depot/pkgs/development/compilers/reason/rtop.nix

{ buildDunePackage, reason, cppo, utop, makeWrapper }:

buildDunePackage {
  pname = "rtop";
  inherit (reason) version src;

  nativeBuildInputs = [
    makeWrapper
    cppo
  ];

  propagatedBuildInputs = [ reason utop ];

  postInstall = ''
    wrapProgram $out/bin/rtop \
      --prefix PATH : "${utop}/bin" \
      --prefix CAML_LD_LIBRARY_PATH : "$CAML_LD_LIBRARY_PATH" \
      --prefix OCAMLPATH : "$OCAMLPATH:$OCAMLFIND_DESTDIR"
  '';

  meta = reason.meta // {
    description = "Toplevel (or REPL) for Reason, based on utop";
    mainProgram = "rtop";
  };
}