{
inputs.nixpkgs.url = "flake:nixpkgs";
inputs.flake-utils.url = "github:numtide/flake-utils";
outputs = inputs: inputs.flake-utils.lib.eachDefaultSystem (system: let
  pkgs = inputs.nixpkgs.legacyPackages.${system};
  ghc = null; # ghc901
  haskellPackages =
    if ghc == null
    then pkgs.haskellPackages
    else pkgs.haskell.packages.${ghc};
  hs = haskellPackages.extend (with pkgs.haskell.lib;
    packageSourceOverrides {
      aoc2020 = ./.;
    });
  in rec {
    defaultPackage = hs.aoc2020;
    devShell = hs.shellFor {
      packages = p: [ p.aoc2020 ];
      withHoogle = true;
      nativeBuildInputs = [
        hs.cabal-install
        hs.haskell-language-server
        #hs.hpc
      ];
      buildInputs = [
        #hs.ghcid
        #hs.ormolu
        #hs.hlint
        #pkgs.nixpkgs-fmt
      ];
    };
    apps = {
      "Day04" = { type = "app"; program = defaultPackage + "/bin/Day04"; };
    };
  }
);
}