[core libraries] RE: Data.Dynamic: Any vs existential

[Roman Cheplyaka]

Adding a phantom type to TypeRep itself will break a lot of code (the untyped
TypeRep is used quite a bit) without any real benefit (it's essentially the same
as Tagged a TypeRep).

* Carter Schonwald <carter.schonwald at gmail.com> [2014-03-10 13:44:13-0400]
> +++++1000 to dan doels strawman suggestion.
> 
> I'm a huge fan of indexed types style apis like that one.
> 
> 
> On Mon, Mar 10, 2014 at 11:54 AM, Dan Doel <dan.doel at gmail.com> wrote:
> 
> > On Mon, Mar 10, 2014 at 7:26 AM, Simon Peyton Jones <simonpj at microsoft.com
> > > wrote:
> >
> >> Dear Core Libraries committee
> >>
> >> I think Roman is right here. Moreover we have known this for at least I
> >> think seven years
> >> http://comments.gmane.org/gmane.comp.lang.haskell.cafe/20097, where I
> >> wrote:
> >>
> >> | Yes, Dynamic preceded the Typeable class, I think.
> >> | Were we to do it today, I think we'd have
> >> |
> >> | data Dynamic = forall a . (Typeable a) => Dynamic a
> >> |
> >> | Whether it's worth changing, I'm not sure.  It's a library so,
> >> | if a change desirable, anyone could take a lead.
> >>
> >> The new representation for Dynamic would be good because it's less
> >> insecure than all this "Obj" nonsense, instead relying on Typeable, which
> >> is pretty good these days.
> >>
> >> Pedro is the most recent visitor to this territory and may have views.
> >>
> >> Roman's point about the method for the Typeable class is a good one too,
> >> and not one I've seen discussed.
> >>
> >> Over to you
> >>
> >
> > I, at least, am in favor of this. I've thought for a while that the ideal
> > form of all this is something like:
> >
> >     data TypeRep a -- indexed by a
> >
> >     class Typeable a where
> >       typeRep :: TypeRep a
> >
> >     data Dynamic = forall a. Dynamic (TypeRep a) a
> >
> >
> > Where, ideally, TypeRep is an indexed type, and you can match on it to
> > refine its argument. Then Typeable just becomes the implicit version of
> > this evidence, for convenience. TypeRep versions of cast would also have to
> > be primitive probably (since one can not, a priori, enumerate the diagonal
> > of all possible monotypes, now and in the future).
> >
> > Anyhow, I don't know if this entire interface is or will be implementable
> > in GHC (the idea comes from a document about JHC way back, although I don't
> > know if even it implements this), but using Typeable in an existential
> > captures the Dynamic part of it, at least.
> >
> > Also, I hadn't seen the Tagged version of Typeable before (and hadn't
> > thought of it myself). I expect it's an oversight that this wasn't the form
> > chosen, as it seems superior to me. In fact, it seems like every situation
> > where the 0-width Proxy# could be used, Tagged could also be used instead,
> > and is automatically 0-width, and has better sharing behavior? Unless you
> > explicitly don't want sharing, that is.
> >
> > It seems like any relative inconvenience could probably be alleviated by
> > the proxy function in tagged, while keeping the nicer behavior.
> >
> > -- Dan
> >
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> >
> >

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