Nix builtins

Nix exposes a minimal set of unprefixed builtins to avoid a crowded namespace. These functions, and all other builtins, are also available under the builtins namespace.

Since the set builtins contains all the built-in functions and other values, it can be used with Nix operators^ᛦ like ? to provide graceful fallback settings for older versions of Nix.

Unprefixed

• abort s - abort evaluation with message s
• baseNameOf s - basename^ᛦ of string s
• derivation attrs - Nix derivations^ᛦ
• dirOf s - dirname^ᛦ of string s
• fetchTarball url - fetchTarball^ᛦ
• import path - Nix import keyword^ᛦ
• map f list - apply f to each element of list
• removeAttrs set list - Remove the attributes with keys in list from set
• throw s - Throw an error message s. Usually works like abort, but won’t break nix-env
• toString e - Convert the expression e to, where e is one of:
  • a string - no change
  • a path - quotes the path
  • a set containing a __toString = self: ... attribute
  • an integer
  • a list - joined with spaces
  • a boolean - "" for false, "1" for true
  • null - ""

Prefixed

• Nix numeric builtins^ᛦ
• Nix boolean builtins^ᛦ
• Nix string builtins^ᛦ
• Nix path builtins^ᛦ
• Nix list builtins^ᛦ
• Nix set builtins^ᛦ

Functions

• functionArgs f - Return a set whose keys are the formal parameters of f and whose values are booleans indicating whether a default is set for each one
• isFunction e - true if e is a function, otherwise false

Fetching

• fetchurl url - Download the URL to the Nix store^ᛦ and return the path
• fetchGit - fetchGit^ᛦ

Packaging

• compareVersions s1 s2 - Compare two version strings. -1 if s1 < s2, 0 if s1 == s2, 1 if s1 > s2
• filterSource e1 e2 - filterSource^ᛦ
• hashFile type p - Return a base-16 representation of the cryptographic hash of the file at path p under algorithm type, one of md5 or sha1/256/512
• parseDrvName s - split s into { name, version }
• splitVersion s - split a version string into its components

Values

• currentSystem - platform identifier for the system on which the expression is being evaluated
• getEnv s - gets the environment variable s
• typeOf e - return a string representing the type of e as in Nix values^ᛦ

Evaluation

• deepSeq e1 e2 - like seq, but e1’s elements are evaluated recursively if it’s a list or set
• seq e1 e2 - Evaluate e1, then evaluate and return e2; this ensures strict evaluation in the value of e1
• trace e1 e2 - evaluate e1, print its abstract syntax to stderr, then return e2
• tryEval e - return { success ? false, value ? false }, with sucess set true and value set to e if e evaluates successfully