21628f7ad4
Change-Id: I2008d4d5d92dc02f3955828ba93f748282948f07 Reviewed-on: https://cl.snix.dev/c/snix/+/30447 Autosubmit: Florian Klink <flokli@flokli.de> Reviewed-by: Vova Kryachko <v.kryachko@gmail.com> Tested-by: besadii
79 lines
3 KiB
Markdown
79 lines
3 KiB
Markdown
---
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title: Specification of the Nix Language
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slug: language-spec
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description: ""
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summary: ""
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date: 2025-05-01T13:10:37+02:00
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lastmod: 2025-05-01T13:10:37+02:00
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draft: false
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weight: 50
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toc: false
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---
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{{< callout context="caution" title="Caution" icon="outline/alert-triangle" >}}
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This document is a work in progress. Please keep an eye on
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[`topic:nix-spec`](https://cl.snix.dev/q/topic:nix-spec) for ongoing
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CLs.
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{{</callout>}}
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Nix is a general-purpose, functional programming language which this
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document aims to describe.
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## Background
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Nix was designed and implemented as part of the [Nix package
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manager](https://nixos.org/nix). It is primarily used for generating
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so-called [*derivations*](#derivations), which are data structures
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describing how to build a package.
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The language has been described in the
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[thesis](https://edolstra.github.io/pubs/phd-thesis.pdf) introducing
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the package manager, but only on a high-level. At the time of writing,
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Nix is informally specified (via its only complete implementation in
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the package manager) and there is no complete overview over its -
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sometimes surprising - semantics.
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The primary project written in Nix is
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[nixpkgs](https://github.com/NixOS/nixpkgs/). Uncertainties in the
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process of writing this specification are resolved by investigating
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patterns in nixpkgs, which we consider canonical. The code in nixpkgs
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uses a reasonable subset of the features exposed by the current
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implementation, some of which are *accidental*, and is thus more
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useful for specifying how the language should work.
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## Introduction to Nix
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Nix is a general-purpose, partially lazy, functional programming
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language which provides higher-order functions, type reflection,
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primitive data types such as integers, strings and floats, and
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compound data structures such as lists and attribute sets.
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Nix has syntactic sugar for common operations, such as those for
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attribute sets, and also provides a wide range of built-in functions
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which have organically accumulated over time.
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Nix has a variety of legacy features that are not in practical use,
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but are documented in sections of this specification for the sake of
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completeness.
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This document describes the syntax and abstract semantics of the Nix
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language, but leaves out implementation details about how Nix can be
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interpreted/compiled/analysed etc.
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### Program structure
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This section describes the semantic structure of Nix, and how it
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relates to the rest of the specification.
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Each Nix program is a single [*expression*](#expressions) denoting a
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[*value*](#values) (commonly a [*function*](#functions)). Each value
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has a [*type*](#types), however this type is not statically known.
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Nix code is modularised through the use of the
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[*import*](#builtins-import) built-in function. No separate module
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system exists.
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In addition to chapters describing the building blocks mentioned
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above, this specificiation also describes the [*syntax*](#syntax), the
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available [built-in functions](#builtins), [*error handling*](#errors)
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and known [*deficiencies*](#deficiencies) in the language.
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