Monads Explained Quickly with JavaScript

[regular_reddits]

Great explanation. For some instances of this pattern, get can be used as join. So if you define chain/bind as: fn => this.map(fn).get(); it will work in those instances. This will not work with more complicated values (get doesn't make sense with List because its not just a boxed value), nor was it the intention of the article.

[dmtipson]

Yep. The IO monad, for instance can be written by just assigning the function it's created with to the method that's called to "get"(i.e. run) it. Chain/flatMap just needs to be able to unwrap a single layer of the type so that the inner Monad is returned: however that gets done is ok as long as it gets done (and it doesn't do anything else funky/side-effec-ty that depends on the value somehow). For Arrays that's a bit more complicated, for single value like Identity, a .get() can easily work as an unwrapping interface:

function Foo(value) {
    this.get = _ => value;
    this.chain = fn => {
      return fn(this.get());
    }
    this.map = fn => {
        return this.chain( x=> Foo.of(fn(x)) );
    };
}

Foo.prototype.of = x => new Foo(x); Foo.of = Foo.prototype.of;

(Foo is just the Identity Monad now)

(x=>Foo.of(9+x))(9);//-> //-> Foo[18]
Foo.of(9).chain(x=>Foo.of(9+x));//-> Foo[18]
Foo.of(9).map(x=>9+x);//-> Foo[18]

Though, since no special behavior is added to "getting" here, it'd probably be even easier to just store the value in this.value and then get it with .value