Hands-On Functional Programming with TypeScript

Functors

The Functor type has two main characteristics:

• It holds a value
• It 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:

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

At this point, you may think that the Functor type is not very useful because we have implemented the most basic version possible. The next two sections implement two ...