{ nixpkgs }:

with nixpkgs;
with nixpkgs.lib;

let
  # provided by .envrc
  root = builtins.getEnv "BIZ_ROOT";

  # general functions to put in a lib
  lines = s: strings.splitString "\n" s;
  seq = ls: builtins.filter (x: x!= null) ls;
  depsToPackageSet = packageSet: deps:
    map (s: builtins.getAttr s packageSet) deps;

  # do the build...

  claySrc = pkgs.fetchFromGitHub {
    owner = "sebastiaanvisser";
    repo = "clay";
    rev = "cc7729b1b42a79e261091ff7835f7fc2a7ae3cee";
    sha256 = "1vd67976lvi5l4qq18zy6j44apynkl44ps04p8vwfx4gzr895dyp";
  };

  ghc865_ = pkgs.haskell.packages.ghc865.override (oldAttrs: {
    overrides = with pkgs.haskell.lib; self: super: {
      clay = self.callCabal2nix "clay" claySrc {};
      wai-middleware-metrics = dontCheck super.wai-middleware-metrics;
    };
  });

  # ghcjs-8.6.0.1
  ghcjs_ = pkgs.haskell.packages.ghcjs.override (oldAttrs: {
    overrides = with pkgs.haskell.lib; self: super: {
      QuickCheck = dontCheck super.QuickCheck;
      base-compat-batteries = dontCheck super.http-types;
      clay = dontCheck (self.callCabal2nix "clay" claySrc {});
      comonad = dontCheck super.comonad;
      http-types = dontCheck super.http-types;
      network-uri= dontCheck super.network-uri;
      scientific = dontCheck super.scientific; # takes forever
      servant = dontCheck super.servant;
      tasty-quickcheck = dontCheck super.tasty-quickcheck;
      time-compat = dontCheck super.time-compat;
    };
  });

in {
  buildGhc = main:
    let
      relpath = builtins.replaceStrings ["${root}/"] [""] (builtins.toString main);
      module = builtins.replaceStrings ["/" ".hs"] ["." ""] relpath;
      content = builtins.readFile main;
      exe = builtins.head (lists.flatten (seq
        (map (builtins.match "^-- : exe ([[:alnum:]._-]*)$")
          (lines content))));
      deps = lists.flatten (seq
        (map (builtins.match "^-- : dep ([[:alnum:]._-]*)$")
          (lines content)));
      ghc = ghc865_.ghcWithHoogle (hp: depsToPackageSet hp deps);
    in stdenv.mkDerivation {
      name = module;
      version = "0";
      src = ./.;
      nativeBuildInputs = [ ghc ];
      strictDeps = true;
      buildPhase = ''
        #
        mkdir -p $out/{bin,static} ${baseNameOf relpath}
        #
        # compile with ghc
        #
        ${ghc}/bin/ghc -Werror -i. \
            --make ${main} \
            -main-is ${module} \
            -o $out/bin/${exe}
      '';
      # the install process was handled above
      installPhase = "exit 0";
    } // { env = ghc; };

  buildGhcjs = main:
    let
      relpath = builtins.replaceStrings ["${root}/"] [""] (builtins.toString main);
      module = builtins.replaceStrings ["/" ".hs"] ["." ""] relpath;
      content = builtins.readFile main;
      exe = builtins.head (lists.flatten (seq
        (map (builtins.match "^-- : exe ([[:alnum:]._-]*)$")
          (lines content))));
      deps = lists.flatten (seq
        (map (builtins.match "^-- : dep ([[:alnum:]._-]*)$")
          (lines content)));
      ghcjs = ghcjs_.ghcWithPackages (hp: depsToPackageSet hp deps);
    in stdenv.mkDerivation {
      name = module;
      version = "0";
      src = ./.;
      nativeBuildInputs = [ ghcjs ];
      strictDeps = true;
      buildPhase = ''
        #
        mkdir -p $out/{bin,static} ${baseNameOf relpath}
        #
        # compile with ghcjs
        #
        ${ghcjs}/bin/ghcjs -Werror -i. \
            --make ${main} \
            -main-is ${module} \
            -o ${exe}
        #
        # optimize js output
        #
        ${pkgs.closurecompiler}/bin/closure-compiler \
            ${exe}.jsexe/all.js > $out/static/${exe}
      '';
      installPhase = "exit 0";
    } // { env = ghcjs; };
}