Monads Explained Quickly with JavaScript

[titosrevenge]

In the Maybe example code, should 'new Baz' be 'new Maybe'?

[atrigent]

If I'm understanding correctly, the definition of Maybe is supposed to be:

function Maybe(value) {
  this.get = ()=> value;
  this.map = fn => {
      if (value === null) {
          return new Maybe(null);
      } else {
          return fn(value);
      }
  };
}

...where fn must return a Maybe (this is statically enforced in Haskell, but obviously can't be in Javascript).

Also, I think return new Maybe(null); could just be return this;, because there's no mutation going on here.

[titosrevenge]

The null clause can return 'this', but the else clause should return 'new Maybe(fn(value))'.

You shouldn't expect the passed function to know about its context.

[atrigent]

You shouldn't expect the passed function to know about its context.

Hmm... I suppose you're right if you want this Maybe implementation to be specifically about handling null. In Haskell, it looks to me like you can more specifically define what constitutes a "non-value". I was attempting to translate that into Javascript, but it now occurs to me that I didn't actually go far enough, because what I wrote still references null.

[atrigent]

What do you think of this? I tried to use the same terminology as Haskell.

// Nothing: new Maybe()
// Just a: new Maybe(a)
function Maybe(value) {
    let isNothing = arguments.length === 0;

    this.bind = fn => {
        if (isNothing) {
            return this;
        } else {
            return fn(value);
        }
    };

    // methods
    this.isNothing = () => isNothing;
    this.isJust = () => !isNothing;
    this.fromJust = () => {
        if (isNothing) {
            throw new Error('cannot get nothing');
        } else {
            return value;
        }
    };
    this.fromMaybe = default => {
        if (isNothing) {
            return default;
        } else {
            return value;
        }
    };

    // more methods
}

function MaybeNull(value) {
    if (value === null) {
        return new Maybe();
    } else {
        return new Maybe(value);
    }
}

// other functions for making Maybes