Re: [Haskell-cafe] type variable in class instance

From the point of view of the language, Message () and Message Int and Message Player are all completely distinct types and may have different behavior--there's no way for it to "know" that they all have the same representation that only contains a String.

The derived Typeable instance for "Message m" is really a derived instance of "Typeable1 Message" along with the generic instance "(Typeable1 f, Typeable a) => Typeable (m a)" in Data.Typeable. So you need to specify the type of message you want, or drop the type parameter from Message. A simpler answer, though, would just be to put the functions in the typeclass. class Event e where viewEvent :: e -> IO () instance Event Player where viewEvent (Player a) = putStrLn $ show a instance Event (Message m) where viewEvent (Message s) = putStrLn s In this case, the instance makes it clear that the type parameter is irrelevant and puts no constraints on it. And the type of viewEvent is exactly the same as you were asking for: Event e => e -> IO (). -- ryan On Mon, Sep 10, 2012 at 3:06 PM, Corentin Dupont wrote:

Hi Stephen, I wasn't aware of Data.Dynamic. I tried:

*viewEvent :: Dynamic -> IO () viewEvent event = do case fromDynamic event of Nothing -> return ()

Just (Message s) -> putStrLn $ show s *

But still got the same error (Ambiguous type variable `t0' in the constraint: (Typeable t0) arising from a use of `fromDynamic')...

Best, Corentin

On Mon, Sep 10, 2012 at 11:33 PM, Stephen Tetley

...

wrote:

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Whilst dynamic typing isn't idiomatic for Haskell, it seems like you've decided you want it. So why not use Data.Dynamic rather than roll you're own dynamic typing with Typeable?

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