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)))))))))

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