Felipe Lessa wrote:
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}
Another way to do it is to use an alternative to Maybe, defined like this:
data MaybeFun a = NoFun | Fun (a -> MaybeFun a)
...which gives this definition for walk:
walk :: (a -> MaybeFun a) -> [a] -> [a] walk f [] = [] walk f (x:xs) = case f x of Fun g -> x : walk g xs NoFun -> []