[Haskell-cafe] Typeable and Dynamic

Antoine Latter aslatter at gmail.com
Mon Nov 3 19:58:39 EST 2008

On Mon, Nov 3, 2008 at 4:58 PM, minh thu <noteed at gmail.com> wrote:
> Hi,
> Given a TypeRep and a Dynamic value with type corresponding to the
> TypeRep, I'd like to be able to use fromDyn *without* specifying a type
> (given as an additional 'default' argument) (since it is somewhat known
> from the TypeRep).
> That is, I'd like an undefinedOf that can be used in
> extract :: Typeable a => TypeRep -> Dynamic -> a
> extract typerep dyn = fromDyn (undefinedOf typerep) dyn
> For instance, if TypeRep is Int,
> extract Int dyn = fromDyn (undefined::Int) dyn
> Is it possible to write "extract" ?

Yep!  It's already written:

fromDynamic :: Typable a => Dynamic -> Maybe a

It returns Nothing if the Dynamic value was of the wrong type.




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