Syntax Summary

Below is a summary of the most important syntactic constructs in the Nix expression language. It’s not complete. In particular, there are many other built-in functions. See the Nix manual for the rest.

Example Description

Basic values

"Hello world"

A string

"${pkgs.bash}/bin/sh"

A string containing an expression (expands to "/nix/store/*hash*-bash-*version*/bin/sh")

true, false

Booleans

123

An integer

./foo.png

A path (relative to the containing Nix expression)

Compound values

{ x = 1; y = 2; }

A set with attributes named x and y

{ foo.bar = 1; }

A nested set, equivalent to { foo = { bar = 1; }; }

rec { x = "foo"; y = x + "bar"; }

A recursive set, equivalent to { x = "foo"; y = "foobar"; }

[ "foo" "bar" ]

A list with two elements

Operators

"foo" + "bar"

String concatenation

1 + 2

Integer addition

"foo" == "f" + "oo"

Equality test (evaluates to true)

"foo" != "bar"

Inequality test (evaluates to true)

!true

Boolean negation

{ x = 1; y = 2; }.x

Attribute selection (evaluates to 1)

{ x = 1; y = 2; }.z or 3

Attribute selection with default (evaluates to 3)

{ x = 1; y = 2; } // { z = 3; }

Merge two sets (attributes in the right-hand set taking precedence)

Control structures

if 1 + 1 == 2 then "yes!" else "no!"

Conditional expression

assert 1 + 1 == 2; "yes!"

Assertion check (evaluates to "yes!"). See Warnings and Assertions for using assertions in modules

let x = "foo"; y = "bar"; in x + y

Variable definition

with pkgs.lib; head [ 1 2 3 ]

Add all attributes from the given set to the scope (evaluates to 1)

Functions (lambdas)

x: x + 1

A function that expects an integer and returns it increased by 1

(x: x + 1) 100

A function call (evaluates to 101)

let inc = x: x + 1; in inc (inc (inc 100))

A function bound to a variable and subsequently called by name (evaluates to 103)

{ x, y }: x + y

A function that expects a set with required attributes x and y and concatenates them

{ x, y ? "bar" }: x + y

A function that expects a set with required attribute x and optional y, using "bar" as default value for y

{ x, y, ... }: x + y

A function that expects a set with required attributes x and y and ignores any other attributes

{ x, y } @ args: x + y

A function that expects a set with required attributes x and y, and binds the whole set to args

Built-in functions

import ./foo.nix

Load and return Nix expression in given file

map (x: x + x) [ 1 2 3 ]

Apply a function to every element of a list (evaluates to [ 2 4 6 ])