shell.nix

{ nixpkgs ? import <nixpkgs> {}, compiler ? "default", doBenchmark ? false }:

let

  inherit (nixpkgs) pkgs;

  f = { mkDerivation, base, containers, directory, filepath
      , generic-deriving, lens, mtl, parsec, pretty, stdenv, tasty
      , tasty-hunit
      }:
      mkDerivation {
        pname = "core";
        version = "0.1.0.0";
        src = ./.;
        libraryHaskellDepends = [
          base containers generic-deriving lens mtl parsec pretty
        ];
        testHaskellDepends = [ base directory filepath tasty tasty-hunit ];
        homepage = "https://github.com/themattchan/core#readme";
        description = "Implementing functional languages";
        license = stdenv.lib.licenses.mit;
      };

  haskellPackages = if compiler == "default"
                       then pkgs.haskellPackages
                       else pkgs.haskell.packages.${compiler};

  variant = if doBenchmark then pkgs.haskell.lib.doBenchmark else pkgs.lib.id;

  drv = variant (haskellPackages.callPackage f {});

in

  if pkgs.lib.inNixShell then drv.env else drv