def foldM[U, T](zero: U)(f: (U, T) ⇒ Future[U]): Sink[T, Future[U]] = {
Sink.fold[Future[U], T](Future.successful(zero)) { (fu, t) =>
println(s"inside fold $t")
fu.flatMap { x =>
println(s"inside flatMap $x")
f(x, t)
}
}.mapMaterializedValue(_ flatMap identity)
}
val result = Source(1 to 10).runWith(foldM(0)((a, x) => Future.successful(a +
x)))
println(Await.result(result, 10.minutes))
prints:
inside fold 1
inside fold 2
inside fold 3
inside fold 4
inside fold 5
inside fold 6
inside fold 7
inside fold 8
inside flatMap 0
inside fold 9
inside fold 10
inside flatMap 1
inside flatMap 3
inside flatMap 6
inside flatMap 10
inside flatMap 15
inside flatMap 21
inside flatMap 28
inside flatMap 36
as you can see, the fold produces a stream of futures that are created
instantly (I guess they are not technically unbounded because it's a fold,
but there's no backpressure), while the actual folding operation happens on
the executionContext you have in scope (because of the flatMap).
That is basically the same as iteratively calling flatMap N times and
waiting for the result, because we don't have backpressure.
Roland and everyone else: please correct me if I'm wrong!
Cheers
On Monday, 7 March 2016 21:08:15 UTC+1, Andrew Gaydenko wrote:
>
> On Sunday, March 6, 2016 at 2:28:22 AM UTC+3, Giovanni Alberto Caporaletti
> wrote:
>>
>> Hi Roland,
>> you're right, my solution was a bit naive. I came up with this, I'm
>> pretty sure it can be done in a better way, looking forward to seeing your
>> solution :)
>>
>
> Giovanni, thanks for this graph - I use as example on reading streams doc
> :)
> Have you idea how to provoke those "unbounded amounts of futures without
> back pressure" Roland has wrote about? - to get OOME or.. what?
>
--
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>>>>>>>>>> Check the FAQ:
>>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html
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