Practical example of Lisp's flexibility? [closed]

Answer 1

There are plenty of killer features in Lisp, but macros is one I love particularily, because there's not really a barrier anymore between what the language defines and what I define. For example, Common Lisp doesn't have a while construct. I once implemented it in my head, while walking. It's straightforward and clean:

(defmacro while (condition &body body)
  `(if ,condition
       (progn
         ,@body
         (do nil ((not ,condition))
           ,@body))))

Et voilà! You just extended the Common Lisp language with a new fundamental construct. You can now do:

(let ((foo 5))
  (while (not (zerop (decf foo)))
    (format t "still not zero: ~a~%" foo)))

Which would print:

still not zero: 4
still not zero: 3
still not zero: 2
still not zero: 1

Doing that in any non-Lisp language is left as an exercise for the reader...