[Haskell-cafe] A newbie question ?

[zaxis]

"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 <z_axis at 163.com> 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!
>>
>> -----
>> fac n = let {  f = foldr (*) 1 [1..n] } in f
>> --
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-----
fac n = let {  f = foldr (*) 1 [1..n] } in f 
-- 
View this message in context: http://old.nabble.com/A-newbie-question---tp28383563p28384431.html
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.