? generalizedLit?
      | DOTLIKEOP optInd symbolOrKeyword generalizedLit?
      | '[' optInd exprColonEqExprList optPar ']'
      | '{' optInd exprColonEqExprList optPar '}'
pragma = '{.' optInd (exprColonEqExpr comma?)* optPar ('.}' | '}')
identVis = symbol OPR?  # postfix position
identVisDot = symbol '.' optInd symbolOrKeyword OPR?
identWithPragma = identVis pragma?
identWithPragmaDot = identVisDot pragma?
declColonEquals = identWithPragma (comma identWithPragma)* comma?
                  (':' optInd typeDescExpr)? ('=' optInd expr)?
identColonEquals = IDENT (comma IDENT)* comma?
     (':' optInd typeDescExpr)? ('=' optInd expr)?)
tupleTypeBracket = '[' optInd (identColonEquals (comma/semicolon)?)* optPar ']'
tupleType = 'tuple' tupleTypeBracket
tupleDecl = 'tuple' (tupleTypeBracket /
    COMMENT? (IND{>} identColonEquals (IND{=} identColonEquals)*)?)
paramList = '(' declColonEquals ^* (comma/semicolon) ')'
paramListArrow = paramList? ('->' optInd typeDesc)?
paramListColon = paramList? (':' optInd typeDesc)?
doBlock = 'do' paramListArrow pragma? colcom stmt
routineExpr = ('proc' | 'func' | 'iterator') paramListColon pragma? ('=' COMMENT? stmt)?
routineType = ('proc' | 'iterator') paramListColon pragma?
forStmt = 'for' ((varTuple / identWithPragma) ^+ comma) 'in' expr colcom stmt
forExpr = forStmt
expr = (blockExpr
      | ifExpr
      | whenExpr
      | caseStmt
      | forExpr
      | tryExpr)
      / simpleExpr
simplePrimary = SIGILLIKEOP? identOrLiteral primarySuffix*
commandStart = &('`'|IDENT|literal|'cast'|'addr'|'type'|'var'|'out'|
primary = simplePrimary (commandStart expr (doBlock extraPostExprBlock*)?)?
        / operatorB primary
        / routineExpr
        / rawTypeDesc
        / prefixOperator primary
rawTypeDesc = (tupleType | routineType | 'enum' | 'object' |
                ('var' | 'out' | 'ref' | 'ptr' | 'distinct') typeDesc?)
                ('not' primary)?
typeDescExpr = (routineType / simpleExpr) ('not' primary)?
typeDesc = rawTypeDesc / typeDescExpr
typeDefValue = ((tupleDecl | enumDecl | objectDecl | conceptDecl |
                 ('ref' | 'ptr' | 'distinct') (tupleDecl | objectDecl))
               / (simpleExpr (exprEqExpr ^+ comma postExprBlocks?)?))
               ('not' primary)?
extraPostExprBlock = ( IND{=} doBlock
                     | IND{=} 'of' exprList ':' stmt
                     | IND{=} 'elif' expr ':' stmt
                     | IND{=} 'except' optionalExprList ':' stmt
                     | IND{=} 'finally' ':' stmt
                     | IND{=} 'else' ':' stmt )
postExprBlocks = (doBlock / ':' (extraPostExprBlock / stmt)) extraPostExprBlock*
exprStmt = simpleExpr postExprBlocks?
         / simplePrimary (exprEqExpr ^+ comma) postExprBlocks?
         / simpleExpr '=' optInd (expr postExprBlocks?)
importStmt = 'import' optInd expr
              ((comma expr)*
              / 'except' optInd (expr ^+ comma))
exportStmt = 'export' optInd expr
              ((comma expr)*
              / 'except' optInd (expr ^+ comma))
includeStmt = 'include' optInd expr ^+ comma
fromStmt = 'from' expr 'import' optInd expr (comma expr)*
returnStmt = 'return' optInd expr?
raiseStmt = 'raise' optInd expr?
yieldStmt = 'yield' optInd expr?
discardStmt = 'discard' optInd expr?
breakStmt = 'break' optInd expr?
continueStmt = 'continue' optInd expr?
condStmt = expr colcom stmt COMMENT?
           (IND{=} 'elif' expr colcom stmt)*
           (IND{=} 'else' colcom stmt)?
ifStmt = 'if' condStmt
whenStmt = 'when' condStmt
condExpr = expr colcom stmt optInd
        ('elif' expr colcom stmt optInd)*
         'else' colcom stmt
ifExpr = 'if' condExpr
whenExpr = 'when' condExpr
whileStmt = 'while' expr colcom stmt
ofBranch = 'of' exprList colcom stmt
ofBranches = ofBranch (IND{=} ofBranch)*
                      (IND{=} 'elif' expr colcom stmt)*
                      (IND{=} 'else' colcom stmt)?
caseStmt = 'case' expr ':'? COMMENT?
            (IND{>} ofBranches DED
            | IND{=} ofBranches)
tryStmt = 'try' colcom stmt &(IND{=}? 'except'|'finally')
           (IND{=}? 'except' optionalExprList colcom stmt)*
           (IND{=}? 'finally' colcom stmt)?
tryExpr = 'try' colcom stmt &(optInd 'except'|'finally')
           (optInd 'except' optionalExprList colcom stmt)*
           (optInd 'finally' colcom stmt)?
blockStmt = 'block' symbol? colcom stmt
blockExpr = 'block' symbol? colcom stmt
staticStmt = 'static' colcom stmt
deferStmt = 'defer' colcom stmt
asmStmt = 'asm' pragma? (STR_LIT | RSTR_LIT | TRIPLESTR_LIT)
genericParam = symbol (comma symbol)* (colon expr)? ('=' optInd expr)?
genericParamList = '[' optInd
  genericParam ^* (comma/semicolon) optPar ']'
pattern = '{' stmt '}'
indAndComment = (IND{>} COMMENT)? | COMMENT?
routine = optInd identVis pattern? genericParamList?
  paramListColon pragma? ('=' COMMENT? stmt)? indAndComment
commentStmt = COMMENT
enumDecl = 'enum' optInd (symbol pragma? optInd ('=' optInd expr COMMENT?)? comma?)+
objectWhen = 'when' expr colcom objectPart COMMENT?
            ('elif' expr colcom objectPart COMMENT?)*
            ('else' colcom objectPart COMMENT?)?
objectBranch = 'of' exprList colcom objectPart
objectBranches = objectBranch (IND{=} objectBranch)*
                      (IND{=} 'elif' expr colcom objectPart)*
                      (IND{=} 'else' colcom objectPart)?
objectCase = 'case' declColonEquals ':'? COMMENT?
            (IND{>} objectBranches DED
            | IND{=} objectBranches)
objectPart = IND{>} objectPart^+IND{=} DED
           / objectWhen / objectCase / 'nil' / 'discard' / declColonEquals
objectDecl = 'object' ('of' typeDesc)? COMMENT? objectPart
conceptParam = ('var' | 'out')? symbol
conceptDecl = 'concept' conceptParam ^* ',' (pragma)? ('of' typeDesc ^* ',')?
              &IND{>} stmt
typeDef = identVisDot genericParamList? pragma '=' optInd typeDefValue
varTupleLhs = '(' optInd (identWithPragma / varTupleLhs) ^+ comma optPar ')'
varTuple = varTupleLhs '=' optInd expr
colonBody = colcom stmt postExprBlocks?
variable = (varTuple / identColonEquals) colonBody? indAndComment
constant = (varTuple / identWithPragma) (colon typeDesc)? '=' optInd expr indAndComment
bindStmt = 'bind' optInd qualifiedIdent ^+ comma
mixinStmt = 'mixin' optInd qualifiedIdent ^+ comma
pragmaStmt = pragma (':' COMMENT? stmt)?
simpleStmt = ((returnStmt | raiseStmt | yieldStmt | discardStmt | breakStmt
           | continueStmt | pragmaStmt | importStmt | exportStmt | fromStmt
           | includeStmt | commentStmt) / exprStmt) COMMENT?
complexOrSimpleStmt = (ifStmt | whenStmt | whileStmt
                    | tryStmt | forStmt
                    | blockStmt | staticStmt | deferStmt | asmStmt
                    | 'proc' routine
                    | 'method' routine
                    | 'func' routine
                    | 'iterator' routine
                    | 'macro' routine
                    | 'template' routine
                    | 'converter' routine
                    | 'type' section(typeDef)
                    | 'const' section(constant)
                    | ('let' | 'var' | 'using') section(variable)
                    | bindStmt | mixinStmt)
                    / simpleStmt
stmt = (IND{>} complexOrSimpleStmt^+(IND{=} / ';') DED)
     / simpleStmt ^+ ';'

Order of evaluation

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

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:

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:

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) =

# '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 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.)

import std/strformat

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

proc nextFib(): int =
  result = if fibN < 2:
    fibPrevPrev + fibPrev
  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()}")

  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.


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:

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.
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.
the generic unsigned integer 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.
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:

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

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

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

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:

  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:

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.
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, FloatDivByZeroDefect, FloatOverflowDefect, FloatUnderflowDefect, and FloatInexactDefect. These exceptions inherit from the FloatingPointDefect base class.

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

{.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 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 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:

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:

while p != nil and != "xyz":
  # is not evaluated if p == nil
  p =

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.

Enumeration types

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

  Direction = enum
    north, east, south, west

Now the following holds:

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:

  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:

  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:

  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 and 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.

  E1 = enum
  E2 = enum
    value2 = 4

  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.

# a.nim
type Foo* = enum abc

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

  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:

echo 3 # calls `$` for `int`

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

  Person = object
    name: string
    age: int

proc `$`(p: Person): string = # `$` always returns a string
  result = & " 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 $ 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:

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 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:

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:

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

cstring literals shouldn't be modified.

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:

var x = "123456"
var s: 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.


  IntArray = array[0..5, int] # an array that is indexed with 0..5
  IntSeq = seq[int] # a sequence of integers
  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 command-line switch.

An array constructor can have explicit indexes for readability:

  Values = enum
    valA, valB, valC

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

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

  Values = enum
    valA, valB, valC, valD, valE

  lookupTable = [
    valA: "A",
    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 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.

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

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


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:

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:

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:

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:

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:

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

Produces roughly this C code:

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.

  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 == "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:

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 []:

  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.

  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

  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.

  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.

Object construction

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

# This is an example of how an abstract syntax tree could be modelled in Nim
  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: 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:

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:

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())
  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:

  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")

  # inside the 'cast' section it is allowed to pass 't.kind' to a 'var T' parameter:
  # 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.

  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.

  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

The set type models the mathematical notion of a set. The set's basetype can only be an ordinal type of a certain size, namely:
  • int8-int16
  • uint8/byte-uint16
  • char
  • enum
  • Ordinal subrange types, i.e. range[-10..10]

or equivalent. When constructing a set with signed integer literals, the set's base type is defined to be in the range 0 .. DefaultSetElements-1 where DefaultSetElements is currently always 2^8. The maximum range length for the base type of a set is MaxSetElements which is currently always 2^16. Types with a bigger range length are coerced into the range 0 .. MaxSetElements-1.

The reason is that sets are implemented as high performance bit vectors. Attempting to declare a set with a larger type will result in an error:

  var s: set[int64] # Error: set is too large; use `std/sets` for ordinal types
                    # with more than 2^16 elements

Note: Nim also offers hash sets (which you need to import with import std/sets), which have no such restrictions.

Sets can be constructed via the set constructor: {} is the empty set. The empty set is type compatible with any concrete set type. The constructor can also be used to include elements (and ranges of elements):

  CharSet = set[char]
  x: CharSet
x = {'a'..'z', '0'..'9'} # This constructs a set that contains the
                         # letters from 'a' to 'z' and the digits
                         # from '0' to '9'

The module `std/setutils` provides a way to initialize a set from an iterable:

import std/setutils

let uniqueChars = myString.toSet

These operations are supported by sets:

A + Bunion of two sets
A * Bintersection of two sets
A - Bdifference of two sets (A without B's elements)
A == Bset equality
A <= Bsubset relation (A is subset of B or equal to B)
A < Bstrict subset relation (A is a proper subset of B)
e in Aset membership (A contains element e)
e notin AA does not contain element e
contains(A, e)A contains element e
card(A)the cardinality of A (number of elements in A)
incl(A, elem)same as A = A + {elem}
excl(A, elem)same as A = A - {elem}

Bit fields

Sets are often used to define a type for the flags of a procedure. This is a cleaner (and type safe) solution than defining integer constants that have to be or'ed together.

Enum, sets and casting can be used together as in:

  MyFlag* {.size: sizeof(cint).} = enum
  MyFlags = set[MyFlag]

proc toNum(f: MyFlags): int = cast[cint](f)
proc toFlags(v: int): MyFlags = cast[MyFlags](v)

assert toNum({}) == 0
assert toNum({A}) == 1
assert toNum({D}) == 8
assert toNum({A, C}) == 5
assert toFlags(0) == {}
assert toFlags(7) == {A, B, C}

Note how the set turns enum values into powers of 2.

If using enums and sets with C, use distinct cint.

For interoperability with C see also the bitsize pragma.

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).

Nim distinguishes between traced and untraced 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:

  Node = ref NodeObj
  NodeObj = object
    le, ri: Node
    data: int

  n: Node
new(n) = 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:

# 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:

  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 module contains further information.


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:

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.


p[].field = 3

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:

  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:

# free the memory:

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.

Procedural type

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


proc printItem(x: int) = ...

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

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

  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':

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:

is the default convention used for a Nim proc. It is the same as fastcall, but only for C compilers that support fastcall.
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.
This is the stdcall convention as specified by Microsoft. The generated C procedure is declared with the __stdcall keyword.
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.
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.
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.
Fastcall means different things to different C compilers. One gets whatever the C __fastcall means.
This is the thiscall calling convention as specified by Microsoft, used on C++ class member functions on the x86 architecture.
The syscall convention is the same as __syscall in C. It is used for interrupts.
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 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 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:

  Dollar = distinct int
  Euro = distinct int

  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:

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:

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 has been designed to solve this problem; in principle, it generates the above trivial implementations:

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.

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) =
    typ* = distinct base
  multiplicative(typ, base)

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:

  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:

import std/strutils

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

  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:

  SQL = distinct string

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

  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:

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 of modules like db_sqlite.

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:

proc returnsInt(): auto = 1984

For parameters it currently creates implicitly generic routines:

proc foo(a, b: auto) = discard

Is the same as:

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:

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:

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.

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

  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) the routine p that matches best is selected. If multiple routines match equally well, the ambiguity is reported during semantic analysis.

Every arg in args needs to match. There are multiple different categories how an argument can match. 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.

There are two major methods of selecting the best matching candidate, namely counting and disambiguation. Counting takes precedence to disambiguation. In counting, each parameter is given a category and the number of parameters in each category is counted. The categories are listed above and are in order of precedence. 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.

In the following, 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:

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"
    return false
return "ambiguous"

When counting is ambiguous, disambiguation begins. Parameters are iterated by position and these parameter pairs are compared for their type relation. The general goal of this comparison is to determine which parameter is more specific. The types considered are not of the inputs from the callsite, but of the competing candidates' parameters.

Some examples:

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 this algorithm returns "ambiguous" further disambiguation is performed: 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:

  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:

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:

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"

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. var T matches better than just T then.

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 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:

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:

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.


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).

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

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.

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.

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 and complex statements. 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, 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


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 pragma:

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:

{.push discardable .}
template example(): string = ""


This template will resolve into "" 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:

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:

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

proc valid*(): string =
  let x = 317

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:

  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.

Typedefault value
any integer type0
any float0.0
ref or pointer typenil
procedural typenil
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 = enumcast[T](0); this may be an invalid value

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

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

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

proc returnUndefinedValue: int {.noinit.} = discard

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

  MyObject {.requiresInit.} = object

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

requiresInit pragma can also be applied to distinct types.

Given the following distinct type definitions:

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

The following code blocks will fail to compile:

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

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

But these will compile successfully:

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

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

Let statement

A let statement declares new local and global single assignment 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.

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:

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

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

This is treated as syntax sugar for roughly the following:

  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.

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

Tuple unpacking can also be nested:

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:

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

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

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:

  echo "echo at compile time"

static can also be used like a routine.

proc getNum(a: int): int = a

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


var name = readLine(stdin)

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

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


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:

  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:

  Foo = ref object
    x: seq[string]

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

var foo = Foo(x: @[])

This can be fixed by explicitly using result or return:

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

When statement


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!"
  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.


proc someProcThatMayRunInCompileTime(): bool =
  when nimvm:
    # This branch is taken at compile time.
    result = true
    # 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


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:

result = expr
return result

return without an expression is a short notation for return result if the proc has a return type. The result 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:

proc returnZero(): int =
  # implicitly returns 0

Yield statement


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


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



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


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:

while expr1:

Is equivalent to:

while expr1:
  block myBlockName:
    break myBlockName

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 '`':

{.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`

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

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

Instead of:

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

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.:

  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:

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:

var favoriteFood = case animal
  of "dog": "bones"
  of "cat": "mice"
  elif animal.endsWith"whale": "plankton"
    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.

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

Table constructor

A table constructor is syntactic sugar for an array constructor:

{"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:

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

let procVar = (proc(x: string))(p)

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.

Exception: Values that are converted to an unsigned type at compile time are checked so that code like byte(-1) does not compile.

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.


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:

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 and bit_cast 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:

let t1 = "Hello"
  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:

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


What most programming languages call methods or functions are called procedures 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.

# 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.

# 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.

# "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 declaration. If the proc returns a value, the procedure body can access an implicitly declared variable named result that represents the return value. Procs can be overloaded. The overloading resolution algorithm determines which proc is the best match for the arguments. Example:

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

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 are procedures with a special operator symbol as identifier:

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):

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

assert `*+`(3, 4, 6) == `+`(`*`(a, b), c)

Export marker

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

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
  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:

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.


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:

# Module asocket
  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: = 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:

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

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

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

`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:

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.


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 and capture for details on how to change this behavior.

Anonymous procedures

Unnamed procedures can be used as lambda expressions to pass into other procedures:

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 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:

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 = 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.

# 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


The func keyword introduces a shortcut for a noSideEffect proc.

func binarySearch[T](a: openArray[T]; elem: T): int

Is short for:

proc binarySearch[T](a: openArray[T]; elem: T): int {.noSideEffect.}


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).

# foo.nim:
var witness* = 0
type Foo[T] = object
proc initFoo*(T: typedesc): Foo[T] = discard
proc `=destroy`[T](x: var Foo[T]) = # type bound operator

# main.nim:
import foo
  var a = initFoo(int)
  doAssert witness == 0
doAssert witness == 1
  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.

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 the definition in the system 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:

proc divmod(a, b: int; res, remainder: var int) =
  res = a div b
  remainder = a mod b

  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:

proc divmod(a, b: int; res, remainder: ptr int) =
  res[] = a div b
  remainder[] = a mod b

  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:

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 to access the tuple's fields:

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:

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:

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:

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.

Memory safety for returning by var T is ensured by a simple borrowing rule: If result does not refer to a location pointing to the heap (that is in result = X the X involves a ptr or ref access) then it has to be derived from the routine's first parameter:

proc forward[T](x: var T): var T =
  result = x # ok, derived from the first parameter.

proc p(param: var int): var int =
  var x: int
  # we know 'forward' provides a view into the location derived from
  # its first argument 'x'.
  result = forward(x) # Error: location is derived from `x`
                      # which is not p's first parameter and lives
                      # on the stack.

In other words, the lifetime of what result points to is attached to the lifetime of the first parameter and that is enough knowledge to verify memory safety at the call site.

Future directions

Later versions of Nim can be more precise about the borrowing rule with a syntax like:

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].


Note: This section describes the current implementation. This part of the language specification will be changed. See for more information.

The return value is represented inside the body of a routine as the special result variable. This allows for a mechanism much like C++'s "named return value optimization" (NRVO). 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.


proc p(): BigT = ...

var x = p()
x = p()

# is roughly turned into:

proc p(result: var BigT) = ...

var 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:

  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
    x = p(8)
  except ValueError:
    doAssert x == [0, 1, 2, 3, 4, 5, 6, 7, 0, 0, 0, 0, 0, 0, 0, 0]


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:

{.push warning[ObservableStores]: on.}

Overloading of the subscript operator

The [] subscript operator for arrays/openarrays/sequences can be overloaded.


Procedures always use static dispatch. Methods use dynamic dispatch. For dynamic dispatch to work on an object it should be a reference 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 =
  result.x = x

proc newPlus(a, b: Expression): PlusExpr =
  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 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.


Note: Starting from Nim 0.20, to use multi-methods one must explicitly pass --multimethods:on when compiling.

In a multi-method, all parameters that have an object type are used for the dispatching:

  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. This is somewhat comparable to the super keyword that traditional OOP languages offer.

  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 statement is an abstract mechanism to iterate over the elements of a container. It relies on an iterator to do so. Like while statements, for statements open an implicit block 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:

# this definition exists in the system module
iterator items*(a: string): char {.inline.} =
  var i = 0
  while i < len(a):
    yield a[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:

var i = 0
while i < len(a):
  var ch = a[i]
  echo ch

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:

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 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 can be passed around more freely:

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


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.
  5. Closure iterators are not supported by the JS backend.

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 system:

# simple tasking:
  Task = iterator (ticker: int)

iterator a1(ticker: int) {.closure.} =
  echo "a1: A"
  echo "a1: B"
  echo "a1: C"
  echo "a1: D"

iterator a2(ticker: int) {.closure.} =
  echo "a2: A"
  echo "a2: B"
  echo "a2: C"

proc runTasks(t: varargs[Task]) =
  var ticker = 0
  while true:
    let x = t[ticker mod t.len]
    if finished(x): break
    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:

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

Instead, this code has to be used:

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:

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:

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:
      let itr = `call`
      for `expr` in itr():

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:

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.


A converter is like an ordinary proc except that it enhances the "implicitly convertible" type relation (see Convertible relation):

# 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


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


# read the first two lines of a text file that should contain numbers
# and tries to add them
  f: File
if open(f, "numbers.txt"):
    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!"

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.

If there is a finally 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:

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:

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:

  # ...
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:

  # ...
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:

  # ...
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:

  # ...
except CatchableError:
  echo getCurrentExceptionMsg()

Custom exceptions

It is possible to create custom exceptions. A custom exception is a custom 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.:

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:

proc main =
  var f = open("numbers.txt", fmWrite)
  defer: close(f)
  f.write "abc"
  f.write "def"

Is rewritten to:

proc main =
  var f = open("numbers.txt")
    f.write "abc"
    f.write "def"

When defer is at the outermost scope of a template/macro, its scope extends to the block where the template/macro is called from:

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)

  safeOpenDefer(f, "/tmp/z01.txt")
  f.write "abc"
  safeOpenFinally(f, "/tmp/z01.txt"):
    f.write "abc" # adds a lexical scope
  var f = open("/tmp/z01.txt", fmWrite)
    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


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.

If no exception name is given, the current exception is re-raised. The ReraiseDefect 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 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).

|-- 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:

  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: cstring
  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"


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 pragma can be used to explicitly define which exceptions a proc/iterator/method/converter is allowed to raise. The compiler verifies this:

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:

proc p(): bool {.raises: [].} =
    result = true
  except CatchableError:
    result = false

A raises list can also be attached to a proc type. This affects type compatibility:

  Callback = proc (s: string) {.raises: [IOError].}
  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 it is ignored that is marked as .effectsOf: f. 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:

proc mydiv(a, b): int {.raises: [].} =
  a div b # can raise an DivByZeroDefect

And so is:

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 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:

proc weDontRaiseButMaybeTheCallback(callback: proc()) {.raises: [], effectsOf: callback.} =

proc doRaise() {.raises: [IOError].} =
  raise newException(IOError, "IO")

proc use() {.raises: [].} =
  # doesn't compile! Can raise IOError!

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:

{.push warningAsError[Effect]: on.}

import algorithm

  MyInt = distinct int

var toSort = @[MyInt 1, MyInt 2, MyInt 3]

proc cmpN(a, b: MyInt): int =

proc harmless {.raises: [].} =
  toSort.sort cmpN

proc cmpE(a, b: MyInt): int {.raises: [Exception].} =

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. 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:

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:

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:
  # 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:

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:
## this is OK because both toBeCalled2 and ProcType1 have the same requirements:
## these are OK because ProcType2 doesn't have any effect requirement:

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 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:

func `+` (x, y: int): int

To override the compiler's side effect analysis a {.noSideEffect.} cast pragma block can be used:

func f() =
    echo "test"

Side effects are usually inferred. The inference for side effects is analogous to the inference for exception tracking.

GC safety effect

We call a proc p GC safe 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 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:

  someGlobal: string = "some string here"
  perThread {.threadvar.}: string

proc setPerThread() =
    deepCopy(perThread, someGlobal)

See also:

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:

proc p(what: bool) =
  if what:
    raise newException(IOError, "IO")
    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 are Nim's means to parametrize procs, iterators or types with type parameters. 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:

  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
    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(,
      if c < 0:
        if it.le == nil:
          it.le = n
        it = it.le
        if it.ri == nil:
          it.ri = n
        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:
      add(stack, n.ri)  # push right subtree onto the stack
      n = n.le          # and follow the left pointer

  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):

The T is called a generic type parameter or a type variable.

Generic Procs

Let's consider the anatomy of a generic proc to agree on defined terminology.

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:

type Foo[T] = object
proc p[H;T: Foo[H]](param: T): H

A constraint definition may have more than one symbol defined by seperating 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:

  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 classmatches
objectany object type
tupleany tuple type
enumany enumeration
procany proc type
iteratorany iterator type
refany ref type
ptrany ptr type
varany var type
distinctany distinct type
arrayany array type
setany set type
seqany seq type
autoany 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:

# 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:

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:

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 of the generic type parameter:

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.

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.

# 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. 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 types. Here is an example taken directly from the system module to illustrate this:

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 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:

type Matrix[T, Rows, Columns] = object

proc `[]`(m: Matrix, row, col: int): Matrix.T =[col * high(Matrix.Columns) + row]

Here are more examples that illustrate implicit generics:

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

proc p(a: Table, b: Table)

# is roughly the same as:

proc p[Key, Value](a, b: Table[Key, Value])

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:

proc p(a: typedesc)

# is roughly the same as:

proc p[T](a: typedesc[T])

typedesc is a "bind many" type class:

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:

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:

proc g[T](f: proc(x: T); x: T) =

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:

  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 (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 declaration:

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):

# Module A
  lastId = 0

template genId*: untyped =
  bind lastId

# 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:

# module A
proc genericA*[T](x: T) =
  mixin init

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

# module C
type O = object
proc init*(x: var O) = discard

# 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 these symbols by a bind statement inside genericB.


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.


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:

template declareInt(x: untyped) =
  var x: int

declareInt(x) # valid
x = 3

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 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:

template withFile(f, fn, mode, actions: untyped): untyped =
  var f: File
  if open(f, fn, mode):
    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:

template t(body: typed) =
  proc p = echo "hey"

  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:

template t(body: untyped) =
  proc p = echo "hey"

  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:

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 macro and so opens a new scope. Most symbols are bound from the definition scope of the template:

# Module A
  lastId = 0

template genId*: untyped =

# 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:

template typedef(name: untyped, typ: typedesc) =
    `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:

# module 'm'

  Lev = enum
    levA, levB

var abclev = levB

template tstLev(abclev: Lev) =
  echo abclev, " ", m.abclev

# produces: 'levA levA'

But the global symbol can properly be captured by a bind statement:

# module 'm'

  Lev = enum
    levA, levB

var abclev = levB

template tstLev(abclev: Lev) =
  bind m.abclev
  echo abclev, " ", m.abclev

# produces: 'levA levB'

Hygiene in templates

Per default, templates are hygienic: Local identifiers declared in a template cannot be accessed in the instantiation context:

template newException*(exceptn: typedesc, message: string): untyped =
    e: ref exceptn  # e is implicitly gensym'ed here
  e.msg = message

# 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 and gensym 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 and for proc, iterator, converter, template, macro is inject. However, if the name of the entity is passed as a template parameter, it is an inject'ed symbol:

template withFile(f, fn, mode: untyped, actions: untyped): untyped =
    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:

{.pragma myInject: inject.}

template t() =
  var x {.myInject.}: int # does NOT work

To get rid of hygiene in templates, one can use the dirty 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

  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:

template tmp(x) =
    T {.gensym.} = int
  echo x.T # invalid: instead use:  'echo T(x)'.


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:

template declareVar(name: untyped) =
  const name {.inject.} = 45

# Doesn't compile:

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.

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.


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.

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:

# 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)

  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:

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.


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) instead of using unbound identifiers. The bindSym builtin can be used for that:

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))

  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:

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) 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.

macro performWithUndo(task, undo: untyped) = ...

performWithUndo do:
  # multiple-line block of code
  # to perform the task
  # 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"
  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:

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:

for item in items([1, 2, 3]):
  echo item

Another example:

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.==):

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
        let cond = newCall("==", selector, it[j])
        result.add newTree(nnkElifBranch, cond, it[^1])
      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


As their name suggests, static parameters must be constant expressions:

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:

  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:

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.


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:

proc new(T: typedesc): ref T =
  echo "allocating ",

var n =
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:

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:

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.

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.

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 operator):

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:

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]


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. Modules enable information hiding and separate compilation. A module may gain access to the symbols of another module by the import statement. Recursive module dependencies 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:

# Module A
  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


# 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:

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 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:

include fileA, fileB, fileC

The include statement can be used outside the top level, as such:

# Module A
echo "Hello World!"

# 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:

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:

import lib/pure/os, "lib/pure/times"

Note that the module name is still strutils and not lib/pure/strutils, thus one cannot do:

import lib/pure/strutils
echo lib/pure/strutils.toUpperAscii("abc")

Likewise, the following does not make sense as the name is strutils already:

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:

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:

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:

# module B
type MyObject* = object

# module A
import B
export B.MyObject

proc `$`*(x: MyObject): string = "my object"

# 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:

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 module is automatically imported in every module.

If a module imports an identifier by two different modules, each occurrence of the identifier has to be qualified unless it is an overloaded procedure or iterator in which case the overloading resolution takes place:

# Module A
var x*: string

# Module B
var x*: int

# Module C
import A, B
write(stdout, x) # error: x is ambiguous
write(stdout, A.x) # no error: qualifier used

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.

# Module A/C
proc fb* = echo "fizz"

# Module B/C
proc fb* = echo "buzz"

import A/C
import B/C

C.fb() # Error: ambiguous identifier: 'fb'

import A/C as fizz
import B/C

fizz.fb() # Works


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, warning, and error messages. An error message is emitted if the compiler encounters any static error.


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:

proc p() {.deprecated.}
var x {.deprecated.}: char

This pragma can also take in an optional warning string to relay to developers.

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:

proc astHelper(n: NimNode): NimNode =
  result = n

Is the same as:

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:

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:

  Node = ref NodeObj
  NodeObj {.acyclic.} = object
    left, right: Node
    data: string

Or if we directly use a ref object:

  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:

  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) or __attribute__((naked)) (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:

## 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:

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:

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 statement. Syntactically it has to be used as a statement:

case myInt
of 0:
  echo "most common case"
of 1:
  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 in a while true statement. Syntactically it has to be used as a statement inside the loop:

  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:
    let instr = instructions[pc]
    case instr
    of enumA:
      echo "yeah A"
    of enumC, enumD:
      echo "yeah CD"
    of enumB:
      echo "yeah B"
    of enumE:


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:

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).

pragmaallowed valuesdescription
checkson|off Turns the code generation for all runtime checks on or off.
boundCheckson|off Turns the code generation for array bound checks on or off.
overflowCheckson|off Turns the code generation for over- or underflow checks on or off.
nilCheckson|off Turns the code generation for nil pointer checks on or off.
assertionson|off Turns the code generation for assertions on or off.
warningson|off Turns the warning messages of the compiler on or off.
hintson|off Turns the hint messages of the compiler on or off.
optimizationnone|speed|size Optimize the code for speed or size, or disable optimization.
patternson|off Turns the term rewriting templates/macros on or off.
callconvcdecl|...Specifies the default calling convention for all procedures (and procedure types) that follow.


{.checks: off, optimization: speed.}
# compile without runtime checks and optimize for speed

push and pop pragmas

The push/pop pragmas are very similar to the option directive, but are used to override the settings temporarily. Example:

{.push checks: off.}
# compile this section without runtime checks as it is
# speed critical
# ... some code ...
{.pop.} # restore old settings

push/pop can switch on/off some standard library pragmas, example:

{.push inline.}
proc thisIsInlined(): int = 42
func willBeInlined(): float = 42.0
proc notInlined(): int = 9

{.push discardable, boundChecks: off, compileTime, noSideEffect, experimental.}
template example(): string = ""

{.push deprecated, used, stackTrace: off.}
proc sample(): bool = true

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.

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:

{.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:

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'
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:

# 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:

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 for more details.


import std/threadpool
{.experimental: "parallel".}

proc threadedEcho(s: string, i: int) =
  echo(s, " ", $i)

proc useParallel() =
    for i in 0..4:
      spawn threadedEcho("echo in parallel", i)


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:

# client.nim
proc useParallel*[T](unused: T) =
  # use a generic T here to show the problem.
  {.push experimental: "parallel".}
    for i in 0..4:
      echo "echo in parallel"

import client

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++.

  mybitfield = object
    flag {.bitsize:1.}: cuint


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.

  EventType* {.size: sizeof(uint32).} = enum

doAssert sizeof(EventType) == sizeof(uint32)

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.

    AtomicFlag* {.importc: "atomic_flag", header: "<stdatomic.h>", size: 1.} = object
    # if AtomicFlag didn't have the size pragma, this code would result in a compile time error.
    echo sizeof(AtomicFlag)

The size pragma accepts only the values 1, 2, 4 or 8.

Align pragma

The align 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.

  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


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 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, 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:

  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:

  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: <>. If no angle brackets are given, Nim encloses the header file in "" 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 in a sizeof expression:

  DIR* {.importc: "DIR", header: "<dirent.h>",
         pure, incompleteStruct.} = object

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:

{.compile: "myfile.cpp".}

The second form is a tuple where the second arg is the output name strutils formatter:

{.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 command-line option to force the recompilation of the file.

Since 1.4 the compile pragma is also available with this syntax:

{.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:

{.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:

{.passc: "-Wall -Werror".}

Note that one can use gorge from the system module to embed parameters from an external command that will be executed during semantic analysis:

{.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:

# 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:

{.passl: "-lSDLmain -lSDL".}

Note that one can use gorge from the system module to embed parameters from an external command that will be executed during semantic analysis:

{.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++ or Objective C code.


{.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, ");"].}


nimbase.h defines NIM_EXTERNC C macro that can be used for extern "C" code to work with both nim c and nim cpp, e.g.:

proc foobar() {.importc:"$1".}
{.emit: """
#include <stdio.h>
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*/ or /*VARSECTION*/ or /*INCLUDESECTION*/:

{.emit: """/*TYPESECTION*/
struct Vector3 {
  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 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++ methods or C++ symbols in general. The generated code then uses the C++ method calling syntax: obj->method(arg). In combination with the header and emit pragmas this allows sloppy interfacing with libraries written in C++:

# Horrible example of how to interface with a C++ engine ... ;-)

{.link: "/usr/lib/".}

{.emit: """
using namespace irr;
using namespace core;
using namespace scene;
using namespace video;
using namespace io;
using namespace gui;

  irr = "<irrlicht/irrlicht.h>"

  IrrlichtDeviceObj {.header: irr,
                      importcpp: "IrrlichtDevice".} = object
  IrrlichtDevice = ptr IrrlichtDeviceObj

proc createDevice(): IrrlichtDevice {.
  header: irr, importcpp: "createDevice(@)".}
proc run(device: IrrlichtDevice): bool {.
  header: irr, importcpp: "".}

The compiler needs to be told to generate C++ (command cpp) for this to work. The conditional symbol cpp is defined when the compiler emits C++ code.


The sloppy interfacing example uses .emit to produce using namespace declarations. It is usually much better to instead refer to the imported name via the namespace::identifier notation:

  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)). (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:

proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "#.CppMethod(@)".}
var x: ptr CppObj
cppMethod(x[], 1, 2, 3)


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:

proc cppMethod(this: CppObj, a, b, c: cint) {.importcpp: "CppMethod".}

Note that the pattern language naturally also covers C++'s operator overloading capabilities:

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* becomes T.) Two stars can be used to get to the element type of the element type etc.

For example:

type Input {.importcpp: "System::Input".} = object
proc getSubsystem*[T](): ptr T {.importcpp: "SystemManager::getSubsystem<'*0>()", nodecl.}

let x: ptr Input = getSubsystem[Input]()


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 operator can be "imported" like this:

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)


x = new Foo(3, 4)

However, depending on the use case new Foo can also be wrapped like this instead:

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); must not be generated but instead Class c(1,2);. For this purpose the Nim proc that wraps a C++ constructor needs to be annotated with the constructor pragma. This pragma also helps to generate faster C++ code since construction then doesn't invoke the copy constructor:

# 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:

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:

  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


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.

      VectorIterator[T] {.importcpp: "std::vector<'0>::iterator".} = object
    var x: VectorIterator[cint]


    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 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:

# 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;

#include <stdio.h>
@implementation Greeter

- (void)greet:(long)x y:(long)dummy
  printf("Hello, World!\n");

#include <stdlib.h>

  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)

The compiler needs to be told to generate Objective C (command objc) 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:

  a {.codegenDecl: "$# progmem $#".}: int

will generate this C code:

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:

proc myinterrupt() {.codegenDecl: "__interrupt $# $#$#".} =
  echo "realistic interrupt handler"

will generate this code:

__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.

const strTemplate = """
  struct $1 {
type Foo {.codegenDecl:strTemplate.} = object
  a, b: int

will generate this code:

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.

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 at compile time.

The implementation currently provides the following possible options (various others may be added later).

intdefineReads in a build-time define as an integer
strdefineReads in a build-time define as a string
booldefineReads in a build-time define as a bool

const FooBar {.intdefine.}: int = 5
echo FooBar

nim c -d:FooBar=42 foobar.nim

In the above example, providing the -d flag causes the symbol FooBar to be overwritten at compile-time, printing out 42. If the -d:FooBar=42 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 is actually just a shortcut for -d:flag=true.

These pragmas also accept an optional string argument for qualified define names.

const FooBar {.intdefine: "package.FooBar".}: int = 5
echo FooBar

nim c -d:package.FooBar=42 foobar.nim

This helps disambiguate define names in different packages.

See also the generic `define` 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.


when appType == "lib":
  {.pragma: rtl, exportc, dynlib, cdecl.}
  {.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:

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:

const tblspace {.strdefine.} = "dev" # switch for dev, test and prod environments

  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 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:

    type MyObj = object
      a {.dontSerialize.}: int
      b {.defaultDeserialize: 5.}: int
      c {.serializationKey: "_c".}: string

  • Adopting type for gui inspector in a game engine:

    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:

template command(name: string, def: untyped) = discard

proc p() {.command("print").} = discard

This is translated to:

  proc p() = discard

  AsyncEventHandler = proc (x: Event) {.async.}

This is translated to:

  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 instead.

Foreign function interface

Nim's FFI (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:

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:

{.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:

The string literal passed to importc can be a format string:

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:

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:

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.

Extern pragma

Like exportc or importc, the extern pragma affects name mangling. The string literal passed to extern can be a format string:

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:

  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 using the Cpp backend, params marked as byref will translate to cpp references &.

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:

proc printf(formatstr: cstring) {.nodecl, 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 instead of a struct 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:

proc gtk_image_new(): PGtkWidget
  {.cdecl, dynlib: "", 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:

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):

The dynlib pragma supports not only constant strings as an argument but also string expressions in general:

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 command-line option. The Compiler User Guide 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:

proc exportme(): int {.cdecl, exportc, dynlib.}

This is only useful if the program is compiled as a dynamic library via the --app:lib command-line option.


The --threads:on command-line switch is enabled by default. The typedthreads module module then contains several threading primitives. See 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: 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 variable; Additionally, this implies all the effects of the global pragma.

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 annotation is introduced to prevent data races.
  2. Every access of a guarded memory location needs to happen in an appropriate locks statement.

Guards and locks sections

Protecting global variables

Object fields and global variables can be annotated via a guard pragma:

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:

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:

template lock(a: Lock; body: untyped) =
  {.locks: [a].}:

The guard does not need to be of any particular type. It is flexible enough to model low level lockfree mechanisms:

var dummyLock {.compileTime.}: int
var atomicCounter {.guard: dummyLock.}: int

template atomicRead(x): untyped =
  {.locks: [dummyLock].}:

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:

import std/locks

  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:

proc incCounters(counters: var openArray[ProtectedCounter]) =
  for i in 0..counters.high:
    {.locks: [counters[i].L].}:
        inc counters[i].v

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 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:

{.locks: [a[i].L].}:
  inc i
  access a[i].v