ppx: allow dropping arbitrary values with [@drop_default]

[zazedd]

This PR allows [@drop_default] to drop default values provided by [@default].
It also changes [@drop_default] to potentially accept a function for comparison. If no function is provided, then structural equality is used.

[@drop_default] with [@option] remains unchanged

Not sure if ppx_compare works with melange. If yes I can change this PR to be closer to how ppx_yojson_conv implements their similar feature

Motivation

It brings melange-json closer to being able to substitute atd, and makes sense as an extension of the [@drop_default] attribute, which currently makes melange-json unable to fully express some records without making fields needlessly optional

[andreypopp]

I wonder if we can do the following:

• have [@drop_default] require equal_<type> to be present for the corresponding types
• have [@drop_default_if_json_equal] uses Melange_json.equal (to be added)

that way we won't be relying on structural equality

[zazedd]

Sounds good.

To be clear, we want to replace structural equality by equal_<type>, which has to be available in the scope if [@drop_default] is used in flag mode with [@default]? Can the user can still provide a comparison function f of his accord using [@drop_default f]?

Melange_json.equal (to be added)

This should be easy to add, but I'll just leave it commented out since Melange_json.equal isn't here yet

[andreypopp]

To be clear, we want to replace structural equality by equal_, which has to be available in the scope if [@drop_default] is used in flag mode with [@default]?

Yes

Can the user can still provide a comparison function f of his accord using [@drop_default f]?

Can keep that too, I guess.

This should be easy to add, but I'll just leave it commented out since Melange_json.equal isn't here yet

Let's add it? Should be pretty easy.

[zazedd]

Updated

• Melange_json.equal and [@drop_default_if_json_equal] implemented
• [@drop_default] with equal_<type> implemented. In the case of parameterized types, the equal function takes the inner type's equal as a parameter, e.g. int list generates equal_list equal_int

Please check correctness of Melange_json.equal for JS. Native is easy enough since we have Yojson's comparison function, but for JS it is always scary to write all those Obj.magic's 😅

[andreypopp]

thanks! (left a small nit comment though)

[andreypopp]

can use https://ocaml-ppx.github.io/ppxlib/ppxlib/Ppxlib/Expansion_helpers/index.html#val-mangle_lid

[zazedd]

Oh nice, I didn't know about this. Updated

[andreypopp]

@anmonteiro @jchavarri @davesnx anything we missed here? If no objections/comments, I'm going to merge in the coming days.

[davesnx]

I might prefer to have [@drop_default] and [@drop_default { equal }] rather than a separate extension, so users could [@drop_default { equal: Melange_json.equal }] or some other fn

but that can also be added/improved later