Learning TypeScript 2.x - Second Edition

Functor

A Functor is an object that holds a value and implements a method named map. The following code snippet declares a class named Container. This class can be considered a Functor:

class Container<T> { 
    private _value: T; 
    public constructor(val: T) { 
        this._value = val; 
    } 
    public map<TMap>(fn: (val: T) => TMap) { 
        return new Container<TMap>(fn(this._value)); 
    } 
} 

We can use the container as follows:

let double = (x: number) => x + x;
let container = new Container(3); 
let container2 = container.map(double); 
console.log(container2); // { _value: 6 } 

At this point, it might feel like a Functor is not very useful because we have implemented the most basic version possible. The next two sections implement two functors known as Maybe and Either ...