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From: charest@ai-cyclops.jpl.nasa.gov
Newsgroups: comp.lang.lisp
Subject: 'once-only' workalike
Message-ID: <1991May30.205119.1147@jpl-devvax.jpl.nasa.gov>
Date: 30 May 91 20:51:19 GMT
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Reply-To: charest@ai-cyclops.jpl.nasa.gov
Organization: NASA/Jet Propulsion Lab
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Does anyone have sources available for a work-alike implementation of the
'once-only' macro provided in Symbolics Common LISP? Recall that once-only,
when used in the body of a defmacro, guarantees to expand exactly once all of
the defmacro variables in its arglist.* The defmacro variables may be bound to
arbitrary LISP forms in the macro call. For example:

(defmacro double (x &environment env)
  (once-only (x &environment env)
    `(+ ,x ,x)))

(macroexpand '(double (foo-accessor bar)))
=> (let ((#:G666 (foo-accessor bar)))
     (+ #:G666 #:G666))

*Note that I am not particularly interested in the 'code-analysis' feature of
Symbolics' version (wherein the value of each defmacro variable given to
once-only is bound to a gensym only if it is 'non-trivial').

Thanks in advance,
Len Charest

"In the rich man's house there is no place to spit but in his face."
     -Diogenes