[Haskell-cafe] Noob error: Type b -> c b Does not match IO a

[Scott Turner]

On 2005 June 25 Saturday 17:49, kynn at panix.com wrote:
>    Simplified:
>    prodList xs = foldl (*) 1 xs
>
> But my original at least made some provision for short circuiting the
> whole operation if the list contained a 0.  As far as I can figure,
> fold, map, etc., are not suitable for any situation in which
> short-circuiting would be desirable (e.g. and, or, etc.).  Am I wrong?

Actually, foldr is applicable to short-circuiting. 
   foldr (&&) True
works fine for detecting whether all elements of a list are True, and does not 
evaluate the list beyond the first occurrence of False.  Similarly, if `m` is 
defined as a short-circuiting multiplication operator, i.e.
   m x y = if x==0 then 0 else x*y
then
   foldr m 1
does not evaluate the list beyond the first occurrence of 0. Unfortunately, 
`m` does not work as well with foldr as &&.  The function (foldr m 1) creates 
a stack containing one (*) operation for every list element to be multiplied, 
which can cause undesirable memory usage.

It's still possible to use fold and get short circuiting with good memory 
usage.
  upTo pred = foldr (\a -> \xs -> if pred a then [a] else a:xs) []
  prodList = foldl' (*) 1 . upTo (== 0)
It might be considered cheating, but AFAICT the test for ==0 needs to be 
separated from the multiplication proper.

Note, foldl' is a more strict, more useful variant of foldl, located in 
Data.List.