[Haskell-cafe] type variable in class instance

[Martijn Schrage]

On 11-09-12 08:53, oleg at okmij.org wrote:
> Corentin Dupon wrote about essentially the read-show problem:
>> class (Typeable e) => Event e
>>
>> data Player     = Player Int         deriving (Typeable)
>> data Message m  = Message String     deriving (Typeable)
>>
>> instance  Event Player
>>
>> instance (Typeable m) => Event (Message m)
>>
>> viewEvent :: (Event e) => e -> IO ()
>> viewEvent event = do
>>      case cast event of
>>          Just (Player a) -> putStrLn $ show a
>>          Nothing -> return ()
>>      case cast event of
>>          Just (Message s) -> putStrLn $ show s
>>          Nothing -> return ()
> Indeed the overloaded function cast needs to know the target type --
> the type to cast to. In case of Player, the pattern
> (Player a) uniquely determines the type of the desired value: Player.
> This is not so for Message: the pattern (Message s) may correspond to
> the type Message (), Message Int, etc.
>
> To avoid the problem, just specify the desired type explicitly
I had the same idea, but it doesn't work. Fixing m to () causes the cast 
to fail for any other type, so viewEvent (Message "yes" :: Message ()) 
will work, but viewEvent (Message "no" :: Message Char)
won't.

Putting viewEvent in the Event class though, like Ryan suggested, seems 
to be an elegant solution that stays close to the original source.

Cheers,
Martijn

>> case cast event of
>>     Just (Message s::Message ()) -> putStrLn $ show s
>>     Nothing -> return ()
> (ScopedTypeVariables extension is needed). The exact type of the
> message doesn't matter, so I chose Message ().
>
> BTW, if the set of events is closed, GADTs seem a better fit
>
>> data Player
>> data Message s
>>
>> data Event e where
>>      Player  :: Int    -> Event Player
>>      Message :: String -> Event (Message s)
>>
>> viewEvent :: Event e -> IO ()
>> viewEvent (Player a)  = putStrLn $ show a
>> viewEvent (Message s) = putStrLn $ show s
>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20120911/e9be59de/attachment.htm>