On Wed, Jul 22, 2009 at 06:00:38PM +0200, Matthias Görgens wrote: ] 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? It's actually simple! The problem is the type of 'f' would be type TypeOfF a = a -> Maybe (TypeOfF a) However, this type synonym isn't valid for the obvious reason (it is "infinite", I don't remember the correct name). Data types, on the other hand, may be recursive.
type TypeOfF a = a -> Maybe (TypeOfF' a) newtype TypeOfF' a = F {unF :: TypeOfF a}
and you may write your function as
walk :: TypeOfF a -> [a] -> [a] walk _ [] = [] walk f (x:xs) = case f x of Just g -> x : walk (unF g) xs Nothing -> []
Some test predicates
takeN :: Num a => a -> TypeOfF b takeN 0 = const Nothing takeN n = const . Just . F . takeN $ n-1
zigZag :: (a -> Bool) -> TypeOfF a zigZag p = go True where go b x | p x == b = Just . F $ go (not b) | otherwise = Nothing
-- Felipe.