Are they? I tried understanding how that worked a few years ago and it led me to something called "stream fusion" which seems like a magical set of code rewriting rules that wasn't even on by default in haskell[1].

For example if you did a map of x times 2 then map again with x times 2, I expect a single map of x times 4 without using temporary buffers or multiple passes for the intermediate map (this is a simple example, but gets more complicate once you throw in other stream operators).

I know java didn't do this when I was looking at their collectors source code back then.

[1] https://stackoverflow.com/questions/578063/what-is-haskells-...