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   Date: Wed, 20 May 92 10:11:40 pdt
   From: Morry Katz <katz@sid.stanford.edu>
   Reply-To: katz@cs.stanford.edu

      Date: Wed, 20 May 92 12:42:30 -0400
      From: "Aubrey Jaffer" <jaffer@martigny.ai.mit.edu>

      The addition of the following syntax to Scheme would allow automatic
      detection of optional features of R5RS.  Currently, optional features
      are frustrating because one can't use them in portable programs.
      DEFINED? would allow uniform detection of feature support.

	 (defined? <symbol>)					syntax

       Equivalent to #t if <symbol> is a syntactic keyword (such as IF), has
       a syntactic definition or binding (macro) or is a symbol with a
       top-level value (such as CAR).  Otherwise equivalent to #f.

   The top-level restriction is problematic in that many inplementations
   do not have a concept of top-level.

That is addressed in this next paragraph:

      My intent here is that DEFINED? clauses can be replaced at macro
      expansion time with #t or #f.  It is also acceptable if DEFINED? of
      lexically bound variables is #t in the scope of the binding.

   I believe that much more than defined is needed to use optional
   features.  One also needs to know the arity of a form an dpossibly
   clues as to its semantics.  

There are only 5 procedures with options on number of arguments:
make-vector, make-string, apply, -, and /.  I have already proposed
making multiarg apply essential.  The are all easily written in
Scheme.  In fact SLIB has code for multiarg -, /, and apply.  The
arity question is a small part of the uncertainty.  As for clues to
its semantics, if an implementation is not compliant with the report
then how can one expect it to support clues?

   I might support a more limited proposal that just allowed for testing
   of the existance of optional features in the report.

When you say features I gather that you mean more than just syntactic
keywords and procedure names.  I resist this because it will require
naming and implementing each switch.  With DEFINED? it would be
automatic.  How about this as a compromise:

	The value of DEFINED? is specified only for arguments
	which are syntactic keywords or procedures in R5RS.