pre-commit manages a set of hooks that are executed by git before committing code:

The goal is to manage these hooks with Nix and solve the following:

• Simpler integration for Nix projects (wires up a few things behind the scenes)

• Provide a low-overhead build of all the tooling available for the hooks to use (naive implementation of calling nix-shell does bring some latency when committing)

• Common package set of hooks for popular languages like Haskell, Elm, etc.

• Two trivial Nix functions to run hooks as part of development and on your CI

1. Create .pre-commit-config.yaml with hooks you want to run in your git repository:

   repos:
   - repo: .pre-commit-hooks/
     rev: master
     hooks:
      -   id: ormolu
      -   id: shellcheck
      -   id: elm-format

2. (optional) Use binary caches to avoid compilation:

   $ nix-env -iA cachix -f https://cachix.org/api/v1/install
   $ cachix use hercules-ci

3. Integrate hooks to be built as part of default.nix:

    let
      inherit (import (builtins.fetchTarball "https://github.com/hercules-ci/gitignore/tarball/master" {})) gitignoreSource;
      nix-pre-commit-hooks = import (builtins.fetchTarball "https://github.com/hercules-ci/nix-pre-commit-hooks/tarball/master");
    in {
      pre-commit-check = nix-pre-commit-hooks.run {
        src = gitignoreSource ./.;
      };
    }

   Run $ nix-build -A pre-commit-check to perform the checks as a Nix derivation.

4. Integrate hooks to prepare environment as part of shell.nix:

    (import <nixpkgs> {}).mkShell {
      inherit ((import ./. {}).pre-commit-check) shellHook;
    }

   Run $ nix-shell to execute shellHook which will:

   • install git hooks
   • symlink hooks to .pre-commit-hooks/
   • provide pre-commit executable that git commit will invoke

• canonix
• nixpkgs-fmt

• ormolu
• hlint
• cabal-fmt

• elm-format

• shellcheck

Everyone is encouraged to add new hooks.

There's no guarantee the hook will be accepted, but the general guidelines are:

• Nix closure of the tool should be small e.g. < 50MB
• The tool must not be very specific (e.g. language tooling is OK, but project specific tooling is not)