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Re: New types; also opacity without passkeys

   Date: Wed, 15 May 1996 22:00:11 +0000
   From: William D Clinger <will@ccs.neu.edu>


   Although this proposal does not explicitly require any opacity,
   the requirement that the predicate P? returns #T if and only if
   its argument was created by a call to U cannot be implemented
   without some kind of opacity.  In particular, this proposal
   cannot be implemented in R4RS Scheme.

I find it fascinating that, on its face, your proposal is simply about
disjointness of types.  Yet, you parlay disjointness into opacity.

My knowledge of mathematical logic does not extend to its connections
with information theory.  Is this a well known result?  Is its
application to CS new?

On the practical level, the link between disjoint-types and opacity
should convince those rrrs-authors in favor of disjoint-types to
tolerate some opacity in Scheme.  Are there rrrs-authors who want
neither disjoint-types nor opacity?