Can we simplify Dynamic?

Richard Eisenberg eir at
Mon Sep 28 22:11:41 UTC 2015

And indeed it's  a good idea! And indeed it's happening!

See, for example,
But that's now a bit out of date.

Simon PJ, Dimitrios Vytiniotis, Stephanie Weirich and I are hard at work writing a paper about all of this, and we expect the results to be in GHC 8.0, with this Dynamic:

data Dynamic where
  Dynamic :: TypeRep a -> a -> Dynamic

Note that TypeRep is now indexed. This is a breaking change, and I'll admit we haven't worked out the migration path. But we're focused on figuring out, precisely, where we're going before worrying too hard about how, precisely, we shall get there.

But, a much better question from my standpoint is:
Why do you care? Why do you use Dynamic? We're actually struggling a bit in the motivation section of the paper and would love to know why you care. :)


On Sep 28, 2015, at 6:03 PM, Roman Cheplyaka <roma at> wrote:

> I suggested this last year, see this thread
> 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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