[@bs.as] for special BS object creation function

[chenglou]

(* invalid syntax, bear with me for now. I dunno how to attach an attribute onto an argument label *)
external makeConfig : (high [@bs.as "high-value"]):int -> ?low:int -> unit -> _ = "" [@@bs.obj]
let c1 = makeConfig ~high:3 ()

It'd generate {"high-value": 3}.

Super useful. The special object creation is the only one that allows optional fields, so we can't directly create the objs on the Reason side through {"high-value": 3}. For example: https://github.com/reasonml/reason-react/blob/7eed3579ebfc7402eebe331b740b0a8bed4e44b5/src/ReactDOMRe.re#L95

cc @rickyvetter @thangngoc89 @yawaramin @glennsl

[glennsl]

Love the idea, but slightly concerned about overloading the meaning of bs.as. With emphasis on "slightly" since it's already overloaded and I haven't noticed any issues with that.

[bobzhang]

The main thing is if we infer the output type still as < high : int; low : int > Js.t then the access is incorrect. If you infer it as < "high-value" : int ; low : int > Js.t, then it does not feel right to me.

The only sensible way to me is require the output to be an abstract type.

By tweaking the spec of bs.deriving abstract, we are quite close to it:

type label = {
   high : int  [@bs.as "high-value"];
   low : int [@bs.optional]; (* better name or just use `int option` *)
} [@@bs.deriving abstract]

In that case, we derive such functions:

val label : high : int -> ?low: int -> unit -> label
val high : label -> int 
val low : label -> int option

So we need implement bs.as and option support.

For bs.as, the rough idea is

external label : high:int -> low: int -> label = "xxx" (* the internal encoding changed to "high-value" *)
external high : label -> ( _ : [@bs.as  "high-value"]) -> int = "" [@@bs.get_index] 

For bs.optional, the rough idea is

external label : high: int -> ?low: int -> label = "" [@@bs.obj]
external low : label -> int = "low" [@@bs.return undefined_to_opt]

The two attributes may interact, this is quite a few work, lets make it for next release

[bobzhang]

relevant to #2583

[bobzhang]

Currently
type x = { hi: int} [@@bs.deriving abstract],
will generate val x : hi:int -> x, we may want to allow customize the name of constructor,
the proposed syntax:

type x = { hi: int} [@@bs.deriving {abstract = constructorName, "make"}]  

What do you think?