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

[Roman Cheplyaka]

Right — I misunderstood what Dan meant on the first reading.

Then it's an even more drastic change to Typeable. And it's not clear (to me, at
least) how it should work.

* Carter Schonwald <carter.schonwald at gmail.com> [2014-03-10 16:04:21-0400]
> isn't it a GADT/singleton/data family more than a phantom?
> 
> 
> 
> 
> On Mon, Mar 10, 2014 at 2:09 PM, Roman Cheplyaka <roma at ro-che.info> wrote:
> 
> > 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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> >
> >
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