niv/foo/default.nix

{ haskellPackages
, stdenv
, lib
}:

{
  buildPackage =
    attrs:
    let
      src = if !lib.isDerivation attrs && lib.isAttrs attrs then attrs.src else attrs;
      root = if !lib.isDerivation attrs && lib.isAttrs attrs then attrs.root else attrs;
      nubdeps = ds: lib.lists.sort (x: y: x < y) (
        lib.unique (
          map (d: lib.head (lib.splitString " " d)) ds
        )
      );
      spec = builtins.fromJSON (builtins.readFile (root + "/package.yaml"));
      commonDeps = spec.dependencies;

      libraryExtraDeps =
        lib.optionals
          (spec ? library && spec.library ? dependencies)
          spec.library.dependencies;
      libraryDeps = nubdeps (commonDeps ++ libraryExtraDeps);

      exeExtraDeps = lib.optionals (spec ? executables) (
        lib.concatMap
          (
            exe: lib.optionals
              (exe ? dependencies)
              exe.dependencies
          )
          (builtins.attrValues spec.executables)
      );
      exeDeps =
        nubdeps
          (
            builtins.filter (x: x != spec.name)
              (commonDeps ++ exeExtraDeps)
          );

      testExtraDeps = lib.optionals (spec ? tests) (
        lib.concatMap
          (
            test: lib.optionals
              (test ? dependencies)
              test.dependencies
          )
          (builtins.attrValues spec.tests)
      );
      testDeps = nubdeps (builtins.filter (x: x != spec.name) (commonDeps ++ testExtraDeps));

      depsFor = depType:
        map (
          d:
          if ! builtins.hasAttr d haskellPackages
          then throw "haskellPackages does not contain dependency '${d}' needed for '${depType}'"
          else
            haskellPackages.${d}
        );

    in
    haskellPackages.callPackage
      (
        { mkDerivation }:
        mkDerivation {
          pname = spec.name;
          inherit (spec) version;
          inherit src;
          isLibrary = builtins.hasAttr "library" spec;
          isExecutable = builtins.hasAttr "executables" spec;
          enableSeparateDataOutput = true;
          libraryHaskellDepends = depsFor "libraryHaskellDepends" libraryDeps;
          libraryToolDepends = [ haskellPackages.hpack ];
          executableHaskellDepends = depsFor "executableHaskellDepends" exeDeps;
          testHaskellDepends = depsFor "testHaskellDepends" testDeps;
          prePatch = "hpack";
          homepage = "https://github.com/${spec.github}#readme";
          description = spec.synopsis;
          license =
            if builtins.hasAttr "license" spec && spec.license == "MIT"
            then lib.licenses.mit
            else throw "Don't know how to handle license: ${builtins.toJSON spec.license}";
        }
      )
      { };
}