(Moving this to core-libs-dev)

Cay,

Regarding 1, Characteristics was a part of the Gatherers-contract for a very 
long time, alas it didn't end up worth its cost. There's a longer discussion on 
the topic here: 
https://mail.openjdk.org/pipermail/core-libs-dev/2024-January/118138.html (and 
I'm sure that there were more, but that's the one which comes to mind)

Regarding 2, I did have a prototype which had a Downstream in the Combiner, but 
created a new dimension of complexity which made it even harder for people to 
move from sequential to parallelizable. The door isn't closed on it, but I 
remain unconvinced it's worth the surface area for performance reasons.

As a bit of a side-note, it's worth knowing that in the reference stream 
implementation, it is not unusual that parallel-capable stages are executed as 
"islands" which means that short-circuiting signals cannot travel across those 
islands. Since parallel-capable Gatherers can be fused to execute in the same 
"island" if we get to a place where "all" intermediate operations are 
parallel-capable Gatherers, there'd be a single end-to-end "island" and hence 
the ability to propagate the short-circuiting would be preserved in all modes 
of execution. Also worth knowing that a `gather(…)` immediately followed by a 
`collect(…)` can also be fused to run together.

Cheers,
√


Viktor Klang
Software Architect, Java Platform Group
Oracle

________________________________
From: jdk-dev <jdk-dev-r...@openjdk.org> on behalf of Cay Horstmann 
<cay.horstm...@gmail.com>
Sent: Friday, 4 October 2024 19:58
To: jdk-...@openjdk.org <jdk-...@openjdk.org>
Subject: Re: New candidate JEP: 485: Stream Gatherers

Hi, I have some belated questions about the design choices in this API that I 
could not find addressed in the JEP.

1. Why aren't characteristics used to express "greediness/short-circuiting" or 
"sequentialness/parallelizability"?

I understand that for the former I use ofGreedy/of, or implement 
Gatherers.Integrator.Greedy/Gatherers.Integrator. And for the latter, I use 
ofSequential/of, or, if I implement the Gatherer interface, have the combiner 
return defaultCombiner() or not.

But it seems a bit complex and less familiar than the characteristics mechanism 
that exists for spliterators, streams, and collectors.

The original design document (https://cr.openjdk.org/~vklang/Gatherers.html) 
used characteristics, so I wonder what motivated the change.

2. Why wasn't the combiner() designed to allow pushing of elements to the end 
of the first range's sink? Then distinctBy could be parallelized without 
buffering the elements. More generally, with some state fiddling one can then 
handle the elements around range splits.

As it is, I don't see how to parallelize such computations other than to buffer 
all elements.

I looked at the project at https://github.com/jhspetersson/packrat that 
implements a number of gatherers. Only one uses a combiner, to join buffers 
until their contents can be pushed in the finisher.

Cheers,

Cay

On 02/09/2024 19:55, Mark Reinhold wrote:
> https://openjdk.org/jeps/485
>
>    Summary: Enhance the Stream API to support custom intermediate
>    operations.  This will allow stream pipelines to transform data in ways
>    that are not easily achievable with the existing built-in intermediate
>    operations.
>
> - Mark

--

Cay S. Horstmann | https://horstmann.com

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