{- Decide whether these three definitions of zipW are identical. If
   not, give an example evaluating differently. -}

zipW1 f _ [] = []
zipW1 f [] _ = []
zipW1 f (x:xs) (y:ys) = f x y : zipW1 f xs ys

zipW2 f (x:xs) (y:ys) = f x y : zipW2 f xs ys
zipW2 f _      _      = []

zipW3 f [] _ = []
zipW3 f _ [] = []
zipW3 f (x:xs) (y:ys) = f x y : zipW3 f xs ys


{- Scroll down for solution! -}
































{-
zipW1 is different from zipW2 and zipW3, which have identical results.

With the input
 (,) undefined []
we get
  [] 
for zipW1 and
  Exception: Prelude.undefined
for zipW2 and zipW3.
-}