New operators

[vxgmichel]

Several operators have been discussed in the issue tracker over the years. Some should probably be added to the library, some would fit well in the examples or the documentation. This issue centralizes those operators and their implementation:

• debounce: Rate limit / throttle operator #70 (comment)
• shortest: asyncio.Event for graceful/early termination #97 (comment)
• stop_when: asyncio.Event for graceful/early termination #97 (comment)
• executor_map: asyncio.Event for graceful/early termination #97 (comment)
• split: How to handle stream splitting #98 (comment)
• tee: How to duplicate streams properly? #120 (comment)
• higherzip: How to handle stream splitting #98 (comment)

[kniteli]

One stream I think should be added to the enhancement list is the never stream.

It's been a bit since I've used this library, but I recall coming up with a way to use never to create a "pausable" stream by switchmapping between it and a "valued" stream. I was show-stopped though by the fact that never's values types are None. I think this is just the wrong type, not only because it severely curtails the usefulness of the never stream, but I don't think it's logically sound either.

Never doesn't yield None values, it yields no values whatsoever. It seems to me it should be valid to place any type you want as the value type of that stream, greatly widening the scope it can be used in without any compromise to soundness.

I don't know if my strange construct would have panned out, but I do think there's a lot of value in that type of stream that can't really be explored right now.

[vxgmichel]

@kniteli

but I recall coming up with a way to use never to create a "pausable" stream by switchmapping between it and a "valued" stream.

Interesting! The never operator returns a Stream[Never] since v0.6.0, could you open a new issue with an example of such use if the Never type did not fix your problem?