On Mon, Dec 12, 2016 at 1:31 PM, David Feuer <david.fe...@gmail.com> wrote:
> On Dec 12, 2016 1:15 PM, "Edward Kmett" <ekm...@gmail.com> wrote: > > A few thoughts in no particular order: > > Unlike this proposal, the existing 'reify' itself as core can actually be > made well typed. > > > Can you explain this? > I mean just that. If you look at the core generated by the existing 'reify' combinator, nothing it does is 'evil'. We're allowing it to construct a dictionary. That isn't unsound where core is concerned. Where the surface language is concerned the uniqueness of that dictionary is preserved by the quantifier introducing a new type generatively in the local context, so the usual problems with dictionary construction are defused. Tagged in the example could be replaced with explicit type application if > backwards compatibility isn't a concern. OTOH, it is. > > > Would that help Core typing? > It doesn't make a difference there. The only thing is it avoids needing to make up something like Tagged. > > On the other other hand, if you're going to be magic, you might as well > go all the way to something like: > > reify# :: (p => r) -> a -> r > > > How would we implement reify in terms of this variant? > That I don't have the answer to. It seems like it should work though. and admit both fundep and TF forms. I mean, if you're going to lie you > might as well lie big. > > > Definitely. > > There are a very large number of instances out there scattered across > dozens of packages that would be broken by switching from Proxy to Tagged > or explicit type application internally. (I realize that this is a lesser > concern that can be resolved by a major version bump and some community > friction, but it does mean pragmatically that migrating to something like > this would need a plan.) > > > I just want to make sure that we do what we need to get Really Good Code, > if we're going to the trouble of adding compiler support. > That makes sense to me. -Edward
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