Re: Something out for which to watch

[haible (Bruno Haible)]

[This message is from David Loewenstern <loewenst@paul.rutgers.edu>.]

.. On Sat, 1 May 93 21:33:45 +0200, Peter Dudey <pdudey@jupiter.willamette.edu> said:
} If you use a function like this:

} (defun foo (x) (delete x '(a b c d e f)))

} the quoted list '(a b c d e f) is NOT immune to the delete!  Running the
} function could actually change it!

Yes.  I'm fairly certain that CLtL2 makes this trap specific.

} I'm not sure if this is a bug in CLISP or not--I was under the impression
} that a quoted, explicit list like that was a constant, and would be
} "re-created" every time the function was run.

It isn't.  I know Allegro and Symbolics CL show the same behavior.

It would be pretty space-inefficient to have the compiler force every
constant list (or array, or structure, I presume?) to be recopied
every time it was used.  I shudder to think that every construct that
looks like 
(CASE x 
  ((a b c d e f g h i) (foo x))
  ((j k l) (bar x)))

really disguises a huge amount of consing, since typically CASE is
implemented as expanding into MEMBER tests:
(LET ((#:G0001 x))
  (COND ((MEMBER #:G0001 '(a b c d e f g h i)) (foo x))
	((MEMBER #:G0001 '(j k l))) (bar x)))

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