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I suggest to separate the vacuous from the proper instances, and to expose only the former via Data.Generics. That way, the convenience is only an import away, but doesn't get in the way of non-standard applications.

The problem with this is that it leads to conflicting instances if you import both.

I was thinking of splitting Data.Generics.Instances, moving some of its Data instances to Data.Generics.DummyInstances, and not importing/exporting the latter from Data.Generics. Importing both Data.Generics and Data.Generics.DummyInstances would give the current situation, no conflicting instances. And those who need application-specific instances instead of the dummies can avoid importing the dummy instances. In particular, the instances instance (Data a, Data b) => Data (a -> b) instance Typeable a => Data (IO a) instance Typeable a => Data (Ptr a) instance Typeable a => Data (StablePtr a) instance Typeable a => Data (IORef a) instance Typeable a => Data (ForeignPtr a) instance (Typeable s, Typeable a) => Data (ST s a) instance Typeable a => Data (TVar a) instance Typeable a => Data (MVar a) instance Typeable a => Data (STM a) instance (Data a, Integral a) => Data (Ratio a) claim to have polymorphic/generic components, but use the defaults gfoldl _ z = z gmapT f x = unID (gfoldl k ID x) where k (ID c) x = ID (c (f x)) meaning that gmapT (=id) will never get access to those component types. Contrast this with the conventional instances for [], Maybe, Either, (,..) in the same module, and especially the abstraction- preserving instance for Array a b. And compare the not very consistent results (sometimes the transformation is applied, sometimes not): > everywhere (mkT ((+1)::Int->Int)) (0,1)::(,) Int Int (1,2) > everywhere (mkT ((+1)::Int->Int)) (0%1)::Ratio Int 0%1 > everywhere (mkT ((+1)::Int->Int)) (return 0::[] Int) [1] > everywhere (mkT ((+1)::Int->Int)) (return 0::IO Int) 0 > everywhere (mkT ((+1)::Int->Int)) (return 0::() -> Int) () 0 > everywhere (mkT ((+1)::Int->Int)) (array (0,0) [(0,0)] :: Array Int Int) array (0,0) [(0,1)] There isn't any straightforward "proper" Data instance for those types, either. But if, say, I'd knew that, for a specific application context, I'd want to apply my transformation (b->b) to the range, and only to the range, of all functions (a->b), via post-composition, how would I even do that, given the existing instance Data (a->b)? I can bypass monomorphic functions using extT, but what about polymorphic functions? > (\(f,g)->(f (),g 0)) $ everywhere (mkT (((+1).)::(()->Int)->(()->Int))) (\()->(0::Int),\_->(0::Int)) (1,0) Is there a way to do this without removing the existing Data instance (which would allow me to define my own)? Claus