extra lambda list keywords

[David C. Plummer <DCP@QUABBIN.SCRC.Symbolics.COM>]

    Date: Thu 10 Jul 86 14:18:40-MDT
    From: SANDRA <LOOSEMORE@UTAH-20.ARPA>

    I just noticed the bit on p. 65 of the manual where it says:

    "Implementations of Common Lisp are free to provide additional lambda-list
    keywords."

    Is this *really* useful to any implementation?  Although you can supposedly
    find out about the additional keywords from the lambda-list-keywords constant,
    allowing implementation-specific crud to appear in lambda lists seems like
    another good way to frustrate would-be implementors of portable code walkers.
    In particular, I'm concerned that if implementations try to extend the syntax
    of what can appear in lambda lists, code walkers would have a very hard time
    identifying default values and other bits of evaluable code in the lambda
    list.  Not to mention that a routine that tries to parse lambda lists could
    get very confused if, for example, it found a strange keyword &foo sitting
    where it expected to find only an &rest or &key.

What's this talk about portable code walkers.  You can't write portable
code at all if you use implementation-specific lambda-list keywords, let
alone portable code walkers.

Historically, I think LispMs did (some may still) have lambda list
keywords &SPECIAL and &LOCAL, whose effects are somewhat obvious by
their names.  There was also &QUOTE, which effectively defined a special
form, because the argument was automatically quoted.  There was also an
&FUNCTIONAL which I think was like &QUOTE except that it wasn't quoted
as data, it was quoted as a function.  For example, this is how it soft
of behaved:
	(defun foo1 (&quote bar1 &functional baz1)
	  (list bar1 baz1))
	(foo1 car cdr) =more-or-less= (list 'car #'cdr)
There was also a &LIST-OF for macros.  See some old LispM documentation
if you really care.

The above may not be completely accurate, but you get the idea.  I agree
that extending lambda-list keywords should be done carefully and for
very good reasons.