Re: RFR[8238286]: 'Add new flatMap stream operation that is more amenable to pushing’

[Peter Levart]

Hi Patric, Julia,

On 7/2/20 6:45 PM, Patrick Concannon wrote:
> http://cr.openjdk.java.net/~pconcannon/8238286/webrevs/webrev.03/  <http://cr.openjdk.java.net/~pconcannon/8238286/webrevs/webrev.03/>


Then I'd like to discuss cancelation (shortcut Streams). Currently 
mapMulti does not do anything to optimize processing if the downstream 
decides it does not need more elements but just keeps pushing. This is a 
decision I guess to keep the API surface clean and simple. All 
short-cutting ops or even intermediate (like limit) must therefore 
ignore the surplus elements that are emitted by mapMulti and they do so. 
If the intermediate operations up-stream mapMulti are respecting the 
cancelation, this results in at most one invocation of mapMulti function 
producing surplus elements, but if they don't, then multiple invocations 
of mapMulti function is producing surplus elements.

For example:


     someStream

         .mapMulti((e, sink) -> { .... sink.accept(x); ... })

         .filter(x -> ...)

         .mapMulti((x, sink) -> { ... sink.accept(y); ...})

         .limit(10)

         ...


Here the 1st mapMulti emits elements and each of them is filtered and 
then maybe passed to 2nd mapMulti. the 2nd mapMulti could skip calling 
its function if the downstream limit(10) has already passed 10 elements.


WDYT?


Regards, Peter