Replace camlp4 with ppx

[rgrinberg]

I'm evaluating macaque for my own project which already moved to ppx. Unfortunately that means that macaque's camlp4 makes it incompatible. Are there any plans to port macaque to ppx?

[gasche]

It should not be terribly hard to implement this in a minimal way. Macaque was intentionally designed to avoid extending the OCaml grammar, it almost exclusively relies on <:foo< .. >> quotations, which directly translate into [%foo {| ... |}] extensions. The easy way to do this is to keep using the Camlp4-based parsers to parse the in-extension quotation, which means the result would be a ppx extension that depends on Camlp4 as a library. One still has to update the ast-production facility at the end of the chain to produce OCaml Ast directly instead of Camlp4's representation of the AST, but that should not be a lot of work.

There is one (optional) part of Macaque that changes the OCaml grammar, it is pa_banana that allows chaining of structured row data access, using the foo#!bar#!baz syntax. I don't recall the exact detail of what was integrated in the OCaml syntax to allow this, but I think the exact syntax or something very close (possibly ##! and ##?). In any case, this small surface syntax is non-essential and could also be dropped.

However, I don't have time to implement this in the short-term. I would be happy to accept contributions and provide code review and recomendations, though; it is in that spirit that I tried to provide a detailed forward path above.

Finally, as I have already discussed with several Macaque users, I would be interested in eventually transitioning from the set-comprehension syntax to something closer to the actual SQL syntax (to avoid having to do big syntactic brainstorming each time we want to add a new SQL feature; just reuse the damn thing). @Drup started working on that but nothing conclusive emerged so far, and I think his work on this is stalled. @hnrgrgr has developped a SQL parser in OCaml that I think we could reuse. Developing a ppx extension means changing the syntax, so it could be a good time to think about also providing a SQL-based one -- but there is no hurry on this one.

[chrismamo1]

Has there been any motion on this? If not, I'm currently looking at a project where I may be needing something like this, in which case I'd be happy to do the footwork.

[gasche]

No, I don't think anyone has worked on it since my last message above, sorry. If you are willing to implement something, I would be happy to help review and integrate it. (I don't remember anything about the implementation, so we would be on an equal footing :-)

[Drup]

there are now several SQL query parsers in the community which we could rely on directly, in order to apply @gasche's plan.

Although, to be really serious about this, I think one would need to evaluate all the other SQL type encoding that have emerged since then, notably Oleg's one.

[XVilka]

Camlp4 release for OCaml 4.08 is the last release ever - see the official announcement.