Hands-On Functional Programming with TypeScript

Maybe

The following Maybe data type is a Functor and an Applicative, which means that it contains a value and implements the map method. The main difference with the preceding implementation of Functor is that the value contained is optional:

class MayBe<T> {    public static of<TVal>(val?: TVal) {        return new MayBe(val);    }    private _value!: T;    public constructor(val?: T) {        if (val) {            this._value = val;        }    }    public isNothing() {        return (this._value === null || this._value === undefined);    }    public map<TMap>(fn: (val: T) => TMap) {        if (this.isNothing()) {            return new MayBe<TMap>();        } else {            return new MayBe<TMap>(fn(this._value));        }    }    public ap<TMap>(c: MayBe<(val: T) => TMap>) {        return c.map(fn => this.map(fn));    }}

As we can see in the preceding ...