Re: [pro] The Best Examples of "Code is Data"

What happens if you do

(defun foo (x) (plus x 3))

?

Luís Oliveira wrote:

On Sun, Sep 5, 2010 at 9:59 PM, Scott L. Burson Scott@sympoiesis.com wrote:

...

As tempting as it is to call EVAL for various purposes, the reality is that correct uses of EVAL are quite rare. In fact, unless you're writing your own read-eval-print loop of some kind, the best rule of thumb is that if you're calling EVAL explicitly, you've made a mistake. I have trouble coming up with any exceptions to this rule other than a REPL.

One of the many tricks I've picked up from James Bielman (I hope he's subscribed to this mailing list :-)) was using CONSTANTP and EVAL in compiler macros. Here's a simple example:

(defun plus (x y)
  (+ x y))

(define-compiler-macro plus (&whole form x y)
  (if (and (constantp x) (constantp y))
      (+ (eval x) (eval y))
      form))

Execution examples:

(compiler-macroexpand-1 '(plus 1 1)) => 2

(compiler-macroexpand-1 '(plus '1 '1)) => 3

(compiler-macroexpand-1 '(plus (* 2 (/ 4 2)) (+ 3 2))) => 9

(defconstant +1+ 1)
(compiler-macroexpand-1 '(plus +1+ 2)) => 3