This change is motivated by my wish to create a development environment for OCaml with a version of the OCaml compiler with additional features enabled (like the Flambda optimization engine, or support for American Fuzzy Lop).

If I understand correctly, mkOcamlPackages was previously exported, but then that change was reverted. See the commit message for more details.

Before this PR is considered for merging, I have a question which I think needs to be addressed.

I have a file shell.nix in my project directory that I would have assumed to accomplish my goals, but I get the following error about a missing version attribute and I'm not sure why:

$ nix-shell
error: attribute 'version' missing, at /home/jhaberku/src/nixpkgs/pkgs/top-level/ocaml-packages.nix:1020:32
(use '--show-trace' to show detailed location information)

(In the "real" shell.nix I'm using a separate clone of nixkgs under the hakuch name, but that's not important here.)

I don't understand why the ocaml record doesn't have the version attribute in this context.

Here's the shell.nix file:

let
  pkgs = import <nixpkgs> {};
  
  ocaml = pkgs.callPackage <nixpkgs/pkgs/development/compilers/ocaml/4.11.nix> {
    flambdaSupport = true;
  };
  
   ocamlPackages = pkgs.ocaml-ng.mkOcamlPackages { inherit ocaml; };
in

pkgs.mkShell {
  buildInputs = [
    ocaml
    pkgs.ocamlformat
  ] ++ (with ocamlPackages; [
    dune_2
    findlib
    ppx_expect
    odoc
    utop
  ]);
}