Proposal: Simplify/Generalize Data.Dynamic

[Edward Kmett]

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At the time, I offered a solution that is no longer applicable in a world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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