What does "this function will fuse" mean?

Question

The documentation for scanl says "this function will fuse". What exactly does fuse mean here? Is it bad?

Answer 1

http://www.haskell.org/haskellwiki/Short_cut_fusion

(and, for more information: http://www.haskell.org/haskellwiki/Correctness_of_short_cut_fusion)

If a function will fuse, that is a good thing. It means that a chain of functions can be merged into one function, which means less allocation, less stack, and more speed! Awesome!

Here's a trivial fusion: map f . map g ----> map (f . g).

As detailed above, there are many others that get applied by the RULES of the standard library as well.

Answer 2

It's kind of like coroutines - if you compose two functions they can execute on a fused stream instead of one being fully "evaluated" first then the second.

What is Haskell's Stream Fusion