I think I got this idea from JHC, but I'm unable to find it documented in the JHC manual at the moment. The idea is that when _ appears on the RHS of an equation it is another name for undefined. Here is a hypothetical example, inspired by code I've seen in FFI definitions: \begin{code} {-# LANGUAGE EmptyDataDecls #-} data Bar instance Storable Bar where sizeOf _ = sizeOf #{size struct Bar} alignment _ = alignment (_ :: CDouble) -- here we could have just as succinctly said alignment (1 :: CDouble) data Foo -- the struct Foo on the C side contains a Bar instance Storable Foo where sizeOf _ = sizeOf #{size struct Foo} alignment _ = alignment (_ :: Bar) -- we can't instantiate a Bar \end{code} Is this something people would be interested in having as an extension in GHC? Or is it just too fluffy for anyone to really care? Thanks, Jason