Variable references in lisp

Question

Another newbie (Common) LISP question:

Basically in most programming languages there\'s a mean for functions to receive references to variables instead of just value

Answer 1

Of course, in Lisp you can make your own way make variable references, if you want to. The simplest approach is like this:

(defstruct reference getter setter)

(defmacro ref (place)
  (let ((new-value (gensym)))
    `(make-reference :getter (lambda () ,place)
                     :setter (lambda (,new-value)
                               (setf ,place ,new-value)))))

(defun dereference (reference)
  (funcall (reference-getter reference)))

(defun (setf dereference) (new-value reference)
  (funcall (reference-setter reference) new-value))

And then you can use it:

(defun increase-by-one (var-ref)
  (incf (dereference var-ref)))

(defun test-inc-by-one (n)
  (let ((m n))
    (increase-by-one (ref m))
    (values m n)))

(test-inc-by-one 10) => 11, 10