Re: Why would anyone want opacity?

[Aubrey Jaffer <jaffer@martigny.ai.mit.edu>]

   Date: Tue, 30 Apr 1996 16:13:13 +0000
   From: William D Clinger <will@ccs.neu.edu>

...
   It appears to me that, in order to satisfy Gerry, a compiler must
   treat all variables as if they were (pardon the expression) VOLATILE.
   This is a considerable price to pay for an abstraction-breaking
   facility that not even Gerry is likely to use very often.

Whoa!  We seem to be confusing the existence of opacity in a language
with procedures necessarily being opaque in that language.

I would like to have an opaque data type; I would also like to have
standard methods for getting arity and other information from
procedures.  I have uses for opacity, but closing procedures is not
one of them.

If we make opacity a first-class datatype, then it can be applied when
desired, rather than thrust upon every closure, even when unwanted.
If this is truly what our impasse is about, then a plausable solution
presents itself:

 * Let those who desire opacity refine Ziggy's proposal.

 * Let those who advocate translucent procedures propose constructs to
   enable such access.

Scheme would attain a large step in utility if we could adopt both
features.