Road to monad: functor

functor.ts

import { Function1 } from 'fp-ts/lib/function';
import { Option, none, some } from 'fp-ts/lib/Option';

// T -> T
const identity = <T>(v: T): T => v;

// (B -> C, A -> B) -> A -> C
const compose = <A, B, C>(f: (Function1<B, C>), g: Function1<A, B>) => (x: A): C => f(g(x));

const plusFive = (n: number): number => n + 5;
const numberToString = (n: number): string => `${n}`;
const plusFiveToNumber = compose(numberToString, plusFive);

// Array
// (A -> B) -> Array<A> -> Array<B>
const map = <A, B, C extends Array<A>>(f: Function1<A, B>) => (U: C): Array<B> => U.map(f);

const array = [1, 2, 3];

// Identity Law
const identityArray = map(identity)(array); // [1, 2, 3]

// Compose Law
const composeArrays = compose(map(numberToString), map(plusFive))(array); // ['6', '7', '8']
const composeArrays2 = map(plusFiveToNumber)(array); // ['6', '7', '8']


// Option = Some | None
const data = some(100);

// Identity Law
const identityOption = data.map(identity); // some(100)

// Compose Law
const composeOptions = data.map(plusFive).map(numberToString); // some('105')
const composeOptions2 = data.map(value => plusFiveToNumber(value)); // some('105')

// Example
type TPerson = { name: string, avatarUrl: Option<string> };

const person: TPerson = { name: 'Oleg', avatarUrl: none };
const getAvatarBG = (url: string): string => `url(${url})`;

const avatarBG = person.avatarUrl.map(getAvatarBG);


// logs
{
    console.group('Array');
    console.group('Identity law');
    console.log('initial data: ', array);
    console.log('after identity: ', identityArray);
    console.groupEnd();
    console.group('Compose law');
    console.log(composeArrays);
    console.log(composeArrays2);
    console.groupEnd();
    console.groupEnd();

    console.group('Option');
    console.group('Identity law');
    console.log('identityOption: ', data);
    console.log('after identity: ', identityOption);
    console.groupEnd();
    console.group('Compose law');
    console.log(composeOptions);
    console.log(composeOptions2);
    console.groupEnd();
    console.groupEnd();
}