Nix Expression Language

From NixOS Wiki
Jump to: navigation, search

This discussion article covers the syntax, semantics, typing, compilation, tooling and libraries of the Nix Expression Language.

The Nix expression language is a pure, lazy, functional language. Purity means that operations in the language don't have side-effects (for instance, there is no variable assignment). Laziness means that arguments to functions are evaluated only when they are needed. Functional means that functions are “normal” values that can be passed around and manipulated in interesting ways. The language is not a full-featured, general purpose language. Its main job is to describe packages, compositions of packages, and the variability within packages.

— From the Nix manual

The language was designed especially for the Nix Package Manager.

Learning resources

The manual provides a reference of the Nix language. All language constructs you may use in nix are defined here, together with code snippets.

Nix By Example is a step-by-step tutorial. The nix pills also provide a lot of insight into the language and functional package management in general. The Nix Tour is an interactive tour that uses the actual package manager to learn you the language by example, in the browser.

Language Paradigms


Not all expressions in nixpkgs will be evaluated and instantiated as nix performs evaluation only when needed for a finished output. In the following example abort will never be triggered as the variable it belogs to is unused:

  a = abort "will never happen";
  b = "hello";
  c = "world"
in b + c


Functional Programming is a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions or declarations instead of statements.

see also: [1]


A pure function is a function where the return value is only determined by its input values, without observable side effects. In Nix, all build operations try to be as pure as possible to achieve reproducible builds. This means that wherever you build the packages as few side effects as possible should have an impact onto the build.

Language Features

This section describes the main language features of the nix expression language


When Nix tutorials talk about Nix Expressions they typically mean the definition of a function with multiple inputs which as a result in a derivation. However a Nix expression can be everything, from a simple string, to function to a set of expressions.


The nix language provides a number of basic types:

Type Description Example
Strings Strings either start with double quotes or double single quotes. They also support antiquotation (templating). Leading spaces are stripped with double single quotes. "Say ${}",
''first line
  second line
Integers There is no float in the nix language 5
Path relative paths will become to absolute when evaluated,paths must contain a slash. <nixpkgs/pkgs> is also possible and will resolve to the folder incl. subfolders in your NIX_PATH
> /abs/path/to/hello/world
> /path/to/your/nixpkgs/lib
Boolean true,false
Null null
Lists items are not separated by comma, can contain any other type
[ 1 ./example.bin { hello="world"; }]
Sets In other languages called dicts(py),objects(js) hashes(ruby), essentially a list of key-value pairs
{ hello="world"; }.hello
> "world"
Functions see below pattern: body

A detailed description of all types can be found in The Nix manual


Functions have the following form: pattern: body

There are 3 patterns in how functions can be written:

  1. f = a: a*a; a single identifier, can be extended with multiple identifiers as f = a: b: a*b; as a partial function would be returned.
  2. { x, y ? "foo", z ? "bar", ... }: z + y + x a set pattern, which can also set defaults. ellipse means that the function may receive extra arguments.
  3. args@{ x, y, z, ... }: z + y + x + args.a an @ pattern which can be used to match extra arguments and store them in the args set.


Lower precedence means a stronger binding; ie. this list is sorted from strongest to weakest binding, and in the case of equal precedence between two operators, the associativity decides the binding.

Prec Abbreviation Example Assoc Description
1 SELECT e . attrpath [or def] none Select attribute denoted by the attribute path attrpath from set e. (An attribute path is a dot-separated list of attribute names.) If the attribute doesn’t exist, return default if provided, otherwise abort evaluation.
2 APP e1 e2 left Call function e1 with argument e2.
3 NEG -e none Numeric negation.
4 HAS_ATTR e ? attrpath none Test whether set e contains the attribute denoted by attrpath; return true or false.
5 CONCAT e1 ++ e2 right List concatenation.
6 MUL e1 * e2 left Numeric multiplication.
6 DIV e1 / e2 left Numeric division.
7 ADD e1 + e2 left Numeric addition, or string concatenation.
7 SUB e1 - e2 left Numeric subtraction.
8 NOT !e left Boolean negation.
9 UPDATE e1 // e2 right Return a set consisting of the attributes in e1 and e2 (with the latter taking precedence over the former in case of equally named attributes).
10 LT e1 < e2 left Less than.
10 LTE e1 <= e2 left Less than or equal.
10 GT e1 > e2 left Greater than.
10 GTE e1 >= e2 left Greater than or equal.
11 EQ e1 == e2 none Equality.
11 NEQ e1 != e2 none Inequality.
12 AND e1 && e2 left Logical AND.
13 OR e1 || e2 left Logical OR.
14 IMPL e1 -> e2 none Logical implication (equivalent to !e1 || e2).

Source: Gist of joepie91


import loads, parses and imports the nix expression stored in path. This keyword is essentially a builtin of nix but not a part of the language itself.


  x = import <nixpkgs> {};
  y = trace x;

Notable constructs

Nix looks a lot like JSON with functions but also provides a number of very specialized constructs which can help you build clean and easy to read expressions. In this sub-chapter the most notable constructs will be shown by example:

with statement

The with statement introduces the lexical scope of a set into the expression which follows. Common usages are: On top of expressions:

You will see the with statement a lot at the beginning of expression definition. Most of the time it is used to load the lib functions into the namespace for quick access.

{lib, ... }:

with lib;
  options = {
    networking.hosts = mkOption {
      type = with types; attrsOf ( listOf str);
      default = {};

instead of:

{lib, ... }:
  options = {
    networking.hosts = lib.mkOption {
      type = lib.types.attrsOf ( lib.types.listOf lib.types.str);
      default = {};

In package input definitions:

  buildInputs = with pkgs; [ curl php coreutils procps ffmpeg ];

Instead of :

  buildInputs = [ pkgs.curl pkgs.php pkgs.coreutils pkgs.procps pkgs.ffmpeg ];

In the package meta tag:

{stdenv, ...}:
  meta = with stdenv.lib; {
    license = with licenses; [ lgp3 gpl3 ];
    maintainers = with maintainers; [ adisbladis lassulus ];

Instead of :

{stdenv, ...}:
  meta = {
    license = [ stdenv.lib.licenses.lgp3 stdenv.lib.licenses.gpl3 ];
    maintainers = [ stdenv.lib.maintainers.adisbladis stdenv.lib.maintainers.lassulus ];

In a default.nix of an external package:

with import <nixpkgs> {};
stdenv.mkDerivation rec {
    name = "mytool-env";
    src = ./.;
    buildInputs = with pkgs;[

    shellHook =''
      export HISTFILE=$PWD/histfile
    '' ;

let ... in statement

With let you can define local variables which can also reference to self without the need of the rec construct. This feature is use inside expressions to prepare variables which become part of an output. The usage of let is comparable to the Haskell let expression

  a = 1;
  b = 2;
in  a + b
=> 3

inherit statement

The inherit expression can be used to copy variables from the surrounding lexical scope. A typical use case is to declare the version or name of a derivation in the expression and reuse this parameter in the function to fetch the source.

This is a typical python package derivation as the fetchPypi function also requires pname and version as input:

buildPythonPackage rec {
  pname = "hello";
  version = "1.0";
  src = fetchPypi {
    inherit pname version;
   sha256 = "01ba..0";

rec statement

The rec expression turns a basic set into a set where self-referencing is possible. This can be used when the let expression would create too much clutter. It is often seen in package derivation descriptions.

Sample usage:

rec {
  x = y - 100;
  y = 123;
=> 23

Development tools

Syntax highlighting & editor modes

Nix language has decent syntax highlighting (SH) support among popular code editors, but refactoring/autocomplete is still rare.

Reference: Editor Modes for Nix Files