Generalized isomorphic deriving
A newtype is isomorphic to the type that it is based on. For example, newtype A = A Int is isomorphic to Int, so it is possible to automatically derive all of Int's instance for A.
It would be nice if this could be extended to other datatypes that were isomorphic to each other. For example, this would work
data K a = K a a a instance C (K a) where ... data L a = L a a a deriving (Eq,C<-K,Show)
C<-K notation means "derive an instance of C for this type that is 'the same as' the instance of C for K". The compiler would have to check that K and L were actually isomorphic.
I don't know if this is possible with the current internals of GHC, but it would be cool if it would be done. My main usage scenario would be deriving things for single-argument data constructors based on tuples; for example for serialization classes.