#f = #none ?

[Mark Nahabedian <naha@RIVERSIDE.SCRC.Symbolics.COM>]

    Date: Tue, 24 Nov 1992 16:19 EST
    From: Stavros Macrakis <macrakis@osf.org>

       Our original design of Dylan had an object called $unsupplied.  We ended
       up abandoning it for exactly the reasons Bob is pointing out.  People got
       tripped up by it too often....

Perhaps a special form ARGUMENT-SUPPLIED-P (or some appropriate name)
could be introduced.  It would take the name of a keyword argument as
argument and return T iff the argument were supplied.

    Clearly there are many ways to trip up with $unsupplied.  But what is
    the logic behind identifying the boolean value false with the absence
    of an argument, or, worse perhaps, non-#f-ness with the boolean value
    true?  Surely there are many ways to trip up with all that as well?

       I think that it's extremely unlikely that we would reconsider this decision
       at this time.  There are other solutions to the problem, and there are
       plenty of other problems in Dylan which don't yet have existing solutions.

    I'm sure many of us would be interested in hearing about these
    problems.