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Hi Tyler,</div>
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Thank you for the kind words -- they are much appreciated. And you have a very good question indeed!<br>
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My thinking behind making <i>fold</i> a Gatherer is that I think that it is strictly more powerful than "only" having it as a collector (It wouldn't be able to be a Collector since you'd need a combiner for it, but also being able to compose it with other operations,
choosing the output type at a later stage in the process, and so forth).<br>
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Also, if you think about it -- single-element Streams are just as valid as empty Streams, N-sized Streams, or even unbounded Streams, and conceptually, there's no difference between the following two Streams:</div>
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var a = Stream.of("1234")<br>
var b = Stream.of(1,2,3,4).gather(fold(() -> "", (str, next) -> str + next))</div>
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So that was my thinking—allow developers to stay within Stream processing for as long as they want, and choose the terminal operation when they need to break out from the Stream.</div>
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Cheers,<br>
√</div>
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<b><br>
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<b>Viktor Klang</b></div>
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Software Architect, Java Platform Group<br>
Oracle<br>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Tyler Kindy <me@tylerkindy.com><br>
<b>Sent:</b> Monday, 30 October 2023 12:14<br>
<b>To:</b> Viktor Klang <viktor.klang@oracle.com><br>
<b>Cc:</b> core-libs-dev@openjdk.org <core-libs-dev@openjdk.org><br>
<b>Subject:</b> [External] : Re: Update on JEP-461: Stream Gatherers (Preview)</font>
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<div dir="ltr">Thanks for the JEP and your talk, Viktor! I think `Stream::gather` will be super useful in my day-to-day as a Java developer.
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<div>I’m curious why `<font face=".AppleSystemUIFont">fold`</font> is being implemented with gatherers. I recognize `Gatherer` is designed to support intermediate operations, but `fold` feels inherently like a terminal operation to me since it, like `reduce`
or `collect`, consumes all the elements in the stream and produces a single result.</div>
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<div>Is there a technical limitation to making `fold` a terminal operation? For example, does `Collector` inherently presume parallelization in a way that `Gatherer` does not?</div>
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<div>Or is the idea mainly to demonstrate the power of gatherers, and we could also make `fold` a terminal operation with the current `Stream` API?</div>
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<div>Thank you!</div>
<div>Tyler Kindy<br id="x_lineBreakAtBeginningOfMessage">
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<div>On Oct 27, 2023, at 9:50 AM, Viktor Klang <viktor.klang@oracle.com> wrote:</div>
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Greetings,</div>
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As you may have already seen,<span class="x_Apple-converted-space"> </span><b>Stream Gatherers</b><span class="x_Apple-converted-space"> </span>is now a<span class="x_Apple-converted-space"> </span><a href="https://openjdk.org/jeps/461" title="https://openjdk.org/jeps/461" id="LPlnk995248" class="x_OWAAutoLink">Preview
JEP with Candidate status</a></div>
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Work-in-progress (interfaces, implementation, tests, benches, and documentation) for JEP-461 is currently available<span class="x_Apple-converted-space"> </span><a href="https://urldefense.com/v3/__https://github.com/viktorklang-ora/jdk/tree/gatherer__;!!ACWV5N9M2RV99hQ!Lz2sC02xW35XpuwoaqBvD_iR80Xrzkbj-60oOKuQklUOm8e69-O3WC9N93leBFbMkFmULJmsat9k1dmt$" title="https://github.com/viktorklang-ora/jdk/tree/gatherer" id="OWA8ab01dac-cb46-bda0-009f-d3a4a0a6e81e" class="x_OWAAutoLink x_ContentPasted0">here</a>.<br>
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While<span class="x_Apple-converted-space"> </span><a href="https://cr.openjdk.org/~vklang/Gatherers.html" title="https://cr.openjdk.org/~vklang/Gatherers.html " id="LPlnk332869" class="x_OWAAutoLink">the design</a><span class="x_Apple-converted-space"> </span>has
held up well, there are some important improvements made since the original design document was written.<br>
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Notable changes (without any particular order):</div>
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<li style="list-style-type:disc"><b>Stream::gather()</b><span class="x_Apple-converted-space"> </span>now has a default implementation.<br>
<br>
<span></span></li><li style="list-style-type:disc"><b><span>Gatherer</span><span></span>::supplier()<span class="x_Apple-converted-space"> </span></b>was renamed to<span class="x_Apple-converted-space"> </span><b>Gatherer::initializer()</b><span class="x_Apple-converted-space"> </span>to
better reflect intent.<br>
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</li><li style="list-style-type:disc"><b>Gatherer.Sink<R></b><span class="x_Apple-converted-space"> </span>was renamed to<span class="x_Apple-converted-space"> </span><b>Gatherer.Downstream<R></b><span class="x_Apple-converted-space"> </span>to better signal what
it represents.<br>
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</li><li style="list-style-type:disc"><b>Gatherer::collect(Collector)</b><span class="x_Apple-converted-space"> </span>and its companion type<span class="x_Apple-converted-space"> </span><b>Gatherer.ThenCollector</b><span class="x_Apple-converted-space"> </span>was
dropped due to compatibility concerns with existing code which operates on<span class="x_Apple-converted-space"> </span><b>Collector</b>.<br>
<br>
</li><li style="list-style-type:disc">
<div><span><b>Gatherer.Characteristics</b><span class="x_Apple-converted-space"> </span>have been eliminated and superseded by having default values that are used as sentinels.<br>
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(discussed further down the list)<br>
<div><span>This is important because with the Characteristics-model keeping alignment between Characteristics and actual implementation proved brittle, and under composition of Gatherers computing the union was inefficient and mostly lead to an empty set in
the end.<br>
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</li><li style="list-style-type:disc"><span><b>Gatherer.defaultInitializer()</b>,<span class="x_Apple-converted-space"> </span><b>Gatherer.defaultCombiner()</b>, and<span class="x_Apple-converted-space"> </span><b>Gatherer.defaultFinisher()</b><span class="x_Apple-converted-space"> </span>were
added as static methods—these are the sentinels used to elide calling the initializer, to elide calling the combiner (avoid parallelization), and to elide calling the finisher, respectively.<br>
<br>
</span></li><li style="list-style-type:disc"><span><b>Gatherer::initializer()</b>,<span class="x_Apple-converted-space"> </span><b>Gatherer::combiner()</b>, and<span class="x_Apple-converted-space"> </span><b>Gatherer::finisher()</b><span class="x_Apple-converted-space"> </span>default
implementations return the respective sentinels.<br>
<br>
</span></li><li style="list-style-type:disc"><span>A subtype of<span class="x_Apple-converted-space"> </span><b>Integrator</b><span class="x_Apple-converted-space"> </span>named<span class="x_Apple-converted-space"> </span><b>Greedy</b><span class="x_Apple-converted-space"> </span>was
added, together with a factory method to guide lambda-to-nominal-type conversion. This allows creators of Gatherers to signal that an<span class="x_Apple-converted-space"> </span><b>Integrator</b><span class="x_Apple-converted-space"> </span>will never initiate
a short-circuit (but may relay one from downstream), and that is available during evaluation to determine if the operation can short-circuit or not.</span>
<div style="list-style-type:disc"><br>
<span></span></div>
</li><li style="list-style-type:disc"><span>Factories for creating anonymous Gatherers were expanded upon to include<span class="x_Apple-converted-space"> </span><b>Gatherer.of()</b><span class="x_Apple-converted-space"> </span>and<span class="x_Apple-converted-space"> </span><b>Gatherer.ofSequential()</b><span class="x_Apple-converted-space"> </span>with
different sets of parameters, primarily to make it more ergonomical and easier to read and write the code using those factories.</span>
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<span></span></div>
</li><li style="list-style-type:disc"><span>A curated set of initial built-in Gatherers is located in<span class="x_Apple-converted-space"> </span><b>java.util.stream.Gatherers</b><br>
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I recently presented<span class="x_Apple-converted-space"> </span><a href="https://urldefense.com/v3/__https://www.youtube.com/watch?v=8fMFa6OqlY8__;!!ACWV5N9M2RV99hQ!Lz2sC02xW35XpuwoaqBvD_iR80Xrzkbj-60oOKuQklUOm8e69-O3WC9N93leBFbMkFmULJmsam2MP-_S$" title="https://www.youtube.com/watch?v=8fMFa6OqlY8" id="LPlnkOWALinkPreview_2" class="x_OWAAutoLink">Gatherers
at Devoxx</a>, which I'd recommend watching for an introduction to the feature.<br>
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Have you ever wanted to perform an operation on a java.util.stream.Stream only to find that the existing set of operations didn't provide what you needed—forcing you to break out early from the Stream and perform the logic outside of it? As a matter of fact,
java.util.stream was the first JDK API designed with lambdas in mind and was ...</div>
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<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt">Cheers,<br>
√</div>
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</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt"><b><br>
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<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt"><b>Viktor Klang</b></div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif; font-size:12pt">Software Architect, Java Platform Group<br>
Oracle</div>
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