data (Eq a, Show a) => State a = Start | Stop | State a deriving Show
Putting the class context on the data constructor like that wasn't
doing you any more good than this way, and was causing ghc to think
that the constructor was existentially quantified.
- Cale
On 08/12/05, Joel Reymont
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.
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