[Belt] Add List.filter/WithIndex with deprecation warning and point to List.keep/WithIndex

[kutyel]

Hi, I found this pretty confusing since the native List OCaml does contain a filter method, I guess every js dev would expect Belt.List to have exactly that naming as well.

This also came up during @ryyppy workshop in Alicante and I noticed some people were confused as well.

For now can we consider to have an alias to that fns? @IwanKaramazow also mentioned to me something about a deprecation flag in OCaml but I have literally no idea how to do that 😄

[chenglou]

We should have a filter with an unusable signature, plus a deprecation warning that redirects you to keep. You'd add a [@@ocaml.deprecated "foo"] before the declaration in the mli file.

cc @bobzhang

Edit: or maybe just the deprecation and an alias

[kutyel]

@chenglou hi! Do you mean to mark as deprecated the keep function in favor of filter or the other way round? 😄

[chenglou]

The other way around, so exactly what you did, but with a [@@deprecated "message here"] in the mli file. Also, no need to test it.
We should do the same for other modules too.

[kutyel]

I think this is ready 🚀 @chenglou @bobzhang 😉

[bobzhang]

For the record, the attribute should be put behind the val declaration (not in front of)

[chenglou]

Sorry that's my bad, I was thinking in Reason syntax. @kutyel could you make the change please? And I imagine that when you use filter there's no warning currently?

[bobzhang]

@chenglou I already did, no worries