composability-example08.js

const IO = f => ({
  runIO: f,
  map: g =>
    IO(() => g(f())),
  chain: g =>
    IO(g(f()).runIO)
})

const log = label => x =>
  IO(() => (console.log(label, x), x))

const $ = x =>
  IO(() => document.querySelector(x))

const getText = e =>
  e.textContent

const main = selector =>
  $(selector)
    .map(getText)
    .chain(log('A'))
    .map(s => s.toUpperCase())
    .chain(log('B'))
    .runIO()

main('title')
// A hello world
// B HELLO WORLD