Unfold promises

In this example we use most.unfold to help deal with promises.

We build up a stream of urls by providing most.unfold creator with a seed or an array of urls.

The unfolding will continue building up the stream each time removing one value off the array, invoking the unfold function with the remaining values of the array.

Once the array is empty tuple.done becomes true and the unfolding will stop.

The interesting part is that we fetch one url at a time, process it before returning a tuple with a new seed.

If we were to run the code below we would observe the 1st url gets processed instantly while the 2nd waits for about 3 seconds before continuing to resolve the last.

import { unfold } from 'most'

const fetch = url => {
	// ... fetch url content and return a promise
	return Promise.resolve('...')
}

const urls = ['http://reqres.in/api/users?page=2',
            'http://reqres.in/api/users?delay=3',
            'http://reqres.in/api/users?page=3']

unfold(urls =>
    urls.length === 0
    ? { done: true }
    : fetch(urls[0]).then(content => {
        return { value: content, seed: urls.slice(1) }
	}, urls))
.observe(x => console.log(x));