Overlapping instances
Hi, I've got the following code which tries to implement a TShow class, which is equivalent to Show, except it is supposed to work on TVar types as well. import GHC.Conc createEngine :: String -> Int -> Int -> IO Engine createEngine name major minor = do tUsers <- newTVarIO [] return $ Engine { engineName = name , version = EngineVersion { major = major , minor = minor } , users = tUsers } class TShow a where tshow :: a -> IO String instance Show (TVar a) where show a = "%" instance (Show a) => TShow a where tshow a = return $ show a instance (Show a) => TShow (TVar a) where tshow ta = do a <- atomically (readTVar ta) return $ show a data User = User { userName :: String } deriving Show data EngineVersion = EngineVersion { major :: Int , minor :: Int } deriving Show data Engine = Engine { engineName :: String , version :: EngineVersion , users :: TVar [User] } instance TShow Engine where tshow a = do users <- atomically (readTVar (users a)) return $ "Engine { " ++ "engineName = " ++ show (engineName a) ++ ", " ++ "version = " ++ show (version a) ++ ", " ++ "users = %" ++ show users ++ " }" When I run it however, I get this: *Main> te <- createEngine "Hello" 1 2 *Main> s <- tshow te <interactive>:1:5: Overlapping instances for TShow Engine arising from a use of `tshow' at <interactive>:1:5-12 Matching instances: instance (Show a) => TShow a -- Defined at fxmain.hs:(26,0)-(27,27) instance TShow Engine -- Defined at fxmain.hs:(51,0)-(58,41) In a stmt of a 'do' expression: s <- tshow te I'm not seeing how instance (Show a) => TShow a in the above error message is applicable here since Engine is not an instance of Show. Why is it complaining? Thanks, -John
The problem is that Engine *could* be made an instance of Show (remember that any user of the module can create that instance later). What you need is the overlappinginstances extension: {-# LANGUAGE OverlappingInstances #-} With this extension, the most specific instance will be used, i.e. "instance TShow Engine" for Enginge's, no matter if it is an instance of Show. /Tobias ________________________________ From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of John Ky Sent: den 8 december 2008 13:32 To: Haskell Cafe Subject: [Haskell-cafe] Overlapping instances Hi, I've got the following code which tries to implement a TShow class, which is equivalent to Show, except it is supposed to work on TVar types as well. import GHC.Conc createEngine :: String -> Int -> Int -> IO Engine createEngine name major minor = do tUsers <- newTVarIO [] return $ Engine { engineName = name , version = EngineVersion { major = major , minor = minor } , users = tUsers } class TShow a where tshow :: a -> IO String instance Show (TVar a) where show a = "%" instance (Show a) => TShow a where tshow a = return $ show a instance (Show a) => TShow (TVar a) where tshow ta = do a <- atomically (readTVar ta) return $ show a data User = User { userName :: String } deriving Show data EngineVersion = EngineVersion { major :: Int , minor :: Int } deriving Show data Engine = Engine { engineName :: String , version :: EngineVersion , users :: TVar [User] } instance TShow Engine where tshow a = do users <- atomically (readTVar (users a)) return $ "Engine { " ++ "engineName = " ++ show (engineName a) ++ ", " ++ "version = " ++ show (version a) ++ ", " ++ "users = %" ++ show users ++ " }" When I run it however, I get this: *Main> te <- createEngine "Hello" 1 2 *Main> s <- tshow te <interactive>:1:5: Overlapping instances for TShow Engine arising from a use of `tshow' at <interactive>:1:5-12 Matching instances: instance (Show a) => TShow a -- Defined at fxmain.hs:(26,0)-(27,27) instance TShow Engine -- Defined at fxmain.hs:(51,0)-(58,41) In a stmt of a 'do' expression: s <- tshow te I'm not seeing how instance (Show a) => TShow a in the above error message is applicable here since Engine is not an instance of Show. Why is it complaining? Thanks, -John
On Mon, Dec 8, 2008 at 4:43 AM, Tobias Bexelius
{-# LANGUAGE OverlappingInstances #-}
With this extension, the most specific instance will be used, i.e. "instance TShow Engine" for Engine's, no matter if it is an instance of Show.
Of course, down this way madness lies :) For example, this code:
uhoh :: Show a => a -> IO String uhoh x = tshow x
won't compile. The question is, what should this code do?
instance Show (TVar a) where show _ = "TVAR" broken :: TVar a -> String broken x = uhoh x
"broken" will construct the Show dictionary for TVars and pass it to "uhoh", which no longer knows that it is getting called on a TVar. Then uhoh will construct a TShow (TVar a) dictionary using the Show (TVar a) dictionary and the instance "Show a => TShow a", even though there is a more specific instance. So the compiler will just not let "uhoh" compile if overlapping is allowed. You can force it to compile using the wrong instance with the "IncoherentInstances" extension, but it's aptly named; the result is bad because each type is supposed to only have one instance for a particular typeclass. It's worse because it breaks referential transparency; if you inline "uhoh" into "broken", now you get the specific instance! -- ryan
participants (3)
-
John Ky -
Ryan Ingram -
Tobias Bexelius