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From: norvig@bansai.uucp (Peter Norvig)
Newsgroups: comp.lang.lisp
Subject: Downward Funargs no longer anonymous?
Keywords: funarg, dynamic-extent
Message-ID: <26081@pasteur.Berkeley.EDU>
Date: 11 Jul 90 17:04:25 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: norvig@bansai.uucp (Peter Norvig)
Distribution: usa
Organization: University of California at Berkeley
Lines: 26
X-Local-Date: 11 Jul 90 10:04:25 PDT

In several LISPM implementations, we have been able to declare:

#'(lambda (x)
    (declare (sys:downward-function))
    (f x y))

to indicate that even though the lambda expression captures the variable y
from the environment, it will only be used by functions below, and hence can
be stack-allocated, without copying the enviornment.

In CLtL2, the declaration DYNAMIC-EXTENT has been added, ostensibly to cover
this and other cases.  Unfortunately, this means we can no longer use
anonymous lambda expressions.  I have to say instead:

(flet ((f (lambda (x)
            (declare (dynamic-extent f))
            (f x y))))
   ...
   #'f
   ...
   )

where the "..." indicate the surrounding environment.  Thus, the
dynamic-extent declaration does not seem to do the job of
sys:downward-function very well.  Was this problem discussed
when dynamic-extent was adopted?