Formal comment #136 (enhancement)

Trivial Enhancement of macros in v5.91: capture-syntax
Reported by:	Lynn Winebarger

Component: 	macros
Version: 	5.91

Summary:

The macro system needs a way of redefining core forms without losing their
primitive definitions.

Description:

Scheme is a language without reserved words.  However, while it is
possible to redefine core forms such as lambda, doing so will render the
system useless, because the original primitive transformer will be lost. 
The v 5.91 draft has admitted that syntactic transformation is
accomplished by closures.  These closures should be accessible.  The
following code illustrates the idea.

(define-syntax primitive-lambda (capture-syntax lambda))
(define-syntax lambda (capture-syntax trace-lambda))
<code to be traced>
(define-syntax lambda (capture-syntax primitive-lambda))

A more serious example might extend the definition of lambda to take
keyword arguments or more patterns in the argument than a simple list. 
The capture-syntax form should be trivial to implement.

RESPONSE:

It is already possible using the import syntax within a library or program
(5.91 script) to rename any keyword and provide an alternative definition
of it in terms of the renamed binding.  Thus a new feature is not needed
to achieve the stated goals for portable code, which is the only code for
which the draft R6RS attempts to give a syntax or semantics.

Since the draft R6RS does not attempt to describe the syntax or semantics
of a top level, implementations are free to support some mechanism for
redefining keywords, e.g., import with renaming and the ability to
redefine keywords, at top level, and we suspect that most will do so.