From http://en.wikibooks.org/wiki/Haskell/YAHT/Type_basics#Recursive_Datatypes

list.hs

data List a =
	Nil |
	Cons a (List a)

listLength :: Num b => List a -> b
listLength Nil = 0
listLength (Cons x xs) = 1 + listLength xs

listHead :: List a -> a
listHead (Cons x xs) = x

listTail :: List a -> List a
listTail (Cons x xs) = xs

listFoldl :: (a -> b -> a) -> a -> List b -> a
listFoldl f a Nil = a

-- my version
listFoldl f a (Cons x xs) = f (listFoldl f a xs) x

-- from the tutorial solution at http://en.wikibooks.org/wiki/Haskell/YAHT/Type_basics/Solutions#Recursive_Datatypes
--listFoldl f y (Cons x xs) = listFoldl f (f y x) xs

listFoldr :: (a -> b -> b) -> b -> List a -> b
listFoldr f a Nil = a
listFoldr f a (Cons x xs) = f x (listFoldr f a xs)