On Friday 05 November 2004 15:07, Benjamin Franksen wrote:

instance Typeable a => Typeable (MVar a) where typeOf x = mkAppTy (mkTyCon "Control.Concurrent.MVar.MVar") [typeOf y] where y = unsafePerformIO $ do z <- newEmptyMVar >>= readMVar return (z `asTypeOf` x)

which is wrong because it also passes the typeOf of the MVar and not the content. This one is correct, I hope: instance Typeable a => Typeable (MVar a) where typeOf x = mkAppTy (mkTyCon "Control.Concurrent.MVar.MVar") [typeOf v] where v = unsafePerformIO $ do y <- newEmptyMVar readMVar (y `asTypeOf` x) On Friday 05 November 2004 16:44, Koji Nakahara wrote:

instance Typeable a => Typeable (MVar a) where typeOf (x :: MVar a) = mkAppTy (mkTyCon "Control.Concurrent.MVar.MVar") [typeOf (undefined::a)]

Yes, that's it. The above is a lot more convoluted but has a small advantage: it doesn't need -fglasgow-exts. I understand now, why pattern signatures were deemed a useful feature! Thanks to all who helped. Cheers, Ben

On Friday 05 November 2004 15:51, you wrote:

On Fri, Nov 05, 2004 at 01:57:53PM +0100, Benjamin Franksen wrote:

...

Hello Experts,

I need MVar and Chan to be instances of Typeable. Any hint on how this is most easily done would be greatly appreciated. I could change the libraries and add 'deriving Typeable' but I hesitate to do so.

The easiest way is to hide type constructor Chan:

import Control.Concurrent import Data.Generics

newtype MyChan a = MyChan (Chan a) deriving Typeable

Of course, you can also write the instance for Chan by hand.

This might be the easiest way, but is otherwise inconvenient. I tried to write the instances by hand. My first attempt was: instance Typeable a => Typeable (MVar a) where typeOf x = mkAppTy (mkTyCon "Control.Concurrent.MVar.MVar") [typeOf (undefined::a)] but unfortunately this doesn't work. Ghc complains about Ambiguous type variable `a1' in the top-level constraint: `Typeable a1' arising from use of `typeOf' at Helpers.hs:8 The reason is apparently that inside the definition of typeOf the type variable 'a' is not unified with the 'a' from the instance header. I could write typeOf (MVar x) = mkAppTy (mkTyCon "Control.Concurrent.MVar.MVar") [typeOf y] where y = undefined `asTypeOf` x but the doc says that typeOf should be written without evaluating its argument, so that is ca be passed 'undefined'. What I need is a trick that enables me to get at the type of the 'a' in the instance header for use inside definition of 'typeOf'. Ben