dewey92

type Tree<A> = Leaf<A> | Branch<A>;
type Leaf<A> = { _f: 'tree', kind: 'leaf', value: A };
type Branch<A> = { _f: 'tree', kind: 'branch', left: Tree<A>, right: Tree<A> };

function mapTree<A, B>(map: (val: A) => B, tree: Tree<A>): Tree<B> {
  switch (tree.kind) {
    case 'leaf':
      return leaf(map(tree.value));
    case 'branch':
      return branch(

dewey92

fillNames :: String -> String -> String -> String
fillNames firstN middleN lastN = firstN ++ middleN ++ lastN

a :: String -> String -> String
a = fillNames "Jihad"

b :: String
b = a " Dzikri" " Waspada"
b == "Jihad Dzikri Waspada" -- True

dewey92

const fillNames = firstN => middleN => lastN =>
  firstN + middleN + lastN

const a = fillNames('Jihad');
const b = a(' Dzikri')(' Waspada')

b === 'Jihad Dzikri Waspada' // true

dewey92

const add = x => y => x + y

const identity = add(0)
const increment = add(1)
const decrement = add(-1)

identity(7)  // 7
increment(7) // 8
decrement(7) // 6

dewey92

const fillNames = (firstN, middleN, lastN) =>
  firstN + middleN + lastN

const a = fillNames.bind(null, 'Jihad');
const b = a(' Dzikri', ' Waspada')

b === 'Jihad Dzikri Waspada' // true

dewey92

const FORM_ID = 'personal-details'

// Factory
const onChange = (fieldName: string) => e => {
  notifyParentOnFieldChange(FORM_ID, fieldName)

  // actual computaion
  const value = e.target.value
  ...
}

dewey92

const liftA2 = f => (m, n) => m.map(f).ap(n);

const add = x => y => x + y;
const monadicAdd = liftA2(add);

add(5, 6) // 11
monadicAdd(Just(5), Just(6)) // Just(11)

dewey92

const memoize = fn => {
  const cache = {}
  return (…args) => {
    const stringifiedArgs = JSON.stringify(args)
    const result = cache[stringifiedArgs] || fn(…args)
    cache[stringifiedArgs] = result
    return result
  }
}

dewey92

const numAddition = createMonoid({
  indentity: 0,
  semigroup: (a, b) => a + b,
});

log(numAddition.id) // => 0
log(numAddition.append(1, 2)) // => 3
log(numAddition.append(3, numAddition.id)) // => 3

dewey92

const numMult = createMonoid({
  indentity: 1,
  semigroup: (a, b) => a * b,
});

log(numMult.id) // => 1
log(numMult.append(1, 999)) // => 999
log(numMult.append(999, numMult.id)) // => 999