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Lean 4 Nix

Nix flake build for Lean 4.

Features:

  • Lean overlay
  • Automatically read toolchain version
  • Convert lake-manifest.json into Lean build

Example

The default template is a good starting point for projects requiring manual dependency management:

nix flake new --template github:lenianiva/lean4-nix ./minimal

The .#dependency template shows an example of using lake-manifest.json to fetch dependencies automatically.

nix flake new --template github:lenianiva/lean4-nix#dependency ./dependency

Flake outputs

Overlay

The user must decide on a Lean version to use as overlay. The minimal supported version is v4.11.0, since it is the version when Lean's official Nix flake was deprecated. There are a couple of ways to get an overlay. Each corresponds to a flake output:

  • readSrc { src; bootstrap; }: Builds Lean from a source folder. A bootstrapping function must be provided.
  • readFromGit{ args; bootstrap; }: Given parameters to builtins.fetchGit, download a git repository
  • readRev { rev; bootstrap; } : Reads a revision from the official Lean 4 repository
  • readToolchainFile: Reads the toolchain from a file. Due to Nix's pure evaluation principle, this only supports leanprover/lean4:{tag} based lean-toolchain files. For any other toolchains, use readRev or readFromGit.
  • tags.{tag}: Lean4 tags. See the available tags in manifests/

Then apply the overlay on pkgs:

pkgs = import nixpkgs {
  inherit system;
  overlays = [ (lean4-nix.readToolchainFile ./lean-toolchain) ];
};

and pkgs.lean will be replaced by the chosen overlay.

pkgs.lean

This attribute set has properties

  • lean: The Lean executable
  • lean-all: lean, lake, and the Lean library.
  • example: Use nix run .#example to see an example of building a Lean program.
  • Init, Std, Lean: Lean built-in libraries provided in the same format as buildLeanPackage

and the function buildLeanPackage, which accepts a parameter set { name; roots; deps; src; }. The complete parameter set can be found in Lean 4's nix/buildLeanPackage.nix file. In general:

  • src: The source directory
  • roots: Lean modules at the root of the import tree.
  • deps: A list of outputs of other buildLeanPackage calls.

This is a form of manual dependency management.

lake2nix

Use lake2nix = lean4-nix.lake { inherit pkgs; } to generate the lake utilities.

lake2nix.mkPackage { src; roots; } automatically reads the lake-manifest.json file and builds dependencies.

  • src: The source directory
  • manifestFile: Path to the manifest file. Defaults to ${src}/lake-manifest.json
  • roots: Lean modules at the root of the import tree. Defaults to the project name from manifestFile
  • deps: Additional dependencies. Defaults to [ Init Std Lean ].

Troubleshooting

attribute '"{Lean,Init}.*"' is missing

If you see this error, add these packages to deps in either buildLeanPackage or mkPackage.

{
  deps = with pkgs.lean; [ Init Std Lean ];
}

Only leanprover/lean4:{tag} toolchains are supported

The Lean version is not listed in the manifests/ directory. Use readRev or readFromGit instead.

Development

Use nix flake check to check the template builds.

Update the template lean-toolchain files when new Lean versions come out.

All code must be formatted with alejandra before merging into main. To use it, execute

nix run .#formatter.$SYSTEM -- .

replace $SYSTEM with the current system.