Proposal: Simplify/Generalize Data.Dynamic

[Simon Peyton Jones]

Yes, this would be a great improvement.  Often proposed, but never executed!

Simon

From: Libraries [mailto:libraries-bounces at haskell.org] On Behalf Of Edward Kmett
Sent: 25 August 2014 16:15
To: Haskell Libraries
Subject: Proposal: Simplify/Generalize Data.Dynamic

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At the time, I offered a solution that is no longer applicable in a world with polykinded Typeable.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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