README.md

Fourmolu-nix

fourmolu-nix makes a wrapper for fourmolu with settings already pass to it so you no longer need fourmolu.yaml or even to be in root of project to have these options set for you.

Getting started

For creating the wrapper, you can use mkWrapper function from lib which takes pkgs and configuration and returns wrapped fourmolu with the provided configuration.

Flakes

# flake.nix
{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
    utils.url = "github:numtide/flake-utils";
    fourmolu-nix.url = "github:jedimahdi/fourmolu-nix";
  };

  outputs = {
    nixpkgs,
    utils,
    fourmolu-nix,
    ...
  }:
    utils.lib.eachDefaultSystem (
      system: let
        pkgs = nixpkgs.legacyPackages.${system};
        fourmoluWrapper = fourmolu-nix.lib.mkWrapper pkgs {
          settings = {
            indentation = 4;
            comma-style = "leading";
            record-brace-space = true;
            extensions = ["OverloadedRecordDot", "RecordWildCards"];
          };
        };
      in {
        devShells.default = pkgs.mkShell {
          nativeBuildInputs = [
            fourmoluWrapper
          ];
        };
      }
    );
}

Flake-parts

With flake-parts you don't need to call mkWrapper, just add inputs.fourmolu-nix.flakeModule to list of imports.

Fourmolu wrapper will be accessible via config.fourmolu.wrapper.

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
    flake-parts.url = "github:hercules-ci/flake-parts";
    fourmolu-nix.url = "github:jedimahdi/fourmolu-nix";
  };

  outputs = inputs:
    inputs.flake-parts.lib.mkFlake {inherit inputs;} {
      imports = [
        inputs.fourmolu-nix.flakeModule
      ];
      systems = ["x86_64-linux" "aarch64-linux" "aarch64-darwin" "x86_64-darwin"];
      perSystem = {
        config,
        pkgs,
        ...
      }: {
        fourmolu.settings = {
          indentation = 4;
          import-export-style = "diff-friendly";
          extensions = ["ImportQualifiedPost" "OverloadedRecordDot"];
        };

        devShells.default = pkgs.mkShell {
          nativeBuildInputs = [
            config.fourmolu.wrapper
          ];
        };
      };
    };
}

Note: fourmolu-nix will automatically set the wrapper package for treefmt-nix fourmolu program.

Configuration

See options for the full list of configs.