How do I write something like the following in Haskell:
showSquare :: (Show a, Num a) => a -> String
showSquare x = "The square of " ++ (show x) ++ " is " ++ (show (x * x))
showSquare :: (Show a, not Num a) => a -> String
showSquare x = "I don't know how to square " ++ (show x)
Basically, something like boost::enable_if in C++.
GHC extensions are ok.