The problem lies in the definition of your class:
class AFun f where afun :: Data a => f -> ([Dynamic] -> a)
You are saying that afun can return any type "a" that the user wants as long as it is an instance of "Data", whereas here
instance Data v => AFun v where afun f [] = f afun _ _ = error "Too many arguments"
you are restricting the type returned by afun. What you probably want is a class that looks more like class Data r => AFun f r where afun :: f -> ([Dynamic] -> r) so that the return type is explicitly included in the definition of the type. Also, you might consider requiring that the argument and result be instances of Binary rather than Data, since this gives you fast binary serialization for free. Then you could write a class like class (Binary a, Binary b) => AFun a b where afun :: f -> (a -> b) And your code can take care of the serialization/deserialization and then just hand the values of the correct type over to afun. Cheers, Greg