It seems to be a fairly popular proposal. =)
Indeed. Ben Price (now at Strathclyde) did a short internship at MSR working on this; see
https://ghc.haskell.org/trac/ghc/wiki/TypeableT
Dimitrios, Stephanie, Richard, and I are working on a paper about it, which should be out in a couple of weeks.
At that point we’ll be in a position to have a proper core-libraries-committee discussion about the right API etc; I hope we can make the result part of GHC 8.0.
But I want to get the paper out first since it gives a reasonably articulate exposition of the ideas and rationale, so that we all start on the same page.
Simon
From: Libraries [mailto:libraries-bounces@haskell.org] On Behalf Of Edward Kmett
Sent: 29 September 2015 16:53
To: Roman Cheplyaka
Cc: Haskell Libraries
Subject: Re: Can we simplify Dynamic?
It seems to be a fairly popular proposal. =)
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-Edward
On Mon, Sep 28, 2015 at 6:03 PM, Roman Cheplyaka
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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