findlib files in package metadata should be documented or removed

[whitequark]

Everyone can see these files in the OPAM repository. However, no one knows what function they serve. They are not documented in manual, not mentioned in packaging documentation, do not support optional findlib packages and do not even bother to list these, as in case of lwt. Thus I posit that there is no such function nor they even can meaningfully serve any function.

Multiple times (#2187, #2254) people have asked for clarification on the mechanism behind the findlib files but @AltGr has carefully avoided giving any clarification on that part, going as far as silently closing #2187 as he is wont to do.

Well, I would like to finally have an answer on this, because one of the contributors to opam-cross-android says that the lack of findlib files breaks something yet I don't see any reason why that would be the case, nor is the presence of findlib files even appropriate there because opam-cross-android does not install into %{lib}%.

cc @dbuenzli @Chris00 who were similarly mystified

[dra27]

Looking at the code, I think this may be more one for @samoht?

Virtually everything to do these with these files has been deleted in opam2.

[avsm]

It serves no function in OPAM1 for the main package repository, and CI does not test anything to do with them. I have no objection to removing these files from the main repository.

[whitequark]

@avsm ocaml/opam-repository#9328

[lpw25]

For reference: IIRC these files were used by the old opam-doc prototype for determining which findlib libraries corresponded to which packages. In the newer documentation work this role is handled by odig which does not rely on these files. I think they were also intended to allow some opam support for finding which package contained a particular library but that never materialised.

[AltGr]

I confirm that they have no purpose, and are not used in any way, by the opam tool itself.
They may have a use for some external tools, though.

[whitequark]

Thanks @AltGr @lpw25 @avsm!

[Chris00]

oasis2opam uses them to determine which findlib libraries are provided by an opam package—there is no certain way to do that when the package is not installed. Now that OPAM is widely deployed, it would be IMHO good if there would only be two layers: opam packages providing one or several OCaml libraries (.cma, .cmxa, .cmxs,...).

[whitequark]

@Chris00 I'm not especially convinced, for example, the OCaml packages in Debian do not go through opam.

[Chris00]

But they use ocamlfind — so opam could become a dependency instead of findlib. Of course, that may require opam to have a few additional capabilities (like being able to consider packages coming from the system) but, especially with the trend towards the removal of optional dependencies, opam package dependencies will closely match findlib ones (and .install files give locations and could slowly become mandatory). I think it is a good time to revisit the idea of having less layers in our build systems.

[whitequark]

especially with the trend towards the removal of optional dependencies

Which trend?

[Chris00]

https://discuss.ocaml.org/t/ann-cstruct-3-0-0-with-packaging-changes/284