Differentiate between fn(.) and fn(.())

[ryyppy]

Extracted from this issue:

Currently they are all de-sugared into OCaml syntax fn () [@bs]

The thing is that in BuckleScript, since there is no syntax space for arity zero, so we make a special form that fn () [@bs] is treated arity zero application.

I wonder Reason could make a difference here which feels more intuitive:

fn (.) --> arity zero -> fn () [@bs]
fn (. ()) --> arity 1 -> let unit = () in fn unit of course, some care need to be taken to avoid name collision

Example where this causes an issue:

Js.Promise.make((~resolve, ~reject as _) => Js.Global.setTimeout(() => {
      resolve(. ())
}, 1500)->ignore)

See Playground Link