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nixpkgs/nixos/doc/manual/configuration/summary.section.md

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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 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 ])