Optimizing "counting" GADTs

David Feuer david.feuer at gmail.com
Sun Jun 19 14:06:12 UTC 2016

I meant reflection in the sense of the reflection package. Sorry for the
On Jun 19, 2016 4:28 AM, "Ben Gamari" <ben at smart-cactus.org> wrote:

> "Edward Z. Yang" <ezyang at mit.edu> writes:
> snip
> >> Dictionaries are harder to come by,
> >> but reflection might be an option.
> >
> > If I understand correctly, even if you have a Typeable dictionary you
> > don't necessarily have a way of constructing the other dictionaries
> > that are available at that type.  Maybe that is something worth fixing.
> >
> Right; a Typeable dictionary gives you nothing more than the identity of
> the type. You cannot get any further dictionaries from it. Honestly
> fixing this seems quite non-trivial (essentially requiring that you
> construct a symbol name for the desired dictionary and do a symbol table
> lookup to find it, hoping that the linker didn't decide to drop it due
> to being unused).
> Moreover, it seems possible that providing this ability may have
> consequences on parametricity. Reflection already comes dangerously
> close to compromising this property; we are saved only by the fact that
> a Typeable constraint is needed to request a representation. I'd imagine
> that allowing the user to produce arbitrary dictionaries from a
> representation may pose similar issues.
> Cheers,
> - Ben
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