Re: [Haskell-cafe] A newbie question ?

"proxy type" is indeed much clear! thanks Ryan Ingram wrote:

Remember, "fromJust res" won't be evaluated unless typeOf needs it. But it does get typechecked at compile time, and the type is used to select which instance of typeOf to call.

Fortunately, legal instances of Typeable require typeOf to not inspect its argument.

This is a somewhat old design; modern variations on this tend to use a "proxy type" to make it clear that the argument's value cannot be evaluated. In modern Haskell, I would write it like so:

data Proxy a = Proxy proxy :: a -> Proxy a

class Typeable a where typeOf :: Proxy a -> TypeRep

-- uses ScopedTypeVariables extension lookup :: forall a. Typeable a => TypeMap -> Maybe a lookup (TypeMap mp) = liftM (fromJust . fromDynamic) $ Map.lookup (typeOf (Proxy :: Proxy a)) mp

-- ryan

On Tue, Apr 27, 2010 at 5:17 PM, zaxis wrote:

...

newtype TypeMap = TypeMap (Map.Map TypeRep Dynamic)

lookup :: Typeable a => TypeMap -> Maybe a lookup (TypeMap mp) = res    where res = liftM (fromJust . fromDynamic) $ Map.lookup (typeOf $ fromJust res) mp

It seems that the `res` in  `fromJust res`  has not been defined ?

Sincerely!

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