Seamless integration of pre-commit git hooks with Nix

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

Installation & Usage

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

   $ nix-env -iA cachix -f https://cachix.org/api/v1/install
   $ cachix use pre-commit-hooks

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

    let
      nix-pre-commit-hooks = import (builtins.fetchTarball "https://github.com/cachix/nix-pre-commit-hooks/tarball/master");
    in {
      pre-commit-check = nix-pre-commit-hooks.run {
        src = ./.;
        hooks = {
          elm-format.enable = true;
          ormolu.enable = true;
          shellcheck.enable = true;
        };
      };
    }

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

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

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

   Add /.pre-commit-config.yaml to .gitignore.

   Run $ nix-shell to execute shellHook which will:

   • build the tools and .pre-commit-config.yaml config file symlink which references the binaries, for speed and safe garbage collection
   • provide the pre-commit executable that git commit will invoke

Optional

Direnv + Lorri

.envrc

eval "$(lorri direnv)"

# Use system PKI
unset SSL_CERT_FILE
unset NIX_SSL_CERT_FILE

# Install pre-commit hooks
eval "$shellHook"

Hooks

Nix

• nixpkgs-fmt
• nixfmt

Haskell

• ormolu
• hindent
• hlint
• cabal-fmt

Elm

• elm-format

Shell

• shellcheck

Terraform

• terraform-format: built-in formatter

Contributing hooks

Everyone is encouraged to add new hooks.

Have a look at the existing hooks and the options.

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)
• The tool needs to live in a separate repository (even if a simple bash script, unless it's a oneliner)