style.hs

Obsidian Systems' shared Haskell code style, packaged as a Nix flake that wraps fourmolu with a common fourmolu.yaml.

Point any of our Haskell projects at this flake and they all format the same way, without each repo having to vendor and maintain its own copy of the config.

Usage

Run it directly

# Format in place, using the bundled Obsidian Systems config.
nix run github:obsidiansystems/style.hs -- --mode inplace src/

# Check formatting without writing (e.g. in CI).
nix run github:obsidiansystems/style.hs -- --mode check src/

Everything after -- is forwarded straight to fourmolu, so any flag fourmolu accepts works here too.

Add it to a project

Reference the flake as an input and use the fourmolu package as your formatter, for example in a dev shell:

{
  inputs.style.url = "github:obsidiansystems/style.hs";

  outputs = { self, nixpkgs, style, ... }:
    let system = "x86_64-linux";
    in {
      devShells.${system}.default =
        nixpkgs.legacyPackages.${system}.mkShell {
          packages = [ style.packages.${system}.fourmolu ];
        };
    };
}

Inside that shell, plain fourmolu already uses the Obsidian Systems style, with no --config needed.

Vendor it as a git submodule

Add this repo as a submodule and symlink your project's fourmolu.yaml to the vendored one. fourmolu discovers fourmolu.yaml from the project root automatically, so editors, formatters, and CI all pick up the style with no flags:

git submodule add https://github.com/obsidiansystems/style.hs style.hs
ln -s style.hs/fourmolu.yaml fourmolu.yaml

Pull in later changes to the shared style by updating the submodule:

git submodule update --remote style.hs

Use fourmolu.yaml as an executable

fourmolu.yaml is both the config and a runnable Nix shebang script. Copy it into a project and run it directly to format files using itself as the config:

./fourmolu.yaml --mode inplace src/

This needs nix on PATH with flakes enabled.

Choosing a config

The wrapper resolves which config to use as follows:

• No --config: the bundled fourmolu.yaml (the default).
• --config <path> / --config=<path>: your own config file. Repeated --config flags collapse to the last one, since fourmolu itself rejects duplicates.
• --config -: read the config from stdin.

What's in the repo

File	Purpose
fourmolu.yaml	The shared style config; also a runnable Nix shebang script.
fourmolu.nix	Builds the fourmolu wrapper that bundles the config and handles --config.
flake.nix	Exposes the fourmolu (and default) package for every supported system.
inputs.nix	flake-compat shim so non-flake Nix can consume the inputs.