Confusing type error when transforming from OCaml's stdlib Array to RescriptCore.Array

[ryyppy]

Minimal example:

open RescriptCore

let arr = [1,2,3]

Array.map((a) => a + 1, arr)

Yields the following type error:

[E] Line 5, column 10:
This has type: \"function$"<'a, [#Has_arity1]>
  Somewhere wanted: array<'b>

Playground Link

The playground comes with uncurried set to true.

I assume \"function$"<'a, [#Has_arity1]> is the internal name for an uncurried callback function?

[cristianoc]

Self contained:

let foo = (x: array<_>) => ()
let x = foo(a => a + 1)

[cristianoc]

This is the parse tree (in ml syntax for readability):

let foo = ((Function$ (fun (x : _ array) -> ()))[@res.arity 1])
let x = ((foo ((Function$ (fun a -> a + 1))[@res.arity 1]))[@res.uapp ])

[cristianoc]

What's going on is that the Function$ processing produces type $function<funtype, arity> but since the type error happens before unification, funtype is still 'a.
So all is known to the type system is that this is a function.