GHC suggests -fglasgow-exts, and not {-# LANGUAGE #-} pragmas or "Extensions:" cabal field

Instead of:

Pdp1Emu.hs:14:8:
    Can't make a derived instance of `Monad Pdp1'
      (`Monad' is not a derivable class
       Try -fglasgow-exts for GHC's newtype-deriving extension)
    When deriving instances for `Pdp1'

I'd like to see something like:

Pdp1Emu.hs:14:8:
    Can't make a derived instance of `Monad Pdp1'
      (`Monad' is not a derivable class
       Try -fglasgow-exts or GeneralizedNewtypeDeriving for GHC's newtype-deriving extension)
    When deriving instances for `Pdp1'

I'm not sure if this message makes it quite clear enough how to use GeneralizedNewtypeDeriving, though... (seeing as it sheds no light on the subject whatsoever ;-)