>>16-18
This is not about random, it's just an example. And even then, the -DRND_VAL approach wouldn't work if you want to use the macro more than once. But comparing the macro directly doesn't even make sense to begin with, since C doesn't have list primitives.
The point of the example was to focus on the =(assert (symbolp list))=, a compile-time assertion about the kind of argument that was passed into the macro. >>19 doesn't fit that bill either, because it just generates code that does the checks at run-time.
And since I'm seeing that you're having so much fun with translating Lisp to C, here's one I recently had fun writing:
(defmacro roll (&body events)
"Chance-based cond."
(let* ((random (gensym))
(vars (loop for (chance . _) in events
collect (list (gensym) chance)))
(body (loop for (chance . action) in events
for (var . _) in vars
collect var into sum
collect
(cons (if (eq chance t) t
`(< ,random (+ ,@(copy-list sum))))
action))))
`(let ((,random (random 1d0)) ,@vars)
(declare (ignorable ,@(mapcar #'car vars)))
(cond ,@body))))which lets me write something like
(roll
(0.3 'a) ; return symbol a with a 30% chance
(0.5 'b) ; return symbol b with a 50% chance
(t 'c)) ; return symbol c with a 20% (= 100% - 30% - 50%) chanceI'm looking forward to what you'll come up with, void.