[Haskell-cafe] Generalizing takeWhile

I need to take some elements from the front of a list.  However the criteria are somewhat complex.

walk f [] = [] walk f (x:xs) = case f x of Just g -> x : walk g xs Nothing -> []

For each item the `predicate' f either returns Nothing, when it thinks we should not take any more elements, or return Just another `predicate' to apply to the next element. However the type system does not like my function. How can I mollify it?