{ lib, inputs, ... }:
{
  imports = [ inputs.treefmt-nix.flakeModule ];
  perSystem =
    { self', pkgs, ... }:
    {
      treefmt.projectRootFile = "flake.nix";
      treefmt.programs.shellcheck.enable = true;

      treefmt.programs.mypy.enable = true;
      treefmt.programs.mypy.directories = {
        "pkgs/clan-cli".extraPythonPackages = self'.packages.clan-cli.testDependencies;
        "pkgs/clan-app".extraPythonPackages =
          # clan-app currently only exists on linux
          (self'.packages.clan-app.externalTestDeps or [ ]) ++ self'.packages.clan-cli.testDependencies;
      };

      treefmt.settings.formatter.nix = {
        command = "sh";
        options = [
          "-eucx"
          ''
            # First deadnix
            ${lib.getExe pkgs.deadnix} --edit "$@"
            # Then nixpkgs-fmt
            ${lib.getExe pkgs.nixfmt-rfc-style} "$@"
          ''
          "--" # this argument is ignored by bash
        ];
        includes = [ "*.nix" ];
        excludes = [
          # Was copied from nixpkgs. Keep diff minimal to simplify upstreaming.
          "pkgs/builders/script-writers.nix"
        ];
      };
      treefmt.settings.formatter.python = {
        command = "sh";
        options = [
          "-eucx"
          ''
            ${lib.getExe pkgs.ruff} check --fix "$@"
            ${lib.getExe pkgs.ruff} format "$@"
          ''
          "--" # this argument is ignored by bash
        ];
        includes = [ "*.py" ];
      };
      treefmt.settings.formatter.vale =
        with pkgs;
        let
          vocab = "cLAN";
          style = "Docs";
          config = writeText "vale.ini" ''
            StylesPath = ${styles}
            Vocab = ${vocab}

            [*.md]
            BasedOnStyles = Vale, ${style}
            Vale.Terms = No
          '';
          styles = symlinkJoin {
            name = "vale-style";
            paths = [
              accept
              headings
            ];
          };
          accept = writeTextDir "config/vocabularies/${vocab}/accept.txt" ''
            Nix
            NixOS
            Nixpkgs
            clan.lol
            Clan
            monorepo
          '';
          headings = writeTextDir "${style}/headings.yml" ''
            extends: capitalization
            message: "'%s' should be in sentence case"
            level: error
            scope: heading
            # $title, $sentence, $lower, $upper, or a pattern.
            match: $sentence
          '';
        in
        {
          command = "${vale}/bin/vale";
          options = [ "--config=${config}" ];
          includes = [ "*.md" ];
          # TODO: too much at once, fix piecemeal
          excludes = [
            "docs/*"
            "clanModules/*"
            "pkgs/*"
          ];
        };
    };
}