[comp.lang.lisp] 'once-only' workalike

charest@ai-cyclops.jpl.nasa.gov (05/31/91)

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

gregor@parc.xerox.com (Gregor Kiczales) (06/01/91)

In article <1991May30.205119.1147@jpl-devvax.jpl.nasa.gov> charest@ai-cyclops.jpl.nasa.gov writes:

   Does anyone have sources available for a work-alike implementation of the
   'once-only' macro provided in Symbolics Common LISP?

There is one of these in the PCL sources.  In the file macros.lisp I believe.