[nine_k]

In Python, such steps like map() and filter() would execute concurrently, without large intermediate arrays. It lacks the chaining syntax for them, too.

Java streams are the closest equivalent, both by the concurrent execution model, and syntactically. And yes, the Java debugger can show you the state of the intermediate streams.

[maleldil]

> would execute concurrently

Iterators are not (necessarily) concurrent. I believe you mean lazily.

[nine_k]

Concurrent, not parallel.

That is, iterators' execution flow is interspersed, with the `yield` statement explicitly giving control to another coroutine, and then continuing the current coroutine at another yield point, like the call to next(). This is very similar to JS coroutines implemented via promises, with `await` yielding control.

Even though there is only one thread of execution, the parts of the pipeline execute together in lockstep, not sequentially, so there's no need for a previous part to completely compute a large list before the following part can start iterating over it.