Did someone actually try compiling this? Here are the results: data State a = Start | Stop | (Show a, Eq a) => State a deriving Show foo.hs:1:5: Can't make a derived instance of `Show (State a)' (`State' has existentially-quantified constructor(s)) When deriving instances for type `State' I do not want to do it the way Tomasz and John are suggesting below. I need the user of my library to supply their own a and be able to pattern match on it. In the library itself I just need Show and Eq a. The following does the trick (compiles, works) but sucks in its verbosity: data State a = Start | Stop | (Show a, Eq a) => State a instance Eq a => Eq (State a) where (State a) == (State b) = a == b Start == Start = True Stop == Stop = True _ == _ = False instance Show a => Show (State a) where show (State a) = show a show Start = "Start" show Stop = "Stop" On Dec 8, 2005, at 8:36 AM, John Meacham wrote:
On Thu, Dec 08, 2005 at 09:13:10AM +0100, Tomasz Zielonka wrote:
Shouldn't it be:
data State = Start | Stop | forall a . (Show a, Eq a) => State a
ah. you are right. my bad.