Switch to ppxlib

[pveber]

This is my shot at converting the lwt_ppx to ppxlib instead of ppx_tools_versioned, which is going to be deprecated. This new version passes all tests performed after a dune runtest invocation, I only see this warning:

$dune runtest
File "test/ppx/main.ml", line 9, characters 0-46:
9 | let%lwt structure_let_result = Lwt.return true
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 22: let%lwt should not be used at the module item level.
Replace let%lwt x = e by let x = Lwt_main.run (e)

which is I believe is expected.

I tried to stay as close as possible to the original code, by sticking to a whole-module transformation. An alternative might have been to resort to ppxlib's context-free rules which are preferable but maybe not expressive enough here, I'm not sure.

One recurrent difference with the original code is that I think there is no equivalent of the [@metaloc ] attribute in ppxlib.metaquot which forced me to introduce all quotations with a let loc in ... prelude.

I'd be surprised if there were nothing to change to this patch, so if you think it can lead to something, I'd be happy to work on it further.

[raphael-proust]

I'm not familiar with the ppx (and especially with the ppx implementation), I'll review the MR next week, but I might have to ask for additional review.

[Drup]

At the very least, I suggest we wait for the new ppxlib to be released, which is probably going to break compat one way or the other.

And yeah, if we switch to ppxlib, might as well try to use their fancy hook system...

[pveber]

Waiting for the next release of ppxlib certainly is an option, but in that case it would be better that ppx_tools_versioned released a version compatible with ocaml-migrate-parsetree 2.0.0. Otherwise lwt would hold back other libraries that switched to ppxlib.

[pveber]

Seems there won't be any release of ppx_tools_versioned compatible with OMP 2.0.0, see ocaml-ppx/ppx_tools_versioned#28

[kit-ty-kate]

There is currently no release of ppxlib compatible with both omp2 and OCaml 4.12. The latest release is compatible with OCaml 4.12 but requires omp2.

This makes packages relying on both lwt_ppx and some other ppxlib-based ppxs, uninstallable when using the soon to come OCaml 4.12.
It would be very helpful to have this out sooner rather than later.

[aantron]

Thanks. This looks correct up to some nits.

Could you apply this patch to get opam pin/development installation working again? Related: aantron/bisect_ppx#327. This should also (help) fix the CI.

diff --git a/lwt.opam b/lwt.opam
index 277dea245..0fe007bfd 100644
--- a/lwt.opam
+++ b/lwt.opam
@@ -28,7 +28,8 @@ depends: [
   "result" # result is needed as long as Lwt supports OCaml 4.02.
   "seq" # seq is needed as long as Lwt supports OCaml < 4.07.0.

-  "bisect_ppx" {dev & >= "2.0.0"}
+  # Until https://github.com/aantron/bisect_ppx/pull/327.
+  # "bisect_ppx" {dev & >= "2.0.0"}
   "ocamlfind" {dev & >= "1.7.3-1"}
 ]

[aantron]

AFAICT we are now using ocaml-compiler-libs.common (e.g. new line 268).

[pveber]

done

[aantron]

Please reduce formatting changes like here, where the change on old line 268 also wrongly appears to affect old line 267. This applies to many of the other diff hunks also, parts of which have changed only in indentation or splitting of tokens across lines. The formatting in this file is not great, but we should change it in separate work because this change is a bit scary and keeping it focused will be much easier for review, and for debuging later, if necessary, if this ends up as someone's git bisect endpoint :)

[pveber]

Yes I understand it very well, sorry about that. I hope I fixed all superfluous changes.

[pveber]

I tried to fix the various problems and I rebased the PR on master.

[aantron]

Thanks!

Note that lwt_ppx now implicitly requires OCaml 4.04. It previously required only 4.02.

I've adjusted the CI accordingly.

[pveber]

No problem, thanks for the careful review!