gencopy fails to build on OCaml 4.13.1

[rwmjones]

+ dune build -j48
File "tools/gencopy.ml", line 79, characters 43-74:
79 |     Pat.construct ?loc ?attrs (lid ?loc s) (may_tuple ?loc Pat.tuple args)
                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Error: This expression has type Parsetree.pattern option
       but an expression was expected of type
         (str list * Parsetree.pattern) option
       Type Parsetree.pattern is not compatible with type
         str list * Parsetree.pattern 

[rwmjones]

This patch is what we're carrying in Fedora. It allows compilation but I've no idea if it's correct:

https://src.fedoraproject.org/rpms/ocaml-migrate-parsetree/blob/rawhide/f/0001-tools-gencopy.ml-Fix-compilation.patch

[pitag-ha]

Ok, thanks for reporting!

It would still be good to keep on using may_tuple since the variant constructor might have no or only one parameter (whereas if you use Pat.tuple always, you're assuming that it has more than one parameter), so I think something like

Pat.construct ?loc ?attrs (lid ?loc s) (Option.map (fun tuple -> ([], tuple)) (may_tuple ?loc Pat.tuple args))

would work.

If this isn't urgent now, I'll look into it in more detail when we need to generate the migration modules for 4.14. We'll need to generate them for ppxlib directly, not for OMP anymore, but we'll still use this same tooling.

[rwmjones]

Sure, no problem at the moment. I have enough of a workaround for now.

[pitag-ha]

Cool! By the way, out of curiosity: What do you need the gencopy for?

[rwmjones]

We just package it in Fedora. I don't have any insight into whether anyone uses it.

[pitag-ha]

This has been fixed in #119.