"Cancelable" streams

[Brian Goetz]

> The main issue is that your example uses side effect,
>    () -> cancelFlag.set(true)
> which goes against the model

The model is not "side effects are illegal"; we support forEach() and 
into() which are side-effectful.  The model is more "don't use 
side-effects where they are not needed."

>  > Here, you might want to process until some threshold has been reached
>  > (find the first N results), until some external event has occured
>  > (process for five minutes and take the best result; process until the
> asked to shut down.)
>
> find the N results => use limit()

Parallel limit has some serious limitations that make it pretty 
unsuitable for this case.  While these may be fixable, the effort and 
distortion involved is far, far greater than what is being suggested 
here.  In fact, I'm on the fence about whether to keep limit at all in 
its current state; I worry people will expect more of it than it can 
deliver, and be unhappy.

> fitting the whole word and the kitchen sink into the Stream API is not a
> goal.

No, but this is a pretty lightweight cancelation mechanism -- far more 
lightweight than limit as currently implemented.  We've talked to 
customers who are very interested in using this for processing infinite 
event streams.  The only thing missing is "how do I make it stop."