Specification of the Nix Language

WARNING: This document is a work in progress. Please keep an eye on topic:nix-spec for ongoing CLs.

Nix is a general-purpose, functional programming language which this document aims to describe.

Background

Nix was designed and implemented as part of the Nix package manager. It is primarily used for generating so-called derivations, which are data structures describing how to build a package.

The language has been described in the thesis introducing the package manager, but only on a high-level. At the time of writing, Nix is informally specified (via its only complete implementation in the package manager) and there is no complete overview over its - sometimes surprising - semantics.

The primary project written in Nix is nixpkgs. Uncertainties in the process of writing this specification are resolved by investigating patterns in nixpkgs, which we consider canonical. The code in nixpkgs uses a reasonable subset of the features exposed by the current implementation, some of which are accidental, and is thus more useful for specifying how the language should work.

Introduction to Nix

Nix is a general-purpose, partially lazy, functional programming language which provides higher-order functions, type reflection, primitive data types such as integers, strings and floats, and compound data structures such as lists and attribute sets.

Nix has syntactic sugar for common operations, such as those for attribute sets, and also provides a wide range of built-in functions which have organically accumulated over time.

Nix has a variety of legacy features that are not in practical use, but are documented in sections of this specification for the sake of completeness.

This document describes the syntax and abstract semantics of the Nix language, but leaves out implementation details about how Nix can be interpreted/compiled/analysed etc.

Program structure

This section describes the semantic structure of Nix, and how it relates to the rest of the specification.

Each Nix program is a single expression denoting a value (commonly a function). Each value has a type, however this type is not statically known.

Nix code is modularised through the use of the import built-in function. No separate module system exists.

In addition to chapters describing the building blocks mentioned above, this specificiation also describes the syntax, the available built-in functions, error handling and known deficiencies in the language.