Hi! I would like to define a type class for a data structure that supports a lookup and an update function like the following one. class Mapping map key where lookup :: key -> map v -> v update :: key -> (v -> v) -> map v -> map v Now I want to lift this to a class instance for tuples of keys. This should be done by just using a map that contains another map as values. The problem is that I don't know how to express the composition of type constructors like the function (.) for ordinary functions. I know that (map1 . map2) is wrong but it states what I want to express, namely the composition of two type constructors. With this I would like to define an instance definition like the following. instance (Mapping map1 key1, Mapping map2 key2) => Mapping (map1 . map2) (key1, key2) where lookup (key1, key2) = lookup key2 . lookup key1 update (key1, key2) f = update key1 (update key2 f) I am not sure about the practical pertinence of this example cause I will probably only use one instance where the type of the keys is Int but I am curious if this is possible at all. thanks in advance, Jan