nixpkgs: main:

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;

  # turn the file path into a Haskell module name
  relpath = builtins.replaceStrings ["${root}/"] [""] (builtins.toString main);
  module = builtins.replaceStrings ["/" ".hs"] ["." ""] relpath;

  # extract info from special comments
  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)));

  # do the build...

  depsToPackageSet = packageSet: deps:
    map (s: builtins.getAttr s packageSet) deps;

  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;
    };
  });

  ghc = ghc865_.ghcWithPackages (hp: depsToPackageSet hp deps);

in stdenv.mkDerivation {
  name = module;
  version = "0";
  src = ../../.; # this is the git root
  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; }