lisp clos accessor problems

Question

I can't use the clos accessor functions when the class is in a list.

Say I have class a:

(defclass a ()
  ((a :accessor a
      :initarg :a)))

And I make 2 instances:

(defparameter b (make-instance 'a :a 1))
(defparameter c (make-instance 'a :a 2))

and then if I wanted to create a function that would get the a value for each of the instances while in a list i would do

(defun get-a (lst)
  (mapcar #'a lst))

and call it with

(get-a '(b c))

but I do that I get an error:

There is no applicable method for the generic function
  #<STANDARD-GENERIC-FUNCTION A (1)>
when called with arguments
  (B).
    [Condition of type SIMPLE-ERROR]

And it also happens if instead of calling the accessor directly with mapcar, I call a function which contains the accessor. Also I've tried using loops and other things instead of mapcar.

Thanks

Rainer Joswig Rainer Joswig · Accepted Answer · 2014-03-05T20:16:49

If you read the error, you get the explanation.

There is no applicable method for the generic function
  #<STANDARD-GENERIC-FUNCTION A (1)>
when called with arguments
  (B).

So you got a call, which is similar to (a 'b). But b is a symbol, not a CLOS instance.

(b c) is a list of two symbols. You probably wanted to create a list of two CLOS instances. Use LIST to create a list with evaluated arguments.

lisp clos accessor problems

1 Answers