{ nixpkgs }:

with nixpkgs;

let
  ghcCompiler = "ghc865";
  ghcjsCompiler = "ghcjs86";

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

  # general functions to put in a lib
  lines = s: lib.pipe s [
    (builtins.split "\n")
    (builtins.filter (x: builtins.typeOf x == "string"))
  ];
  removeNull = ls: builtins.filter (x: x != null) ls;

  selectAttrs = deps: packageSet:
    lib.attrsets.attrVals deps packageSet;

  # returns true if a is a subset of b, where a and b are attrsets
  subset = a: b: builtins.all
    (x: builtins.elem x b) a;

  haskellDeps = hpkgs: import ./Deps/Haskell.nix hpkgs;

  mkGhcPackageSet = pkgs.haskell.packages.${ghcCompiler}.ghcWithHoogle;
  mkGhcjsPackageSet = pkgs.haskell.packages.${ghcjsCompiler}.ghcWithPackages;

in rec {
  # gather data needed for compiling by analyzing the main module
  analyze = main: rec {
    # path to the module relative to the git root
    relpath = builtins.replaceStrings ["${root}/"] [""]
        (builtins.toString main);
    # Haskell-appropriate name of the module
    module = builtins.replaceStrings ["/" ".hs"] ["." ""] relpath;
    # file contents
    content = builtins.readFile main;
    # search for the ': out' declaration
    out = lib.pipe content [
      lines
      (map (builtins.match "^-- : out ([[:alnum:]._-]*)$"))
      removeNull
      lib.lists.flatten
      builtins.head
    ];
    # collect all of the ': dep' declarations
    deps = lib.pipe content [
      lines
      (map (builtins.match "^-- : dep ([[:alnum:]._-]*)$"))
      removeNull
      lib.lists.flatten
    ];
    # collect ': sys' declarations
    sysdeps = lib.pipe content [
      lines
      (map (builtins.match "^-- : sys ([[:alnum:]._-]*)$"))
      removeNull
      lib.lists.flatten
    ];
  };

  ghcPackageSetFull = mkGhcPackageSet haskellDeps;

  ghc = main:
    let
      data = analyze main;
      ghc = mkGhcPackageSet (hp: selectAttrs data.deps hp);
    in stdenv.mkDerivation {
      name = data.module;
      src = ../../.;
      nativeBuildInputs = [ ghc ] ++ selectAttrs data.sysdeps nixpkgs;
      strictDeps = true;
      buildPhase = ''
        mkdir -p $out/bin
        # compile with ghc
        ${ghc}/bin/ghc \
          -Werror \
          -i. \
          --make ${main} \
          -main-is ${data.module} \
          -o $out/bin/${data.out}
      '';
      # the install process was handled above
      installPhase = "exit 0";
    } // { env = ghc; };

  ghcjs = main:
    let
      data = analyze main;
      ghcjs = mkGhcjsPackageSet (hp: selectAttrs data.deps hp);
    in stdenv.mkDerivation {
      name = data.module;
      src = ../../.;
      nativeBuildInputs = [ ghcjs ];
      strictDeps = true;
      buildPhase = ''
        mkdir -p $out/static
        # compile with ghcjs
        ${ghcjs}/bin/ghcjs \
          -Werror \
          -i. \
          --make ${main} \
          -main-is ${data.module} \
          -o ${data.out}
        # optimize js output
        ${pkgs.closurecompiler}/bin/closure-compiler \
          ${data.out}/all.js > $out/static/${data.out}
      '';
      installPhase = "exit 0";
    } // { env = ghcjs; };

  env = mkShell {
    name = "bizdev";
    buildInputs = [
      # haskell deps
      (mkGhcPackageSet haskellDeps)
      # ghcjs doesn't need everything, and many things fail to build
      (mkGhcjsPackageSet (hp: with hp; [
        aeson
        clay
        containers
        miso
        protolude
        servant
        split
        string-quote
        text
        ghcjs-base
      ]))

      # scheme deps
      nixpkgs.guile
      nixpkgs.inspekt3d
      nixpkgs.libfive

      # python deps
      (nixpkgs.python38.withPackages (p:
        [ p.black p.pylint ]))

      # tools
      nixpkgs.ccze
      nixpkgs.cmark
      nixpkgs.figlet
      nixpkgs.haskell.packages.${ghcCompiler}.fast-tags
      nixpkgs.hlint
      nixpkgs.lolcat
      nixpkgs.niv.niv
      nixpkgs.nixops
      nixpkgs.ormolu
      nixpkgs.python37Packages.black
      nixpkgs.python37Packages.pylint
      nixpkgs.wemux
      (pkgs.writeScriptBin "ftags" (builtins.readFile ../Ide/ftags.sh))
    ];
    shellHook = ". ${./ShellHook.sh}";
  };

  os = cfg: (nixos (args: cfg)).toplevel;
}