Implement "withLatestFrom" as an extra operator

I’m looking for a method to accomplish:

--1----2-----3--------4---
----a-----b-----c--d------
       combineLeft
--1?---2a----3b-------4d--

I believe that it is called withLatestFrom in RxJS. I think it is a really common case in cycle.js development.

For now, I’m using:

let withLatestFrom = (a, b) => {
    let c = xs.create()
    let bValue = ''
    b.addListener({
        next: v => { bValue = v },
        error: () => {},
        complete: () => {}
    })
    a.addListener({
        next: v => c.shamefullySendNext([v, bValue]),
        error: () => {},
        complete: () => {}
    })
    return c
}

1. Is it possible, to easly compose it from existing operators?
2. Maybe it could be implemented as an extra?

[staltz]

@xtianjohns thanks for the initiative. I think we should implement all extra operators with OOP and extending the Operator type. This way we have more control over corner cases, and (important) we get to name each producer/operator with a string that will be shown in the devtools.

[Hypnosphi]

One thing that withLatestFrom as an extra could do is fire events from the main stream even before the first event from the sampler stream. The combinator function should be called with only one argument in those cases.

The recommended B.map(b => A.map(a => f(a, b))).flatten() replacement returns

-------2a----3b-------4d--

instead of

--1?---2a----3b-------4d--