Hi Henrik, I recall there was a thread for EnumeratedStream (https://mail.openjdk.org/pipermail/core-libs-dev/2024-April/thread.html#121871) back in April; it calls for the same feature, to add a stage for indexing entries. This approach of using a gatherer is simple; its disadvantage is that when there is a next stage in the stream, the indices become invalidated and cause confusions for users. This is not a problem in the Rust/Python enumerate examples, as they do not intend to be fluent like Stream.
I had a toy project that explores the consumption of (index, element) pairs: https://github.com/liachmodded/IndexedStream/blob/main/src/main/java/com/github/liachmodded/indexedstream/IndexedGatherers.java This simple example takes predicates and functions that take an (index, element) pair and return a boolean/a next element. In the same package, there is also an Indexed.java API, which allows you to create Collector/Gatherer for an indexed entry, and re-fit that Collector/Gatherer to be general purpose. These are all aimed to avoid propagating no-longer-valid indices to the next stage of the stream operations; and they aren't exactly pretty. Do you have any justifications for the addition of this particular gatherer, so it should be provided in the Java Class Libraries instead of a user library? Regards, Chen Liang ________________________________ From: core-libs-dev <core-libs-dev-r...@openjdk.org> on behalf of Henrik Wall <xehpuk....@gmail.com> Sent: Thursday, December 5, 2024 10:48 AM To: core-libs-dev@openjdk.org <core-libs-dev@openjdk.org> Subject: JEP 473: Proposal for new built-in gatherer `indexed` Hey folks, Not having access to the index of an element of a stream is often a reason to fall back to a traditional loop, at least for me. I'd love to have `Gatherers.indexed()` that looks something like this: public static <TR> Gatherer<TR, ?, Map.Entry<Integer, TR>> indexed() { return Gatherer.ofSequential( () -> new int[1], Gatherer.Integrator.ofGreedy((state, element, downstream) -> downstream.push(Map.entry(state[0]++, element))) ); } (Potentially with a custom pair class to avoid auto-boxing.) In other popular languages like Python or Rust, this is also called `enumerate`. Any chance to get that in a future release? Regards, Henrik
