Experiments with optional parameters in ReasonML

Options.re

/**
 * Trying to figure out types of the optional arguments.
 */


let mandatory = (~s: string) => s;
// let mandatory: (~s: string) => string = <fun>;


let optional = (~s: string=?) => s;
/* Error: This pattern matches values of type string
       but a pattern was expected which matches values of type option('a)

    Which is a pity, because the following is a bit of a mouthful.
*/


let optional = (~s: option(string)=?) => s;
/*
Characters 17-18:
Warning 16: this optional argument cannot be erased.
let optional: (~s: string=?) => option(string) = <fun>;
*/


optional(~s="lkj");
// - : option(string) = Some("lkj")


plainOption = (~s: option(string)) => s;
// let plainOption: (~s: option(string)) => option(string) = <fun>;


let plainWrapper = (~sOut=?, ()) => plainOption(~s=sOut);
/* let plainWrapper: (~sOut: string=?, unit) => option(string) = <fun>;

No need for explicitly passed optional because sOut is already an option, and ~s is an option (not optional) too.

Moreover, if you try the expicit passing...
*/
let plainWrapper = (~sOut=?, ()) => plainOption(~s=?sOut);
/*
Characters 52-56:

Warning 43: the label s is not optional.

let plainWrapper: (~sOut: string=?, unit) => option(string) = <fun>;

It works though:
*/
plainWrapper(~sOut="kj", ());
// - : option(string) = Some("kj")


// No warning with optional:
let optionalWrapper = (~sOut=?, ()) => optional(~s=?sOut);
// let optionalWrapper: (~sOut: string=?, unit) => option(string) = <fun>;