From the things I have read I understand that Accessor functions in CLOS allows for the programmer to get and set variables and it generates a generic function of the name that was give in to the accessor to which you need to define different methods.But I what I want to know is why Don't generic functions work the same way as the accessor functions?
for example
(defclass animal ()
((sound
:initarg :sound
:initform "no sound"
:accessor make-sound)))
I can just define
(defmethod (setf make-sound) ((the-animal animal) value)
(setf (slot-value the-animal 'sound) value))
but if i were to take the accessor away and add in
(defgeneric (setf make-sound) (the-animal value))
then I get an error when after executing the following code.
(setf (make-sound dog) "bark")
unless I redefine the generic function and method as the following
(defgeneric (setf make-sound) (value the-animal))
(defmethod (setf make-sound) (value (the-animal animal))
(setf (slot-value the-animal 'sound) value))
or execute
(setf (make-sound "bark") dog) ;this also works with the accessor
My question is why is this happening?Why can't I achieve the same result with generic functions?
A defmethod form creates a generic function if there isn't one.
CL-USER 7 > (defclass animal ()
((sound
:initarg :sound
:initform "no sound")))
#<STANDARD-CLASS ANIMAL 40200ED09B>
Remember: the new value comes first in a SETF function. That's defined by the Common Lisp standard.
CL-USER 8 > (defmethod (setf make-sound) (value (the-animal animal))
(setf (slot-value the-animal 'sound) value))
#<STANDARD-METHOD (SETF MAKE-SOUND) NIL (T ANIMAL) 40200F098B>
CL-USER 9 > (let ((dog (make-instance 'animal)))
(setf (make-sound dog) "bark")
dog)
#<ANIMAL 402002187B>
CL-USER 10 > (slot-value * 'sound)
"bark"
Seems to work.
In defclass the :accessor slot option defines that it defines a reader method and also a corresponding setf method with the correct parameter list: new value first, then the instance of that class.