I think we probably agree in a lot of concrete cases but discussing in the abstract tends to bring out polarizing statements. That's why I qualified most of my remarks, because I'll happily ham it up with a functional style...in tests, and other places where I don't feel strongly about what's going on.

When you are forced to use some accumulating global state, that leaves you with writing in a different style--loopy, if you will--which maybe is a good signal to the reader that something is weird or different, but then again maybe it isn't.

One thing that bit me using Java streams recently is that it completely broke down when I had a concurrent data structure that needed to have the coarseness of locking tuned for performance. Laziness and functional streaming operators had to just go out the window to even make it clear where a lock was taken and released. So loops it was.

> One thing that bit me using Java streams recently is that it completely broke down when I had a concurrent data structure that needed to have the coarseness of locking tuned for performance. Laziness and functional streaming operators had to just go out the window to even make it clear where a lock was taken and released.

Well yeah, that's very much expected. If you're even talking about locking, stream transformations aren't a good fit (except maybe if you do the microbatching/orchestration style where your stream transformation steps operate on chunks of a few thousand items - and even then your locks should be scoped to a single transformation step).

(Now I'd happily claim that for equal developer time one can usually outperform a locking-and-mutation implementation with a stream-processing implementation - not because the low-level mechanics are faster but because it's easier to understand what's going on and make algorithmic improvements - but that's a rather different matter)