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 ;-)