Question

class  Eq a  where
    (==), (/=)           :: a -> a -> Bool
    x /= y               = not (x == y)
    x == y               = not (x /= y)

deriving instance Eq Bool

I assume it generates

instance Eq Bool where
    True == True = True
    False == False = True

But how do I create a instance from something like

newtype Sink p = Sink {unSink :: MVar (E.Iteratee (Message p) IO ())}

instance Eq (Sink p) where
 ?==? = True

Do I just use deriving and ghc will figure out itself?

deriving instance Eq (Sink p)

http://hackage.haskell.org/packages/archive/websockets/0.7.0.0/doc/html/src/Network-WebSockets-Monad.html#Sink

PS I have read this but it goes beyond my capabilities of understanding
http://www.haskell.org/ghc/docs/7.6.1/html/users_guide/deriving.html

Answer 1

Yes, that will work since there is an instance Eq (MVar a) that tests whether to MVars are the same [1]. The derived instance for Sink will use that. However this might not be what you want since the Eq instance doesn’t compare the contents of the MVars, only whether they are the same MVar in memory.

The answer to your question

But how do I create a instance from something like … Do I just use
deriving and ghc will figure out itself?

is probably “No, you must write an instance that has the properties that you want.”