add nix ocaml setup

default.nix

@@ -0,0 +1,33 @@
+with import <nixpkgs> {};
+
+let
+  ocamlPackages = pkgs.recurseIntoAttrs pkgs.ocamlPackages_latest;
+  ocamlVersion = (builtins.parseDrvName ocamlPackages.ocaml.name).version;
+  findlibSiteLib = "${ocamlPackages.findlib}/lib/ocaml/${ocamlVersion}/site-lib";
+  ocamlInit = pkgs.writeText "ocamlinit" ''
+    let () =
+      try Topdirs.dir_directory "${findlibSiteLib}"
+      with Not_found -> ()
+    ;;
+
+    #use "topfind";;
+    #thread;;
+    #require "core";;
+    #require "core.syntax";;
+  '';
+in
+stdenv.mkDerivation rec {
+  name = "rwo-shell";
+  src = null;
+  buildInputs = with ocamlPackages;
+    [ ocaml
+      core
+      core_extended
+      findlib
+      utop
+    ];
+  shellHook = ''
+    alias utop="utop -init ${ocamlInit}"
+    alias ocaml="ocaml -init ${ocamlInit}"
+  '';
+}