{
# Reuse any local nixpkgs, at cost of reproductibility
#inputs.nixpkgs.url = "flake:nixpkgs";
# Can be overriden by passing --override-input nixpkgs flake:nixpkgs to nix
inputs.nixpkgs.url = "github:NixOS/nixpkgs/8585991bfb629edda1e42c191bef935d9d70d690";
inputs.flake-utils.url = "github:numtide/flake-utils";
outputs = inputs: inputs.flake-utils.lib.eachDefaultSystem (system: let
  pkgs = inputs.nixpkgs.legacyPackages.${system};
  ghc = null; # Use default ghc, but could have been ghc901
  haskellPackages =
    if ghc == null
    then pkgs.haskellPackages
    else pkgs.haskell.packages.${ghc};
  hs = haskellPackages.extend (with pkgs.haskell.lib; hfinal: hprev: {
    "AoC202004" = hfinal.callCabal2nix "AoC2020" ./Day04 {};
  });
  in rec {
    devShell = hs.shellFor {
      packages = p: [ p."AoC202004" ];
      withHoogle = false;
      nativeBuildInputs = [
        hs.cabal-install
        hs.haskell-language-server
        #hs.hpc
      ];
      buildInputs = [
        #hs.ghcid
        #hs.ormolu
        #hs.hlint
        #pkgs.nixpkgs-fmt
      ];
    };
    packages = {
      "Day04" = hs."AoC202004";
    };
    defaultApp = {
      type = "app";
      program = (pkgs.writeShellScript "run" ''
        cd ${inputs.self}
        ${pkgs.gnumake}/bin/make run
      '').outPath;
    };
    apps = pkgs.lib.genAttrs [
        "Day04"
      ] (day: {
        type = "app";
        program = "${packages.Day04}/bin/Day";
      }
    );
  }
);
}