indongyoo

function* filter(f, iter) {
  for (const a of iter) if (f(a)) yield a;
}

function* take(limit, iter) {
  for (const a of iter) if (limit--) yield a;
}

const head = iter => take(1, iter).next().value;

indongyoo

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)

indongyoo

function *entriesLazy(obj) {
  for (const k in obj) yield [k, obj[k]];
}

[...entriesLazy({ a: 1, b: 2 })];
// [['a', 1], ['b', 2]]

function *mapEntriesLazy(f, iter) {
  for (const [k, a] of iter) yield [k, f(a)];
}

indongyoo

// series
const posts = await getPosts();
const users = await getUsers();

// concurrency
const [posts, users] = await Promise.all([
  getPosts(),
  getUsers()
]);

indongyoo

function f1(max) {
  let i = -1, total = 0;
  while (true) {
    let a = ++i;
    a = a * a;
    if (!(a < max)) break;
    total += a;
  }
  return total;
}

indongyoo

const isIterable = a => !!(a && a[Symbol.iterator]);

function *flat(iter) {
  for (const a of iter) {
    if (isIterable(a)) yield *a;
    else yield a;
  }
}

function *deepFlat(iter) {

indongyoo

const log = console.log;

const isIterable = a => !!(a && a[Symbol.iterator]);

function *flatten(iter) {
  for (const a of iter) {
    if (isIterable(a)) for (const b of a) yield b;
    else yield a;
  }
}

indongyoo

function reduce(f, acc, iter) {
  if (arguments.length == 2) {
    iter = acc[Symbol.iterator]();
    acc = iter.next().value;
  }
  for (const a of iter) acc = f(acc, a);
  return acc;
}

reduce((a, b) => a + b, function*() {

indongyoo

function reduce(f, acc, iter) {
  if (arguments.length == 2) {
    iter = acc[Symbol.iterator]();
    acc = iter.next().value;
  }
  for (const a of iter) acc = f(acc, a);
  return acc;
}

function *map(f, iter) {

indongyoo

// 선택된 모든 상품 중 할인 금액이 있는 상품의 총 수량 - pipe 이용
pdts.selected.dq = pipe(
  filter(selected),
  filter(discount),
  _ => pdts.total(quantity, _));

console.log( pdts.selected.dq(products) ); // 5

// 선택된 모든 상품 중 할인 금액이 있는 상품의 총 수량 - pipe + and 이용
pdts.selected.dq = pipe(