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

GHC cannot inline recursive functions, so

foldr            :: (a -> b -> b) -> b -> [a] -> b
foldr _ z []     =  z
foldr f z (x:xs) =  f x (foldr f z xs)

cannot be inlined. But

foldr k z = go
      where
        go []     = z
        go (y:ys) = y `k` go ys

is not a recursive function. It is a non-recursive function with a local recursive definition!

This means that, as @bheklilr writes, in map (foldr (+) 0) the foldr can be inlined and hence f and z replaced by (+) and 0 in the new go, and great things can happen, such as unboxing of the intermediate value.