Why does foldr use a helper function?

Question

In explaining foldr to Haskell newbies, the canonical definition is

foldr            :: (a -> b -> b) -> b -> [a] -> b
foldr _ z          


        

Answer 1

One tiny important detail not mentioned in other answers is that GHC, given a function definition like

f x y z w q = ...

cannot inline f until all of the arguments x, y, z, w, and q are applied. This means that it's often advantageous to use the worker/wrapper transformation to expose a minimal set of function arguments which must be applied before inlining can occur.