Proposal: Simplify/Generalize Data.Dynamic

[Milan Straka]

Hi all,

+1 here, including exporting the constructor.

Cheers,
Milan Straka

> -----Original message-----
> From: Edward Kmett <ekmett at gmail.com>
> Sent: 25 Aug 2014, 11:15
>
> I'd like to propose a cleanup to the API of Data.Dynamic.
> 
> By way of (partial) motivation, a couple of years back Lennart posed a
> question on stackoverflow
> <http://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-function-to-a-dynamic-value/10890414#comment39759480_10890414>
> about how to use Dynamic to implement
> 
> apD :: forall f. Typeable1 f =>
>   (forall a. a -> f a) -> Dynamic -> Dynamic
> 
> 
> At the time, I offered a solution that is no longer applicable in a world
> with polykinded Typeable.
> 
> But, if we change the definition of Data.Dynamic to
> 
> data Dynamic where
>   Dynamic :: Typeable a => a -> Dynamic
> 
> from its current magic implementation in terms of unsafeCoerce and a
> manually held typeRep, then
> 
> fromDynamic becomes a trivial application of Data.Typeable.cast and dynApply
> becomes easier,
> 
> This would enable us to implement Data.Dynamic with its full constructor
> exposed without losing safety.
> 
> In lieu of supplying the constructor, we could offer a form of
> 
> withDyn :: Dynamic -> (forall a. Typeable a => a -> r) -> r
> 
> but I'd rather expose the constructor rather than needlessly Church encode
> as the kind of code/user that needs this is the kind that would bemoan
> needless performance barriers in the name of nebulous encapsulation
> benefits.
> 
> Now it becomes possible to write code that does polymorphic things
> underneath the Dynamic wrapper, such as Lennart's example once more, but
> now in a principled way.
> 
> Discusssion Period: 2 Weeks
> 
> *tl;dr* polykinded Typeable took away some power from the users of
> Data.Dynamic, which we can give back in a more principled way, simplifying
> the API as a result.
> 
> -Edward

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