Hi I'm rather fond of fixpoint constructions like this one:
newtype Copy a = Copy a deriving Show
data Wonky f = Wonky | Manky (f (Wonky f)) deriving Show
(Clearly this is an ill-motivated example, but the real example which caused this problem is available on request...) The trouble is that when I ask either hugs -98 or ghci -fglasgow-exts to show (Wonky :: Wonky Copy) the poor compiler's brain explodes. I take it this is a known problem with instance inference, deriving, etc. Of course, it's easy to write my own show for Wonky Copy, but that's a tad annoying. I tried to guess the type of the show instance derived for Wonky. Being a naive sort of chap, I thought it might be show :: (forall a. Show a => Show (f a)) => Wonky f -> String but that's a syntax error. A little more tinkering, and it looks like it might be show :: Show (f (Wonky f)) => Wonky f -> String Is this really the type of show? If so, no wonder there's a problem. I don't want to start an argument about how to solve this problem. I do want to suggest that, for the time being, it would be better to reject `deriving Show' for type constructors like Wonky (ie those with higher-kind parameters) than to generate instances which break the compiler. Or am I just being a spoilsport? Conor