Hello! I'm trying to rewrite some FD classes to use associated types instead. The Port class is for type structures whose leaves have the same type: class Port p where type Leaf p type Struct p toList :: p -> [Leaf p] fromList :: [Leaf p] -> p (Leaf p) gives the leaf type, and (Struct p) gives a canonical representation of the structure regardless of leaf type. Here we just instantiate two leaf types: instance Port Int where type Leaf Int = Int type Struct Int = () toList a = [a] fromList [a] = a instance Port Bool where type Leaf Bool = Bool type Struct Bool = () toList a = [a] fromList [a] = a There's also a function for mapping over ports: mapPort :: ( Port pa , Port pb , Struct pa ~ Struct pb ) => (Leaf pa -> Leaf pb) -> (pa -> pb) mapPort f = fromList . map f . toList The equality constraint makes sure that we're mapping between equal structures. When I try to run this, I get: *Main> mapPort even (5::Int) <interactive>:1:8: No instance for (Integral (Leaf Int)) ... because as it stands, mapPort is not able to infer (pb = Bool) from (Struct pb = ()) and (Leaf pb = Bool). What's the easiest way to encode that pb can be inferred from (Struct pb) and (Leaf pb)? Thanks, / Emil PS. I used to have a class class SameStruct pa a pb b | pa -> a, pa b -> pb, pb -> b, pb a -> pa In the example above, we'd have pa=Int and b==Bool, so the second dependeny would infer pb=Bool.