Page 162

nil!

Exercise for the reader: what's the difference between Graham's nil! and the following:

(defun set-nil (x)
  (setf x nil))

Page 163

Macro functions

The comment at the top of the page about macros being functions is confusing. Macros are not functions, so you can't pass a macro to apply, mapcar, etc.

However, macros work by calling macro expanders to expand the macro call into Lisp code. Macro expanders are functions. You can get the macro expander for a macro using macro-function.

Page 165

rotatef

Why is the macro that swaps two variables called rotatef? Well, the f part is by analogy to setf. rotatef operates on "places" just like setf does. The rotate part is because rotatef can handle any number of places, not just two. See the glossary in Graham for details.

Quicksort

If you do the list-of macro exercise, then you can define quicksort on a list (not a vector) like this:

(defun quicksort (l)
  (cond ((null l) nil)
        (t (let ((p (car l))
                 (r (cdr l)))
             (append (quicksort (list-of x (x :in r) (< x p)))
                     (list p)
                     (quicksort (list-of x (x :in r) (>= x p))))))))

This is pretty inefficient (it conses a lot), but it does capture the essence of the algorithm nicely: choose a pivot p and split the list into those items less than p and greater than p.

Page 172

avg

This is a cute example of compile-time optimization but it's also an example of an evil macro.


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