[Haskell] GADTs are not nice to me ;-)

3 Jul 2006

      Hello everybody,

I wanted to do something like that:

	data Pair :: (* -> *) -> * where
	    Pair :: a b -> b -> Pair a

	data Sel :: * -> * where
	    A :: Sel Bool
	    B :: Sel Integer

	showSnd :: Pair Sel -> String
	showSnd (Pair A bool)
	    = show bool
	showSnd (Pair B integer)
	    = show integer

However, GHC 6.4.1 with -fglasgow-exts complains in the second last and last 
line that there is no instance for (Show b).  I don't really understand this 
since in my opinion it is clear that bool :: Bool and integer :: Integer and 
that therefore bool and integer are showable.  What's the problem with my 
code?

Best wishes,
Wolfgang

[Haskell] GADTs are not nice to me ;-)

Wolfgang Jeltsch