[Haskell-cafe] Typeable and Dynamic

[minh thu]

2008/11/4, Antoine Latter <aslatter at gmail.com>:
> 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.
>
> See:
>
> http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Dynamic.html#v:fromDynamic

Well, it seems you can't really write

(fromJust . fromDynamic) someDynamic

without giving the type of what you expect.

fromDynamic whould achieve the same thing I wrote in the previous
mail if I did not write the ::Int on the right of undefined.
(I would still have to write ::Int later in the code; that's what I'd
like to avoid)

Thanks,
Thu