bs.deriving abstract: support functions

[chenglou]

  54 │ [@bs.deriving abstract] type jsProps = {
  55 │   isDoNotDisturbEnabled: bool,
  56 │   onStatusChange: bool => unit,
  57 │ };
  58 │

  syntactic function type is not allowed when working with abstract bs.deriving, create a named type as work around

This error is thrown when we try to put a function signature in a record with bs.deriving abstract. This is out of interop currying concerns, but I think this constraint should be removed:

• It's not going to be enforced consistently anyway, since lots of use-cases will be onStatusChange: Js.nullable(bool => unit), which doesn't error
• Js objects (especially for UI) tend to contain a lot of callbacks. Aliasing them manually each time becomes very tedious.
• We want to use this on bsb-native, so allowing functions make sense.

[bobzhang]

Instead of hacking on non-robust ppx (I could not find a way to erase intermediate types)

• patch the compiler to allow the arity be internally customized
• relax our arity check for bs.get

This could be generalized a little bit, but it is not needed atm

[chenglou]

Can the first change be upstreamed? cc @cristianoc

[chenglou]

Would that make this snippet work too?

https://reasonml.github.io/en/try.html?rrjsx=true&reason=C4TwDgpgBAhgJnKBeKAKAdFAlgO2ASmQD5s8BuAKAoG0ABAIwGd04IAnLAN1wHNYngbGAGNgAXQqhIUejDbIoAbwBmAe1UAuUsAA0sBAF9KNBswC2quAFcANhDFQ6TdJxg2HEAB7B2ONzLkoLVl5FAAiMOM7YFhGRBQQqAAfTHg4DCgARnxKIA

[bobzhang]

I think we can make it work, but it requires some extra effort:

type add = int -> int [@bs ]
type bar = {
  foo: int;
  add: add;}[@@bs.deriving abstract]
external bar : bar = ""[@@bs.module ][@@bs.val ]
let asd = bar |. add 1 [@bs ]

[chenglou]

So at the end, this is possible right?

type bar = {
  foo: int;
  add: int -> int [@bs ];
}[@@bs.deriving abstract]
external bar : bar = ""[@@bs.module ][@@bs.val ]
let asd = bar |. add 1 [@bs ]

If so, I think we can add some special reason sugar for it as the de-facto way to bind to js modules. It's about time, and bs.deriving abstract seems appropriate.

Edit: what does it take to make all the other bs.string and whatever convenience annotations still work? E.g.:

type bar = {
  add: ([ `utf8 | `useAscii [@bs.as "ascii"]] [@bs.string][@bs ]
} [@@bs.deriving abstract]

[malthe]

Was any sugar added for this ultimately?