Hey all! I'm currently diving into Kotlin on JVM, ...

# stdlib

[Kiryushin Andrey]

Hey all! I'm currently diving into Kotlin on JVM, coming from C# background, and I have some questions about iterables, sequences and streams, can anyone help me? My questions are the following
• I understand the different intentions behind Iterable and Sequence implementations, however I still don't quite understand why two different interfaces with identical signatures are needed. In C# there is only one IEnumerable interface and its extension methods behave like the ones for Sequence - that is, they are evaluated lazily, processing items from the source one by one. Therefore in C# any LINQ method applied to a collection starts a chain of lazy methods operating on IEnumerable and we use ToList in the end if a materialized collection is needed as a result of the pipeline. In Kotlin, then, not only the lazy to eager transition should be done explicitly (by calling toList) but also an eager to lazy one (by calling asSequence). Is this explicitness the purpose of existing separate Sequence and Iterable interfaces? Why is this explicitness in going from eager to lazy evaluation needed? Or does it have something to do with the fact that Iterable comes from Java and has been therefore in some way not suitable for lazy evaluations?
• I also had a glimpse of streams in Java. Am I right that conceptually this is the same as Sequence in Kotlin, so that Sequence should be used everywhere unless Java interop is required? Are there some advantages of streams over sequences or sequences over streams besides those imposed by the fact that one comes from Kotlin and the other from Java?

[rocketraman]

On most small Iterable's, eager evaluation will be faster than lazy. Note also the evaluation order of sequences and iterables are different. See https://stackoverflow.com/questions/35629159/kotlins-iterable-and-sequence-look-exactly-same-why-are-two-types-required.

I believe you are right about streams. I think the only use case for streams in Kotlin is if you want to use their parallel processing mode.

[Zach Klippenstein (he/him) [MOD]]

Yea, the different types are required so the operators know whether to use lazy or eager eval, and so that they can return the same type.

[Zach Klippenstein (he/him) [MOD]]

Kotlin Sequence isn’t exactly like Java Stream, streams have lots of machinery for parallel processing and threading management. Sequences are just lazy Iterables. You may also be interested in Flow, which is kind of like a Sequence that can be asynchronous, hop threads, and has backpressure.

[elizarov]

Also, stream is single-use and can be backed by a closeable resource (in which case the user must rember to use it), while Sequence is multi-use and cold (you don’t need to remember closing it, you can just drop it).