Can we simplify Dynamic?

[Roman Cheplyaka]

I suggested this last year, see this thread
https://mail.haskell.org/pipermail/libraries/2014-March/022287.html

On 09/29/2015 12:51 AM, David Feuer wrote:
> Currently,
> 
> data Dynamic = Dynamic TypeRep Obj
>                deriving Typeable
> where
> type Obj = Any
> 
> As a result, all of the operations must be implemented "by hand" using
> unsafeCoerce. The more obvious representation these days would seem to be
> 
> data Dynamic where
>   Dynamic :: Typeable a => a -> Dynamic
> 
> Most of the operations then become trivial applications of Typeable
> functions.    The only exceptions seem to be  dynApply and dynApp. That
> there are exceptions strikes me as quite unfortunate. The easiest fix is
> inspired by the fact that Data.Dynamic uses
> 
> funResultTy :: TypeRep -> TypeRep -> Maybe TypeRep
> 
> from Data.Typeable to decide whether to coerce. It seems reasonable to
> add a more informative version, something like
> 
> applyTypeable :: (Typeable f, Typeable a) =>
>    proxy f ->
>    proxy a ->
>    (forall b . (Typeable b, f ~ (a -> b)) => r) ->
>    Maybe r
> 
> On the other hand, it would be really cool if there were some more
> general way to get type-level information out of Typeable instances,
> pattern matching on the type constructors.


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