style.hs

Obsidian-style Haskell

one shared fourmolu.yaml · nix run anywhere · consistent formatting across projects

$ nix run github:obsidiansystems/style.hs -- --mode inplace src/   # format your Haskell, Obsidian-style
$ nix run github:obsidiansystems/style.hs -- --mode check   src/   # or just check it, e.g. in CI

A Nix flake that wraps fourmolu with a common fourmolu.yaml. Point any of your 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. The config implements our Haskell Style Guide.

Why style.hs?

• One config, every repo. Reference the flake and your projects format identically.
• Drop in however you like. Run it directly with nix run, add it to a dev shell so plain fourmolu just works, vendor it as a git submodule, or run the config file itself as an executable.

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
STYLE.md	The Obsidian Haskell Style Guide: the reasoning behind the rules.
fourmolu.yaml	The shared style config: also a runnable Nix shebang script.
fourmolu.nix	Builds 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.

About Obsidian Systems

style.hs is built and maintained by Obsidian Systems. We provide frontier engineering for high-assurance systems, and we're long-time stewards of open-source Nix, Haskell, and daml tooling, including Obelisk, Reflex, nix-daml-sdk, and nix-thunk.

If you're building with Haskell or Nix and want a partner to help design, build, or ship it, we'd love to hear from you.

• Website — https://obsidian.systems
• Blog — https://blog.obsidian.systems
• GitHub — https://github.com/obsidiansystems

License

style.hs is released under the BSD-3-Clause License, © 2026 Obsidian Systems LLC. fourmolu and the other tools it wraps are distributed under their own upstream licenses.