maybe-monad.ts

type Nil = null | undefined;

class Maybe<T> {
    private wrappedValue: T | Nil;
    constructor(value: T | Nil) {
        this.wrappedValue = value;
    }

    map<U>(fn: (value: T) => U): Maybe<U> {
        if (!this.wrappedValue) {
            return this as Maybe<any>;
        }
        return new Maybe(fn(this.wrappedValue));
    }

    flatMap<U>(fn: (value: T) => Maybe<U>): Maybe<U> {
        if (!this.wrappedValue) {
            return this as Maybe<any>;
        }
        return fn(this.wrappedValue);
    }

    get<K extends keyof T>(key: K): Maybe<T[K]> {
        return this.map(value => value[key]);
    }

    valueOrDefault(defaultValue: T | Nil): T {
        return this.wrappedValue ? this.wrappedValue : defaultValue;
    }
}

interface Person {
    name: string;
    town: {
        city: {
            country: string;
        }
    }
}

const alice: Person = {
    name: 'Alice',
    town: {
        city: {
            country: 'AliceCountry'
        }
    }
};

const bob: any = {
    name: 'Bob'
};

const maybeAlice = new Maybe(alice);
const aliceCountry = maybeAlice.get('town').get('city').get('country').valueOrDefault('default value');
console.log(aliceCountry); // <= logs 'AliceCountry'

const maybeBob = new Maybe<Person>(bob);
const bobCountry = maybeBob.get('town').get('city').get('country').valueOrDefault('default value');
console.log(bobCountry); // <= logs 'default value'