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

5 Nov 2004


      On Fri, 5 Nov 2004 14:43:55 +0100
Benjamin Franksen  wrote:
<snip>
...
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
You can write:

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


Hope it helps,
Koji Nakahara