A callbag operator that maps items to sources and flattens them to a single output source.

npm install callbag-flat-map-lazy

This is a fork of callbag-flat-map.

The basic difference between the two is that the source returned by callbag-flat-map is always listenable, whereas the source returned by callbag-flat-map is pullable if the inner sources are pullable. For more information on how that is achieved, see Details

const flatMap = require('callbag-flat-map-lazy');
const fromIter = require('callbag-from-iter');
const of = require('callbag-of');
const pipe = require('callbag-pipe');
const toIterable = require('callbag-to-iterable');

// Pullable sources
const iterable = pipe(
  fromIter([1, 2]),
  flatMap(char => fromIter(['a', 'b']), (a, b) => char + num),
  toIterable
);

for (const i of iterable) {
  console.log(i);                // Logs 1a, 1b, 2a, 2b
}

// Listenable sources
pipe(
  of(1, 2),
  flatMap(char => of('a', 'b'), (a, b) => char + num),
  forEach(x => console.log(x))   // Logs 1a, 1b, 2a, 2b
);

The output source returned by callbag-flat-map-lazy forwards pulls to the oldest inner source. When the last inner source completes, the input source is pulled in order to create the next inner source.

To support mapping a pullable source to listenable inner sources, the input source is pulled once initially to establish the first inner source.