I have this little test program: {-# LANGUAGE CPP #-} #ifdef PRAGMA_DERIVE_TYPEABLE {-# LANGUAGE DeriveDataTypeable #-} #else {-# OPTIONS -fglasgow-exts #-} #endif -- This file is Test/Fail.hs. -- ghc --make -optP-DPRAGMA_DERIVE_TYPEABLE -XCPP Test.Fail module Test.Fail where import Data.Generics data Fail = Fail deriving Typeable If compile this wih the command ghc --make -optP-DPRAGMA_DERIVE_TYPEABLE -XCPP Test.Fail then I get this error from ghc-6.10.1: [1 of 1] Compiling Test.Fail ( Test\Fail.hs, Test\Fail.o ) Test\Fail.hs:11:26: Can't make a derived instance of `Typeable Fail' (You need -XDeriveDataTypeable to derive an instance for this class) In the data type declaration for `Fail' I've checked the preprocessor output with -E, and that looks fine. I can compile the preprocessor output with ghc without errors. Alistair