1
2class Monad {
3  // pure :: a -> M a
4  pure = () => { throw "pure method needs to be implemented" }
5  
6  // flatMap :: # M a -> (a -> M b) -> M b
7  flatMap = (x) => { throw "flatMap method needs to be implemented" }
8
9  // map :: # M a -> (a -> b) -> M b
10  map = f => this.flatMap(x => new this.pure(f(x)))
11}
12
13export class Option extends Monad {
14  // pure :: a -> Option a
15  pure = (value) => {
16    if ((value === null) || (value === undefined)) {
17      return none;
18    }
19    return new Some(value)
20  }
21
22  // flatMap :: # Option a -> (a -> Option b) -> Option b
23  flatMap = f => 
24    this.constructor.name === 'None' ? 
25    none : 
26    f(this.value)
27
28  // equals :: # M a -> M a -> boolean
29  equals = (x) => this.toString() === x.toString()
30}
31
32class None extends Option {
33  toString() {
34    return 'None';
35  }
36}
37// Cached None class value
38export const none = new None()
39Option.pure = none.pure
40
41export class Some extends Option {
42  constructor(value) {
43    super();
44    this.value = value;
45  }
46
47  toString() {
48    return `Some(${this.value})`
49  }
50}
51