yield return based on Coroutines

[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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