user interface macros

[David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>]

    Date: Thu, 3 May 90 11:47:44 PDT
    From: David Gray <gray@lucid.com>

    I'm curious about Moon's phrase "future extensions that unify this
    environment with the &environment argument to macros"; isn't that
    unification already dictated by the current definition of FIND-CLASS?
    I have implemented such unified environments on the Explorer and found that
    it works very well, so this is not untested theory.

Here's the context of your quotation:

  Defining NULL to be the predicate that tests whether an environment is the
  run-time environment is not very abstract.  This could run into serious
  problems with future extensions that unify this environment with the
  &environment argument to macros....

There is more than one macroexpansion environment that, regarded as a 
FIND-CLASS environment, represents the global runtime environment.  Only
one of them can be NIL, therefore NULL cannot be the test for whether
an environment represents the global runtime environment.

It's an argument for having a specific predicate for testing whether
an environment has this property, instead of violating abstraction
and using NULL as that predicate.

There is the additional issue of whether such a predicate is needed
at all.  Moon and Cyphers said:

  I was unable to figure out what specific predicates are needed, nor
  what the specific problem is that needs to be solved (alluded to in
  "testing for these cases" above).

Gray said:

  I would want to see the need for that demonstrated first.

So I think we agree that we're not yet convinced such a predicate is
needed.  Gregor hasn't addressed this issue yet.