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nix-lang: Parser and printers for the Nix language

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nix-lang provides a parser, exact printer, canonical RFC 166-oriented formatter, and editable Trees that Grow style AST for the Nix language. It supports lossless round-tripping for parsed syntax while also offering a fresh syntax tree and formatter for generated Nix expressions.

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Modules

  • Nix
    • Lang
      • Nix.Lang.Annotation
      • Nix.Lang.Edit
      • Nix.Lang.ExactPrint
        • Nix.Lang.ExactPrint.Operations
        • Prepare
          • Nix.Lang.ExactPrint.Prepare.Parse
          • Nix.Lang.ExactPrint.Prepare.Rebuild
          • Nix.Lang.ExactPrint.Prepare.Reflow
          • Nix.Lang.ExactPrint.Prepare.Repair
          • Nix.Lang.ExactPrint.Prepare.Types
          • Nix.Lang.ExactPrint.Prepare.Utils
      • Lexer
        • Nix.Lang.Lexer.Text
      • Nix.Lang.Outputable
      • Nix.Lang.Parser
      • QQ
        • Nix.Lang.QQ.Parsed
      • Nix.Lang.RFCPrint
        • Nix.Lang.RFCPrint.LayoutHints
      • Nix.Lang.Span
      • Nix.Lang.Types
        • Nix.Lang.Types.Ps
        • Nix.Lang.Types.Syn
      • Nix.Lang.Utils

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Versions [RSS] 0.1.0.0
Change log CHANGELOG.md
Dependencies base (>=4.20 && <4.21), containers (>=0.7 && <0.8), megaparsec (>=9.7.0 && <9.8), mtl (>=2.3.1 && <2.4), parser-combinators (>=1.3.1 && <1.4), prettyprinter (>=1.7.1 && <1.8), syb (>=0.7.2 && <0.7.5), template-haskell (>=2.22.0 && <2.23), text (>=2.1.3 && <2.2) [details]
Tested with ghc ==9.10.3
License BSD-3-Clause
Copyright Copyright (c) berberman 2022-2026
Author berberman
Maintainer berberman <berberman@yandex.com>
Uploaded by berberman at 2026-07-15T02:58:33Z
Category Nix
Home page https://github.com/berberman/nix-lang
Bug tracker https://github.com/berberman/nix-lang/issues
Source repo head: git clone https://github.com/berberman/nix-lang
Distributions
Reverse Dependencies 1 direct, 0 indirect [details]
Downloads 3 total (3 in the last 30 days)
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Status Docs not available [build log]
All reported builds failed as of 2026-07-15 [all 2 reports]

Readme for nix-lang-0.1.0.0

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nix-lang

nix-lang is a parser, exact printer, and formatter for the Nix language built around a Trees that Grow style AST. Inspired by ghc-exactprint, it supports lossless round-tripping for parsed source while preserving comments, formatting, and multiline structure after modifying the syntax tree. It also provides a fresh syntax tree for generated Nix code together with an RFC 166-oriented formatter for canonical output.

nix-lang-qq provides quasiquoter for building fresh Nix expressions in Haskell file and supports embedding Haskell expressions.

formatExpr [nixQQ| { value = 1; } |]
  -- => {
  --      value = 1; 
  --    }
formatExpr [nixQQ| %%(foldl (\acc _ -> mkApp (mkVar "f") acc) (mkVar "x") [1..10 :: Int])]
  -- => f (f (f (f (f (f (f (f (f (f x)))))))))

Known unsupported syntax

Although it now parses the entire nixpkgs, some valid syntax is not supported.

  • Legacy let 
let { x = 233; body = x; }
  • Floating number starting with a decimal point 
.233
  • Chained has-attribute expressions 
a ? b ? c ? d
  • Operator without white spaces 
1+-1