----- Original Message -----
> From: "Viktor Klang" <vkl...@openjdk.org>
> To: "core-libs-dev" <core-libs-dev@openjdk.org>
> Sent: Tuesday, November 14, 2023 11:32:48 AM
> Subject: Re: RFR: 8319123: Implementation of JEP-461: Stream Gatherers
> (Preview) [v7]
> On Tue, 14 Nov 2023 09:48:13 GMT, Rémi Forax <fo...@openjdk.org> wrote:
>
>> Hello, the relation between a stateless gatherer (default initializer) and
>> the
>> default combiner are not obvious to me.
>>
>> A default initializer means stateless and a default combiner means
>> sequential,
>> so if i want the integrator of my stateless gatherer to be called in
>> parallel,
>> i need to not use the default combiner but at the same time the combiner I
>> will
>> pass as parameter will not be called because the gatherer is stateless ?
>>
>> Having to create a combiner that will be not called seems weird. I'm sure i'm
>> missing something ?
>
> It stems from the observation that the nature of something being stateless
> does
> not mean that it doesn't have restrictions on encounter order (see forEach and
> forEachOrdered as an example).
yes,
> Parallelization means foregoing encounter-order
> (of execution -- not necessarily of output), so by requiring to specify a
> combiner to opt into parallel (unordered execution) means that the default is
> correct and safe under all evaluation modes, and opting into out-of-order
> execution is inspectable (in terms of ocular inspection of code).
I've no problem with intermediary operation having to opt-in to be parallel.
But I found the API surprising, the way to opt-in is to provide a combiner
which does not work well with a stateless intermediary operation which by
definition does not need a combiner.
Why not either using capabilities (like Spliterator and Collector) or
subclasses like Greedy, or any other mechanisms instead of the concept of a
default initializer/combiner ?
The idea of default initializer/combiner is that those are orthogonal, you can
choose the default initializer and the default combiner separatly, but an
initializer and a combiner are related, they work together so the idea of
default initializer/combiner is leaky. A contrario, there is no such isseu with
the default finisher because this one is independant of the others methods of a
Gatherer.
Rémi
>
> -------------
>
> PR Comment: https://git.openjdk.org/jdk/pull/16420#issuecomment-1809940566
