# 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).