It seems to be a fairly popular proposal. =)
https://mail.haskell.org/pipermail/libraries/2014-August/023633.html
-Edward
On Mon, Sep 28, 2015 at 6:03 PM, 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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