# Nix Language
The Nix language is designed for conveniently creating and composing [derivations](@docroot@/glossary.md#gloss-derivation) – precise descriptions of how contents of existing files are used to derive new files.
> **Tip**
>
> These pages are written as a reference.
> If you are learning Nix, nix.dev has a good [introduction to the Nix language](https://nix.dev/tutorials/nix-language).
The language is:
- *domain-specific*
The Nix language is purpose-built for working with text files.
Its most characteristic features are:
- [File system path primitives](@docroot@/language/types.md#type-path), for accessing source files
- [Indented strings](@docroot@/language/string-literals.md) and [string interpolation](@docroot@/language/string-interpolation.md), for creating file contents
- [Strings with contexts](@docroot@/language/string-context.md), for transparently linking files
It comes with [built-in functions](@docroot@/language/builtins.md) to integrate with the [Nix store](@docroot@/store/index.md), which manages files and enables [realising](@docroot@/glossary.md#gloss-realise) derivations declared in the Nix language.
- *declarative*
There is no notion of executing sequential steps.
Dependencies between operations are established only through data.
- *pure*
Values cannot change during computation.
Functions always produce the same output if their input does not change.
- *functional*
Functions are like any other value.
Functions can be assigned to names, taken as arguments, or returned by functions.
- *lazy*
Values are only computed when they are needed.
- *dynamically typed*
Type errors are only detected when expressions are evaluated.
# Overview
This is an incomplete overview of language features, by example.
<table>
<tr>
<th>
Example
</th>
<th>
Description
</th>
</tr>
<tr>
<td>
*Basic values ([primitives](@docroot@/language/types.md#primitives))*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"hello world"`
</td>
<td>
A [string](@docroot@/language/types.md#type-string)
</td>
</tr>
<tr>
<td>
```
''
multi
line
string
''
```
</td>
<td>
<!-- FIXME: using two no-break spaces, because apparently mdBook swallows the second regular space! -->
A multi-line string. Strips common prefixed whitespace. Evaluates to `"multi\n line\n string"`.
</td>
</tr>
<tr>
<td>
`# Explanation`
</td>
<td>
A [comment](@docroot@/language/syntax.md#comments).
</td>
</tr>
<tr>
<td>
`"hello ${ { a = "world"; }.a }"`
`"1 2 ${toString 3}"`
`"${pkgs.bash}/bin/sh"`
</td>
<td>
[String interpolation](@docroot@/language/string-interpolation.md) (expands to `"hello world"`, `"1 2 3"`, `"/nix/store/<hash>-bash-<version>/bin/sh"`)
</td>
</tr>
<tr>
<td>
`true`, `false`
</td>
<td>
[Booleans](@docroot@/language/types.md#type-boolean)
</td>
</tr>
<tr>
<td>
`null`
</td>
<td>
[Null](@docroot@/language/types.md#type-null) value
</td>
</tr>
<tr>
<td>
`123`
</td>
<td>
An [integer](@docroot@/language/types.md#type-int)
</td>
</tr>
<tr>
<td>
`3.141`
</td>
<td>
A [floating point number](@docroot@/language/types.md#type-float)
</td>
</tr>
<tr>
<td>
`/etc`
</td>
<td>
An absolute [path](@docroot@/language/types.md#type-path)
</td>
</tr>
<tr>
<td>
`./foo.png`
</td>
<td>
A [path](@docroot@/language/types.md#type-path) relative to the file containing this Nix expression
</td>
</tr>
<tr>
<td>
`~/.config`
</td>
<td>
A home [path](@docroot@/language/types.md#type-path). Evaluates to the `"<user's home directory>/.config"`.
</td>
</tr>
<tr>
<td>
`<nixpkgs>`
</td>
<td>
A [lookup path](@docroot@/language/constructs/lookup-path.md) for Nix files. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH).
</td>
</tr>
<tr>
<td>
*Compound values*
</td>
<td>
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }`
</td>
<td>
An [attribute set](@docroot@/language/types.md#attribute-set) with attributes named `x` and `y`
</td>
</tr>
<tr>
<td>
`{ foo.bar = 1; }`
</td>
<td>
A nested set, equivalent to `{ foo = { bar = 1; }; }`
</td>
</tr>
<tr>
<td>
`rec { x = "foo"; y = x + "bar"; }`
</td>
<td>
A [recursive set](@docroot@/language/syntax.md#recursive-sets), equivalent to `{ x = "foo"; y = "foobar"; }`.
</td>
</tr>
<tr>
<td>
`[ "foo" "bar" "baz" ]`
`[ 1 2 3 ]`
`[ (f 1) { a = 1; b = 2; } [ "c" ] ]`
</td>
<td>
[Lists](@docroot@/language/types.md#list) with three elements.
</td>
</tr>
<tr>
<td>
*Operators*
</td>
<td>
</td>
</tr>
<tr>
<td>
`"foo" + "bar"`
</td>
<td>
String concatenation
</td>
</tr>
<tr>
<td>
`1 + 2`
</td>
<td>
Integer addition
</td>
</tr>
<tr>
<td>
`"foo" == "f" + "oo"`
</td>
<td>
Equality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`"foo" != "bar"`
</td>
<td>
Inequality test (evaluates to `true`)
</td>
</tr>
<tr>
<td>
`!true`
</td>
<td>
Boolean negation
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.x`
</td>
<td>
[Attribute selection](@docroot@/language/types.md#attribute-set) (evaluates to `1`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; }.z or 3`
</td>
<td>
[Attribute selection](@docroot@/language/types.md#attribute-set) with default (evaluates to `3`)
</td>
</tr>
<tr>
<td>
`{ x = 1; y = 2; } // { z = 3; }`
</td>
<td>
Merge two sets (attributes in the right-hand set taking precedence)
</td>
</tr>
<tr>
<td>
*Control structures*
</td>
<td>
</td>
</tr>
<tr>
<td>
`if 1 + 1 == 2 then "yes!" else "no!"`
</td>
<td>
[Conditional expression](@docroot@/language/syntax.md#conditionals).
</td>
</tr>
<tr>
<td>
`assert 1 + 1 == 2; "yes!"`
</td>
<td>
[Assertion](@docroot@/language/syntax.md#assertions) check (evaluates to `"yes!"`).
</td>
</tr>
<tr>
<td>
`let x = "foo"; y = "bar"; in x + y`
</td>
<td>
Variable definition. See [`let`-expressions](@docroot@/language/syntax.md#let-expressions).
</td>
</tr>
<tr>
<td>
`with builtins; head [ 1 2 3 ]`
</td>
<td>
Add all attributes from the given set to the scope (evaluates to `1`).
See [`with`-expressions](@docroot@/language/syntax.md#with-expressions) for details and shadowing caveats.
</td>
</tr>
<tr>
<td>
`inherit pkgs src;`
</td>
<td>
Adds the variables to the current scope (attribute set or `let` binding).
Desugars to `pkgs = pkgs; src = src;`.
See [Inheriting attributes](@docroot@/language/syntax.md#inheriting-attributes).
</td>
</tr>
<tr>
<td>
`inherit (pkgs) lib stdenv;`
</td>
<td>
Adds the attributes, from the attribute set in parentheses, to the current scope (attribute set or `let` binding).
Desugars to `lib = pkgs.lib; stdenv = pkgs.stdenv;`.
See [Inheriting attributes](@docroot@/language/syntax.md#inheriting-attributes).
</td>
</tr>
<tr>
<td>
*[Functions](@docroot@/language/syntax.md#functions) (lambdas)*
</td>
<td>
</td>
</tr>
<tr>
<td>
`x: x + 1`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) that expects an integer and returns it increased by 1.
</td>
</tr>
<tr>
<td>
`x: y: x + y`
</td>
<td>
Curried [function](@docroot@/language/syntax.md#functions), equivalent to `x: (y: x + y)`. Can be used like a function that takes two arguments and returns their sum.
</td>
</tr>
<tr>
<td>
`(x: x + 1) 100`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) call (evaluates to 101)
</td>
</tr>
<tr>
<td>
`let inc = x: x + 1; in inc (inc (inc 100))`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) bound to a variable and subsequently called by name (evaluates to 103)
</td>
</tr>
<tr>
<td>
`{ x, y }: x + y`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y` and concatenates them
</td>
</tr>
<tr>
<td>
`{ x, y ? "bar" }: x + y`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y`
</td>
</tr>
<tr>
<td>
`{ x, y, ... }: x + y`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y` and ignores any other attributes
</td>
</tr>
<tr>
<td>
`{ x, y } @ args: x + y`
`args @ { x, y }: x + y`
</td>
<td>
A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y`, and binds the whole set to `args`
</td>
</tr>
<tr>
<td>
*Built-in functions*
</td>
<td>
</td>
</tr>
<tr>
<td>
`import ./foo.nix`
</td>
<td>
Load and return Nix expression in given file.
See [import](@docroot@/language/builtins.md#builtins-import).
</td>
</tr>
<tr>
<td>
`map (x: x + x) [ 1 2 3 ]`
</td>
<td>
Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`).
See [`map`](@docroot@/language/builtins.md#builtins-map).
</td>
</tr>
</table>