Examples of what Lisp's macros can be used for

Question

I\'ve heard that Lisp\'s macro system is very powerful. However, I find it difficult to find some practical examples of what they can be used for; things that would be diffi

Answer 1

Anything you'd normally want to have done in a pre-processor?

One macro I wrote, is for defining state-machines for driving game objects. It's easier to read the code (using the macro) than it is to read the generated code:

(def-ai ray-ai
  (ground
   (let* ((o (object))
          (r (range o)))
     (loop for p in *players*
           if (line-of-sight-p o p r)
           do (progn
                (setf (target o) p)
                (transit seek)))))
  (seek
   (let* ((o (object))
          (target (target o))
          (r (range o))
          (losp (line-of-sight-p o target r)))
     (when losp
       (let ((dir (find-direction o target)))
         (setf (movement o) (object-speed o dir))))
     (unless losp
       (transit ground)))))

Than it is to read:

(progn
 (defclass ray-ai (ai) nil (:default-initargs :current 'ground))
 (defmethod gen-act ((ai ray-ai) (state (eql 'ground)))
            (macrolet ((transit (state)
                         (list 'setf (list 'current 'ai) (list 'quote state))))
              (flet ((object ()
                       (object ai)))
                (let* ((o (object)) (r (range o)))
                  (loop for p in *players*
                        if (line-of-sight-p o p r)
                        do (progn (setf (target o) p) (transit seek)))))))
  (defmethod gen-act ((ai ray-ai) (state (eql 'seek)))
            (macrolet ((transit (state)
                         (list 'setf (list 'current 'ai) (list 'quote state))))
              (flet ((object ()
                       (object ai)))
                (let* ((o (object))
                       (target (target o))
                       (r (range o))
                       (losp (line-of-sight-p o target r)))
                  (when losp
                    (let ((dir (find-direction o target)))
                      (setf (movement o) (object-speed o dir))))
                  (unless losp (transit ground)))))))

By encapsulating the whole state-machine generation in a macro, I can also ensure that I only refer to defined states and warn if that is not the case.