(in-package code-walker)

(defvar *transformers* (make-hash-table :test #'equal))

(defmacro deftransform (transformer-name head args &body body)
  `(progn
     (setf (gethash (list ',transformer-name ',head) *transformers*)
           (lambda ,args ,@body))
     (list ',transformer-name ',head)))

(defmacro deftransform-stop (transformer-name head)
  `(progn
     (setf (gethash (list ',transformer-name ',head) *transformers*)
           (lambda (&rest args)
             (declare (ignore args))
             nil))
     (list ',transformer-name ',head)))

(defvar *whole*)
(defvar *transformer-name*)

(defun whole ()
  *whole*)

(defun walk-continue (&optional subform)
  (if subform
      (walk *transformer-name* subform)
      (mapcar (lambda (subform)
                (walk *transformer-name* subform))
              *whole*)))

(defun walk (transformer-name form)
  (let ((*transformer-name* transformer-name)
        (*whole* form))
    (if (listp form)
        (let ((transformer (gethash (list transformer-name (first form))
                                    *transformers* nil)))
          (if transformer
              (apply transformer (rest form))
              (walk-continue)))
        form)))