yield return based on Continuations

[Daniel Schmid]

Thank you for your interest,

I'll also include loom-dev with this mail.

Yours,
Daniel

Am 28.08.2023 um 11:12 schrieb Alan Bateman:
>
> This looks fun! It's probably best to bring this to loom-dev. In its 
> archives you'll find several discussions about generators as several 
> people have been interested in that topic. Even when thread confined, 
> the main concern has been that exotic control flow yields leads to 
> surprising behavior with many of the existing constructs, e.g. in your 
> example think about behavior with finally blocks, try-with-resources, 
> locks, ... when the iterator is not fully consumed.
>
> -Alan
>
> On 28/08/2023 09:43, Daniel Schmid wrote:
>>
>> Hi,
>>
>> After seeing the JVM Language Summit talk on Continuations 
>> (https://www.youtube.com/watch?v=6nRS6UiN7X0), I thought about it 
>> being possible to implement something like "yield return" in 
>> languages like C# (or "yield" in Python) based on Continuations.
>> Kotlin has implemented a similar feature as well: 
>> https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/-sequence-scope/yield.html
>> Now that Continuations are in the JDK, I feel like it can be used as 
>> a good primitive and now is a good time to start about thinking about 
>> adding something like this as a Java feature or the libraries.
>>
>> After my experiments and some discussion with another developer named 
>> Peter Eastham (https://github.com/Crain-32), I was able to come up 
>> with an implementation/proof-of-concept allowing something like the 
>> following:
>>
>> public static void main(String[] args) {
>>      System.out.println("main thread: " + Thread.currentThread());
>>
>>      for (String s : Yielder.create(YieldReturnTest::someMethod)) {
>>          System.out.println("Text: " + s);
>>      }
>> }
>>
>> private static void someMethod(Yielder<String> y) {
>>      y.yield("Hello - " + Thread.currentThread());
>>      System.out.println("between yields");
>>      y.yield("World - " + Thread.currentThread());
>>
>>      for (String s : Yielder.create(YieldReturnTest::otherMethod)) {
>>          y.yield("nested: " + s);
>>      }
>>
>>      y.yield("bye - " + Thread.currentThread());
>> }
>>
>> private static void otherMethod(Yielder<String> y) {
>>      y.yield("it can");
>>      y.yield("also be");
>>      y.yield("nested");
>> }
>>
>> output:
>>
>> main thread: Thread[#1,main,5,main]
>> Text: Hello - Thread[#1,main,5,main]
>> between yields
>> Text: World - Thread[#1,main,5,main]
>> Text: nested: it can
>> Text: nested: also be
>> Text: nested: nested
>> Text: bye - Thread[#1,main,5,main]
>>
>> In this example, the method reference passed to the Yielder.create 
>> method would be run in a Continuation while y.yield would yield the 
>> Continuation and make the value available to the iterator (next() 
>> calls Continuation#run).
>>
>> You can find a simple proof-of-concept of that here: 
>> https://github.com/danthe1st/ContinuationYieldReturn
>>
>> Would it be possible to add something like this to the JDK libraries?
>> I feel like it might be a useful addition to the JDK libraries as it 
>> simplifies creating sequences a lot.
>>
>> Originally, I thought about whether it might be a good idea to add 
>> syntax for this but after building that proof-of-concept, it looks 
>> like it would be sufficient to add this to the libraries and using 
>> methods like this seems pretty natural.
>> One thing I am concerned with this approach (opposed to an approach 
>> that involves changing syntax) is that it would be possible that the 
>> method suddenly runs in a different thread if the 
>> hasNext()/next()-calls of the Iterator chang the thread they are used 
>> in at some point. While Continuations allow this behaviour, it might 
>> seem a weird to developers who don't know how Continuations work.
>> But aside from that issue with iterations switching threads, this 
>> approach seems pretty natural to me.
>>
>> Yours,
>> Daniel
>>
>
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