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

[Carter Schonwald]

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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