Seamless integration of pre-commit git hooks with Nix

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

• Trivial 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 hooks for languages like Python, Haskell, Elm, etc.

• Run hooks as part of development and on your CI

Getting started

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/pre-commit-hooks.nix/tarball/master");
    in {
      pre-commit-check = nix-pre-commit-hooks.run {
        src = ./.;
        # If your hooks are intrusive, avoid running on each commit with a default_states like this:
        # default_stages = ["manual" "push"];
        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 {
       shellHook = ''
        ${(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
• nix-linter

Haskell

• ormolu
• fourmolu
• hindent
• stylish-haskell
• hlint
• cabal-fmt
• brittany
• hpack

Elm

• elm-format
• elm-review
• elm-test

Purescript

• purty

Python

• black
• isort

Rust

• rustfmt
• clippy
• cargo-check: Runs cargo check

Shell

• shellcheck

HTML

• html-tidy

Terraform

• terraform-format: built-in formatter

Spell checkers

• hunspell

Other Formatters

• prettier
• clang-format

You must configure which languages should be formatted by clang_format using clang-format.types_or. For example to check both C and C++ files:

clang-format = {
  enable = true;
  types_or = [ "c" "c++" ];
};

Custom hooks

Sometimes it is useful to add a project specific command as an extra check that is not part of the pre-defined set of hooks provided by this project.

Example configuration:

 let
   nix-pre-commit-hooks = import (builtins.fetchTarball "https://github.com/cachix/pre-commit-hooks.nix/tarball/master");
 in {
   pre-commit-check = nix-pre-commit-hooks.run {
     hooks = {
       # ...

       # Example custom hook for a C project using Make:
       unit-tests = {
         enable = true;

         # The name of the hook (appears on the report table):
         name = "Unit tests";

         # The command to execute (mandatory):
         entry = "make check";

         # The pattern of files to run on (default: "" (all))
         # see also https://pre-commit.com/#hooks-files
         files = "\\.(c|h)$";

         # List of file types to run on (default: [ "file" ] (all files))
         # see also https://pre-commit.com/#filtering-files-with-types
         # You probably only need to specify one of `files` or `types`:
         types = [ "text" "c" ];

         # Exclude files that were matched by these patterns (default: [ ] (none)):
         excludes = [ "irrelevant\\.c" ];

         # The language of the hook - tells pre-commit
         # how to install the hook (default: "system")
         # see also https://pre-commit.com/#supported-languages
         language = "system";

         # Set this to false to not pass the changed files
         # to the command (default: true):
         pass_filenames = false;
       };
     };
   };
 }

Custom hooks are defined with the same schema as pre-defined hooks.

Nix Flakes support

Given the following flake.nix example:

{
  description = "An example project.";

  inputs.pre-commit-hooks.url = "github:cachix/pre-commit-hooks.nix";
  inputs.flake-utils.url = "github:numtide/flake-utils";

  outputs = { self, nixpkgs, pre-commit-hooks, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      {
        checks = {
          pre-commit-check = pre-commit-hooks.lib.${system}.run {
            src = ./.;
            hooks = {
              nixpkgs-fmt.enable = true;
            };
          };
        };
        devShell = nixpkgs.legacyPackages.${system}.mkShell {
          inherit (self.checks.${system}.pre-commit-check) shellHook;
        };
      }
    );
}

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

To run the all the hooks on CI:

nix flake check 

To install pre-commit hooks developers would run:

nix develop

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)