ReScript version: rescript@9.1.4

Consider the following code, and note the difference in optional type annotations:

let f: (~foo: string=?, unit) => string = (~foo: option<string>=?, ()) =>
  switch foo {
  | None => ""
  | Some(x) => x
  }

let _ = f()
let _ = f(~foo="")

The entire function f is annotated as (~foo: string=?) => string. This what code outside of the function sees. foo is a string because outside code will call it like f(~foo="bar").

The inline type annotation for foo is option<string> though, because that’s what code inside the function sees.

Reference

https://forum.rescript-lang.org/t/binding-to-an-option-value/3087/4?u=kevanstannard