Re: [Haskell-cafe] Making MVar and Chan Instances of Typeable

5 Nov 2004

      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