Is there a way to give curried arrow functions a type/tag?

Question

Function encoded types (i.e. nested curried functions) have some drawbacks in Javascript:

• Their representation in the dev console is obfuscated (e.g. [Some(5), None] is displayed as [f, f])
• Nothing stops you from applying a combinator to the wrong type (e.g. eitherMap(f) (Some(5)))
• You cannot inspect their free variables
• You cannot even duck type them

These drawbacks render them useless in real world applications.

I was wondering if there is a way to overcome these shortcomings and came up with the following sketch:

const tag = (name, x, k) => {
  const Cons =
    Function(`return function ${name}() {}`) ();

  Cons.prototype[Symbol.toStringTag] = name;
  Cons.prototype["run" + name] = k;
  Cons.prototype.tag = name;

  const o = new Cons();
  Object.defineProperty(o, "value", {get: () => x});
  return o;
};

const Some = x =>
  tag("Option", x, def =>
    tag("Option", x, k => k(x)));

const None = tag("Option", null, def =>
  tag("Option", def, k => def));

const option = def => k => fx =>
  fx.runOption(def).runOption(k);

const safeInc = option(0) (n => n + 1);

safeInc(Some(5)); // 6
safeInc(None); // 0

const xs = [Some("foo"), None]; // [Option, Option]

/* 
  expanded dev console display:

  1: Option {value: ...} --> expands to "foo"
  2: Otpion {value: ...} --> expands to null
*/

Please note that I am not interested in prototypal inheritance at all.

This approach is both tedious and probably slow, because I apply the Function constructor, which makes the code less predictable. Is there a better way to give curried functions a type (or rather a tag in JS), so that the listed shortcomings are eliminated?

Answer 1

I slightly improved my approach and got rid of the Function call and the repeated creation of a constructor function. It's appropriate for my specific use case (function encoded types) but I failed to address the more general case of arbitrary functions in curried form, because it is still too tedious. Anyway, here it is:

const tag = Cons => (k, ...args) => {
  const o = new Cons();

  Object.defineProperties(o, {
    "value": {get: () => args},
    "runOpt": {value: k}});

  return o;
};


const Opt = tag(
  function Option() {
    Option.prototype[Symbol.toStringTag] = "Option";
    Option.prototype.tag = "Option";
  });


const Some = x =>
  Opt(def => Opt(k => k(x), x), x);

const None = Opt(def => Opt(k => def, def));

const option = def => k => fx =>
  fx.runOpt(def).runOpt(k);

const safeInc = option(0) (n => n + 1);

safeInc(Some(5)); // 6
safeInc(None); // 0

Some(5); // Option {runOption}
[Some("foo"), None]; // [Option, Option]

/*
  expanded dev console display:

  1: Option {runOpt: f, value: ...} --> expands to ["foo"]
  2: Otpion {runOpt: f, value: ...} --> expands to []
*/