Using 6.0.1 or the 20030903 CVS snapshot: ghc-6.1: panic! (the `impossible' happened, GHC version 6.1): Failed binder lookup: GHCziShow.show {- v r4H -} If I try to compile the following code. Commenting out the line defining show (but not the instance line itself) makes the error go away. Cheers, Ganesh module TH2 where code = [d| data Foo = Foo instance Show Foo where show Foo = "foo" |]