Sunny Sachanandani's Profile

Another classic macro-writing macro is once-only,
which is used to generate code that evaluates certain macro arguments
once only and in a particular order. Using once-only, you
could write do-primes almost as simply as the original leaky
version, like this:

(defmacro do-primes ((var start end) &body body)
  (once-only (start end)
    `(do ((,var (next-prime ,start) (next-prime (1+ ,var))))
         ((> ,var ,end))
       ,@body)))

However, the implementation of once-only is a bit too involved
for a blow-by-blow explanation, as it relies on multiple levels of
backquoting and unquoting. If you really want to sharpen your macro
chops, you can try to figure out how it works. It looks like this:

(defmacro once-only ((&rest names) &body body)
  (let ((gensyms (loop for n in names collect (gensym))))
    `(let (,@(loop for g in gensyms collect `(,g (gensym))))
      `(let (,,@(loop for g in gensyms for n in names collect ``(,,g ,,n)))
        ,(let (,@(loop for n in names for g in gensyms collect `(,n ,g)))
           ,@body)))))