basic monad example

monad.js

function square (x) {
  return [x * x, () => console.log(x * x)]
}

function feet (x) {
  return [x * 3.28084, () => console.log(x * 3.28084)]
}

function compose (fn1, fn2) {
  return (x) => fn2(fn1(x))
}

function bind (fn) {
  return ([x, prevEffect]) => {
    const [result, effect] = fn(x)
    return [result, () => { prevEffect(); effect()}]
  }
}

function unit(x) {
  return [x, () => {}]
}

const squareFeet = compose(unit, compose(bind(feet), bind(square)))

const meters = 5

const [result, effect] = squareFeet(meters)