map, flatMap, then.js

const log = a => console.log(a);

// 0.
// map :: Functor f => (a -> b) -> f a -> f b

// 1. Array.prototype.map
// Array r => (a -> b) -> r a -> r b

// [1] : r a
// a => a + 1 : (a -> b)
// rb : r b
// forEach : r b 안에 있는 값 b에 접근
const rb = [1].map(a => a + 1);
rb.forEach(b => log(b)); // 2

const rb2 = [1].map(a => [a + 1]);
rb2.forEach(log); // [2]
rb2.forEach(fb => fb.forEach(log)); // 2

// 2. Array.prototype.flatMap
// Array r => (a -> b) -> r a -> r b
// Array r => (a -> r b) -> r a -> r b
const rb3 = [1].flatMap(a => a + 1);
rb3.forEach(b => log(b)); // 2

const rb4 = [1].flatMap(a => [a + 1]);
rb4.forEach(b => log(b)); // 2

const rb5 = [1].flatMap(a => [[a + 1]]);
rb5.forEach(b => log(b)); // [2]

// 3. Promise.prototype.then
// Promise p => (a -> b) -> p a -> p b
// Promise p => (a -> p b) -> p a -> p b
// Promise p => (a -> p p b) -> p a -> p b

// Promise.resolve(1) : p a
const pb = Promise.resolve(1).then(a => a + 1);
pb.then(b => log(b)); // 2

const pb2 = Promise.resolve(1).then(a => Promise.resolve(a + 1));
pb2.then(b => log(b)); // 2

const pb3 = Promise.resolve(1).then(a => Promise.resolve(Promise.resolve(a + 1)));
pb3.then(b => log(b)); // 2