PPX: Support open variants

[jchavarri]

Sometimes there are type definitions like this:

type status = [> `Foo | `Bar ]

let handle_status (status : status) : string =
  match status with
  | `Foo -> "It's Foo!"
  | `Bar -> "It's Bar!"
  | _ -> "Unknown status."

This is useful because this code is "future compatible", meaning a server could add new cases without the client crashing, or the other way around.

Would be nice to support it in melange-json PPX.

[anmonteiro]

in [> `Foo | `Bar ] there's a type variable like there's a type variable in 'a StringMap.t. it would be even nicer to provide the function for 'a as well to complete the printing :)

[andreypopp]

I was referring to this .atd's open enum feature https://atd.readthedocs.io/en/latest/atdgen.html#field-open-enum which works either for poly or regular variants.

[andreypopp]

Position: on a variant type comprising exactly one constructor with an argument. The type of that argument must be string. All other constructors must have no arguments.

Hm... wondering what's the motivation for this restriction.

[anmonteiro]

Hm... wondering what's the motivation for this restriction.

probably a small optimization? if you don't allocate + create all ints + one string for the one with payload, you save a little bit?