Can't call method when first argument is nil?

Question

(defmethod carpet-append ((this carpet) (rect image-rectangle))
  (destructuring-bind (rect-width . rect-height)
      (rectangle-size rect)
    (destructuring-bind (bitmap-width . bitmap-height)
        (carpet-size this)
      (if this
          (iter:iter
            (iter:with min-area = (* (+ bitmap-width rect-width)
                                     (+ bitmap-height rect-height)))
            (iter:with min-pos = nil)
            (iter:for pos in (awailable-positions this))
            (iter:for test-area = (try-fit rect pos (carpet-bitmap this)))
            (when (and test-area (< test-area min-area))
              (setf min-pos pos))
            (iter:finally
             (let ((new-carpet
                    (make-carpet
                     :bitmap (make-array
                              (list (+ (car min-pos) rect-width)
                                    (+ (cdr min-pos) rect-height))
                              :element-type 'bit)
                     :rectangles (cons rect (carpet-rectangles this)))))
               (copy-bitmap-state this new-carpet)
               (setf (rectangle-position rect) min-pos)
               (place-image new-carpet rect)
               (return new-carpet))))
          (make-carpet
           :bitmap (make-array
                    (list rect-width rect-height)
                    :element-type 'bit)
           :rectangles (list rect))))))

image-rectangle and carpet are structs.

When this method is called like so:

(carpet-append
 nil
 #s(image-rectangle
    :position (0 . 0)
    :size (48 . 76)
    :file "/home/wvxvw/projects/spritesheet/test-images/test-0.png"))

I'm getting a:

#<SIMPLE-ERROR "~@<There is no applicable method for the generic function ~2I~_~S~
 ~I~_when called with arguments ~2I~_~S.~:>"

Is it meant to be like this? Perhaps there is some way to make it accept nil as an argument? How would I specify that only arguments of type nil and carpet are applicable?

Answer 1

If you have an arglist with classes carpet and image-rectangle, the arguments better be of these classes or their subclasses. You can't pass NIL, when your argument is declared to be of class carpet.

Thus (if this does not make sense. If you pass a carpet object, and you can't pass anything else, the test this will always be true.

If you want to write a method for the NIL object and a rectangle, then you can use the class NULL.

(defmethod carpet-append ((this null) (rect image-rectangle))
   ...)

Since CLOS does not have class combinators like OR or AND, you have to write a method for each case.