Hi all, I'm trying to write something like a generic fmap, or a generic natural transformation. The application is this. I have a typed logical variable library which produces arbitrary terms with values of type "Var a", which are references to a value of type "Maybe a", and I want to write a "solve" function which replaces these values with instantiated versions of type "Id a" where newtype Id a = Id a . Furthermore I want this to be reflected in the type of the generic term: solve :: Pred (t Var) -> [t Id] so if I have a type like data Entry k = Entry (k String) (k Int) then I can write some constraint equation with values of type "Entry Var", and get back values of type "Entry Id" - in other words, objects where the unknowns are statically guaranteed to have been filled in. I looked at the generics library. I may be mistaken, but it seems that it doesn't have what I need to do this. The problem isn't the mapping, it's creating a new type which is parameterized by another type. The only options for creating new types are variations on fromConstr :: Data a => Constr -> a but what is needed is something like fromConstr1 :: Data1 a => Constr1 -> a b With something like that it should be possible to define: gmapT1 :: (forall b . Data1 b => b l -> b m) -> a l -> a m Does this make sense? Here I would be treating all instances of Data as possibly degenerate instances of Data1 (which just might not depend on the type variable). If it seems like a good idea, I would be interested in helping out with the implementation. Frederik -- http://ofb.net/~frederik/