onlyWhen.js

import { periodic, sample, filter, multicast } from '../most';

// stream of values we want to filter
// values :: Stream String
const values = periodic(100, 'hi');

// filter condition. For fun, this one just periodically flip-flops
// between true and false, but could be any stream of booleans
// allow :: Stream boolean
const allow = periodic(1000).scan(x => !x, false);

const sentinel = {};

// onlyWhen :: Stream boolean -> Stream a -> Stream a
const onlyWhen = (filterStream, valueStream) => {
	// For now, it's unfortunate that we have to resort to multicast.
	// We can def improve sample() to make this more succinct
	const values = multicast(valueStream);
	return filter(x => x !== sentinel, sample((x, allow) => allow ? x : sentinel, values, values, filterStream));
};

onlyWhen(allow, values).observe(x => console.log(x));