Doing some functional stuff with state data structure. Here's an example of usage.

buy-stuff-state.js

var housePrice = 15000;

var buyCoke = setupPrice('coke', 50);
var buyHotDog = setupPrice('hot-dog', 150);
var buyHouse = setupPrice('house', housePrice);

var salary = 500;

var getPaid = modifyState(sum.bind(null, salary));
var saveForHouse = replicate( Math.floor(housePrice / salary), getPaid);

var initialAmount = 0;
var buySomething = getPaid.then(buyCoke)
                          .then(buyHotDog)
                          .then(buyCoke)
                          .then(getPaid)
                          .then(buyHotDog)
                          .then(buyCoke)
                          .then(saveForHouse)
                          .then(buyHouse);

var amountLeft = evalState(buySomething, initialAmount);

console.log('Trying to buy some stuff. At the end got: ', amountLeft);