Am 16.11.2006 um 13:33 schrieb Tomasz Zielonka:
On Wed, Nov 15, 2006 at 01:20:43PM +0100, Jan Christiansen wrote:
I would work, if you used existential quantification, but I am don't know if it would be what you want:
data Test = forall a . Test [a -> a]
I don't know why you current code doesn't work - sorry.
I want to apply the functions in the list to values of various types for example to a value of type String and to a value of type Int. Therefore an existential type doesn't work as I see. More precisely I want to use a function encoding of a recursive data type in this special case of a binary tree, i.e., a datatype forall b . (b -> (b -> a -> b -> b) -> b) This type gets an additional b and is packed in a list and this list is packed in a data type data UpDownTree a = UpDownTree (Tree a) (forall b . [b -> (b -> a -> b -> b) -> b -> b]) I notice that it would be more straight forward to pull the quantifier to the inside of the list. I have seen that the ghc documentation calls this impredicative polymorphism. I will take a look at the ICFP paper cause at the moment I do not know the difference between [(forall a. a -> a)] and forall a . [a -> a]. My implementation already works if I use null, head and tail. But I would like to use a type class constraint on b and this doesn't work. Furthermore I would like to understand why ghc doesn't report an error when I use null, head and tail but reports errors when I use pattern matching. Regards, Jan