consider combination of drop+serial 'keepLatest'

[deanrad]

As described at Ember Concurrency: http://ember-concurrency.com/docs/task-concurrency#keepLatest

[deanrad]

It occurs to me that each Listener's concurrency mode is a reducer between the current task, and the new one the Listener returned - returning void or a new task..
So maybe we allow for arbitrary reducers somehow, to make keepLatest or whatever- if it follows the rules below:

interface Stoppable {
  stop(): void
}
interface Startable {
  start(o:Observer): Stoppable
}
interface Awaitable {
  then(fn:Function): Awaitable
}
type Task = undefined | (Startable & Stoppable & Awaitable)

const parallel = (current:Task, task:Task) =>
  task.start()

const ignore = (current:Task, task:Task) =>
  current || task.start()

const toggle = (current:Task, task:Task) =>
  (current && current.stop()) || task.start()

const replace = (current:Task, task:Task) => {
  current && current.stop();
  return task.start();
}

const serial = (current:Task, task:Task) =>
  current.then(() => task.start())