function just(x) {
  return {
    empty   : false,
    map     : (f) => just(f(x)), // := Functor
    flatMap : (f) => f(x), // := Monad
    ap      : (a) => a.map(x), // := Applicative Functor
    show    : () => 'just(' + x + ')'
  };
}

function none() {
  return {
    empty   : true,
    map     : (f) => none(), // := Functor
    flatMap : (f) => none(), // := Monad
    ap      : (a) => none(), // := Applicative Functor
    show    : () => 'none()'
  };
}

const log = x => console.log( x.show() )

const fn_curry_add = x => y => x + y;
const fn_increment = fn_curry_add(1);
const five = just(5);

//Using map from Funtor :
const inc_five = five.map(fn_increment);
log(inc_five) // just(6)
const inc_inc_five = inc_five.map(fn_increment);
log(inc_inc_five) // just(7)

//Using ap from Applicative Functor :
const add = just(fn_curry_add);
const three = just(3);
const result_of_add = add.ap(five).ap(three);
log(result_of_add) //just(8)

//Using flatMap from Monad :
const half = x => x%2===0 ? just(x/2) : none()
const eighty = just(80);
const chained = eighty.flatMap(half).flatMap(half).flatMap(half).flatMap(half);
log(chained) //just(5)
log(chained.flatMap(half)) //none()