Develop a generative PPX alongside the GraphQL PoC

[Firobe]

See #10687. A lot of the interface code should be generated, as well as some sugaring for queries. For now, the code for this lives in PRs in https://github.com/ylecornec/graphql_poc.

EDIT: the code now lives in src/lib/graphql_ppx in the tweag/graphql_experiments branch

[Firobe]

I played with different ways of generating the needed code. I think of of the easiest way would be to simply annotate a record type, deriver-style, specify information on fields only when strictly needed.

Potentially, queries for all basic types (int, string, float, ...) could be predefined, so that user-defined schemas always have sub-queries among those.

[Firobe]

I've got a working PPX deriver generating all needed types from a single record with annotations. Mutations are not managed yet, but subqueries are.

The PPX lives locally for now, and will be merged in Yves-Stan's POC soon.

[Firobe]

We discussed with @ylecornec and agreed on the following points:

• rename types res to something like modifier
• remove type out (define it as t res)
• instead of deriving from a single type declaration, derive from a whole module (the interface of the module is subject to change), with at least a type t
• have a global simple switch that triggers generate of simple getter fields values. Otherwise, do not generate them
• if field values are not generated, generate instead hidden values that check whether the user-defined values type-check with the generated GADT
• we will now work together directly on a branch on the mina repo
• a response_of_json function should be generated, using only calls to other response_of_json for subtypes
• we should write a small library of such functions for core scalar types (linked to #10925)

[ylecornec]

I added the modules for core scalar types as well as some documentation in the gql_types.ml file of the tweag/graphql_experiments branch.

The new modules are Gql_int, Gql_string, etc..

[Firobe]

A new version of the PPX is pushed on the tweag/graphql_experiments branch in src/lib/graphql_ppx, with a tentative interface for field attributes (see test/example.ml)

[Firobe]

The PPX now generates typ, correctly rewrites fields (with type annotations), and overall generates something that compiles. Next: correctly generate the query type

[Firobe]

Seeing developments on #11081, I'm pausing development on this, since if we can manage to use latest upstream graphql_ppx, it will subsume this current work as it now generate types.

[Firobe]

Closing since it's not clear it's needed anymore. Will reopen if the need arises.