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?