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An example of one way of defining, expanding, and calling an Interlisp macro similar to WHEN of Common Lisp. Macro definitions are represented by lists stored on the property list of a symbol. Here ARGS is bound to the CDR of the calling form.

#interlisp #lisp

Screenshot of a portion of the black and white desktop of a 1980s graphical workstation environment. The desktop has a gray background pattern and one window with a white background and a title bar with white text on a black background. The window is a Lisp REPL where the following expressions are evaluated, output printed, and values returned:

2/4_ (PUTPROP 'WHEN 'MACRO '(ARGS (LIST 'COND (CONS (CAR ARGS) (CDR ARGS)))))
(ARGS (LIST (QUOTE COND) (CONS (CAR ARGS) (CDR ARGS))))
2/5_ (GETPROP 'WHEN 'MACRO)
(ARGS (LIST (QUOTE COND) (CONS (CAR ARGS) (CDR ARGS))))
2/6_ (SETQ FOO 3)
3
2/7_ (EXPANDMACRO '(WHEN (GREATERP FOO 2)
(PRINT "Greater.")
T))
(COND
((GREATERP FOO 2)
(PRINT "Greater.")
T))
NIL
2/8_ (WHEN (GREATERP FOO 2)
(PRINT "Greater.")
T)
"Greater."
T — Screenshot of a portion of the black and white desktop of a 1980s graphical workstation environment. The desktop has a gray background pattern and one window with a white background and a title bar with white text on a black background. The window is a Lisp REPL where the following expressions are evaluated, output printed, and values returned: 2/4_ (PUTPROP 'WHEN 'MACRO '(ARGS (LIST 'COND (CONS (CAR ARGS) (CDR ARGS))))) (ARGS (LIST (QUOTE COND) (CONS (CAR ARGS) (CDR ARGS)))) 2/5_ (GETPROP 'WHEN 'MACRO) (ARGS (LIST (QUOTE COND) (CONS (CAR ARGS) (CDR ARGS)))) 2/6_ (SETQ FOO 3) 3 2/7_ (EXPANDMACRO '(WHEN (GREATERP FOO 2) (PRINT "Greater.") T)) (COND ((GREATERP FOO 2) (PRINT "Greater.") T)) NIL 2/8_ (WHEN (GREATERP FOO 2) (PRINT "Greater.") T) "Greater." T

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