doc: document the Melange sources / folder structure in CONTRIBUTING.md

[anmonteiro]

No description provided.

[jchavarri]

Thanks for writing this. Looking at the current structure, I can see how much it's been simplified in the last year or two ✨

[jchavarri]

Maybe worth pointing to the current commit in main now, so that at least if folder layout changes in the future, the docs can point to the state where things were written (applies to all links to code in the document).

[anmonteiro]

I was thinking the fact they're on main will let us catch if we ever break them. Meaning the guide should be updated.

[jchavarri]

I was thinking the fact they're on main will let us catch if we ever break them.

I doubt we will know we are breaking them, as there is no mechanism to warn us.

[jchavarri]

One part that confuses me about the current setup is that the PPX is not self-contained right? In the sense that part of the interpretation of these externals happens in the PPX, but another part happens at lambda compilation time.

E.g. the ppx inlines here some int64 constants

melange/ppx/melange_ppx.ml

Line 655 in 1167ca7

pval_prim = External_ffi_types.inline_int64_primitive s;

that are later picked up by lambda

melange/jscomp/core/lam_convert.ml

Line 544 in 1167ca7

| Ffi_inline_const i -> Lam.const i

I am not sure if it's worth mentioning here that melange.ppx is not a traditional ppx, in that sense, but is designed to work with the rest of the compiler.

[anmonteiro]

@jchavarri I made a few changes thanks to your suggestions. Feel free to suggest further improvements.