Support ocaml 4.08 and 4.09

[emillon]

Hi!

This PR adds support to OCaml 4.08 (#41). There are several aspects to this:

• some helpers can be made compatible with more versions of the compiler "for free" by using features of compiler-libs (see the first commits)
• the main bit of work is being able to deconstruct Types.module_type and Types.signature_item from different versions of the compiler. To do so, I created a Utypes module that contains just what we need, and some shallow conversion functions are provided in a compat module that is selected by dune.

This is not perfect but I think it's a nice solution until OMP or a similar library supports Types.

Some additional remarks:

• I think that the first commits make sense to merge even if the rest is not
• 4.09 compatibility is fairly easy to add (part of the init code needs to be put in compat), but this either adds duplication (we'd need a compat file for <= 4.07, one for 4.08, and one for >= 4.09) or complexity (by having one compat module by versioned feature).

Let me know what you think.

Thanks

[ejgallego]

Great, thanks a lot @emillon ! I'll try to review this ASAP.

Main question I have tho is if we should just add support for this in OMP directly as requested in ocaml-ppx/ocaml-migrate-parsetree#52 ?

I'm unsure how much extra work that'd be, just mentioning so this PR is linked to the above issue.

[emillon]

The support here is really ad hoc to what's used in ppx_import. Assuming there's support in OMP, this bit can be removed, but it seems that it would add a lot of code to OMP.

[ejgallego]

The support here is really ad hoc to what's used in ppx_import. Assuming there's support in OMP, this bit can be removed, but it seems that it would add a lot of code to OMP.

Indeed I dunno what OMP maintainers may think; I suggest we ping them. Meanwhile I don't see a problem adding this to ppx_import and removing it later if we migrate the code there.

[ejgallego]

LGTM, tested on my own projects on 4.07.1 and 4.08.1 and so far seems to work! A couple of comments:

• see the note about using %{ocaml_version} and avoiding the gen.ml wrapper altoghether
• I wonder if we want to introduce the UTypes changed constructors, maybe we could indeed just use aliases to the real constructor names as in OMP?

[emillon]

OK! I switched it to use the types from the 4.07 Types module (I replaced one unused argument by unit in order not to pull an unused type) and this seems to be working. Let me know if that works for you, in which case I'll squash that.

As for %{ocaml_version}, I don't think we can do that. It will expand to 4.08.1, but we need a way to map from these versions strings to an actual file like ge_408.ml. I don't think we have access to a useful comparison from dune file, so it seems to me that a tiny generator is required.

[ejgallego]

Sounds good @emillon , thanks! I think this is ready to merge, and do a release.

WDYT folks @gasche @whitequark ?

[ejgallego]

[I also wonder if you want to squash the last commit that undoes the introduction of Utypes as to get a slightly better git history]

[emillon]

Done 👍

[ejgallego]

@emillon I also realized while testing the release that you should add an entry in CHANGES.md

[emillon]

Sure, will do! Should I try to add 4.09 support while I'm at it? with the new way of reusing 4.07 types it might not be too bad.

[ejgallego]

Sure, will do! Should I try to add 4.09 support while I'm at it? with the new way of reusing 4.07 types it might not be too bad.

As you want @emillon ; we can always do that in a separate PR.

I propose to merge tomorrow afternoon if no futher comments arise [but can wait if you want]

[emillon]

• Added 4.09 support. This turns gen.ml into a cppo-like program that includes different fragments.
• I also noticed that metaquot was listed in (libraries) which is not necessary (and potentially harmful).
• I updated the changelog!
• I did not squash in order to make the review easier but I'll do that if you're fine with the changes.

And that should be ready.

Thanks!

[ejgallego]

Thanks a lot @emillon

• Added 4.09 support. This turns gen.ml into a cppo-like program that includes different fragments.

Great!

• I also noticed that metaquot was listed in (libraries) which is not necessary (and potentially harmful).

This is likely some legacy code from the initial port to dune.

• I updated the changelog!
• I did not squash in order to make the review easier but I'll do that if you're fine with the changes.

Changes look fine to me; I'll leave some time for comments, then merge + release to opam.

We should update the .travis.yml file tho; I have pushed the fix myself.

[emillon]

I cleaned the history a bit, should now be all good.