Super simple Task implementation. Lazy evaluation of anything sequentially.

task.js

const Task = run => ({
  // (a -> e) -> (a -> r) -> Task(a) // Might be inaccurate idk
  run,

  // undefined -> undefined
  log: () =>
    run(console.error)
       (console.log),

  // (a -> b) -> Task(b)
  map: f =>
    Task(err => res =>
      run(err)
         (x => res(f(x)) )),

  // (a -> Task(b)) -> Task(b)
  runMap: f =>
    Task(err => res =>
      run(err)
         (x => f(x).run(err)
                       (res) ))
})

/* Sample Usage:

const http = (uri, method = "GET") =>
  Task(err => res => {
    const xhr = new XMLHttpRequest()
    xhr.open(method, uri, true)
    xhr.onload = () =>
      (xhr.status < 300 && xhr.status >= 200)
        ? res(xhr.response)
        : err(xhr.status + " " + xhr.statusText)
    xhr.onerror = err
    xhr.send()
  })

app = http("https://randomuser.me/api")
  .map(JSON.parse)
  .map(x => x.results)

// Run it when you want
app.run(err => console.error(err))
       (res => console.log(res))

// Shorthand for convenience
app.log()
*/