CLOS slot accessors: read but not write - lisp

Question

I have a list of the names of slots of a CLOS object:
(DEFCLASS TRIAL-DATA (STANDARD-OBJECT)
((A-DATUM :ACCESSOR A-DATUM :INITARG :A-DATUM :INITFORM NIL)
(BOTH-DATA :ACCESSOR BOTH-DATA :INITARG :BOTH-DATA :INITFORM 0)
(CUMULATIVE-DATA :ACCESSOR CUMULATIVE-DATA :INITARG :CUMULATIVE-DATA :INITFORM NIL)
(NAME :ACCESSOR NAME :INITARG :NAME :INITFORM VALUE)))
(let* ((td (make-instance 'trial-data))
(slot-lst (mapcar #'slot-definition-name (class-slots (class-of td)))))
I can read the values of these slots:
(let* ((td (make-instance 'trial-data))
(slot-lst (mapcar #'slot-definition-name (class-slots (class-of td)))))
(funcall (symbol-function (nth 0 slot-lst)) td))
==> NIL
But why can I not write new values to these slots? Shouldn't my class definition of trial-data have created an accessor function for each slot?
;; Should set the first slot, a-datum's, value to 42
(let* ((td (make-instance 'trial-data))
(slot-lst (mapcar #'slot-definition-name (class-slots (class-of td)))))
(setf (funcall (symbol-function (nth 0 slot-lst)) td) 42))
==>
;Compiler warnings for "/Users/frank/Documents/NRL/Error/Patrolbot/Patrol Construction Notes & Testing.lisp" :
; In an anonymous lambda form at position 123: Undefined function (SETF FUNCALL)
> Error: Undefined function (SETF FUNCALL) called with arguments (42 #<STANDARD-GENERIC-FUNCTION A-DATUM #x302001D1C5DF> #<TRIAL-DATA #x30200200D95D>) .
> While executing: #<Anonymous Function #x30200200EB7F>, in process Listener-2(5).

Answer 1

The accessor is called a-datum.
The reader:
CL-USER 9 > #'a-datum
#<STANDARD-GENERIC-FUNCTION A-DATUM 406000091C>
The writer:
CL-USER 10 > #'(setf a-datum)
#<STANDARD-GENERIC-FUNCTION (SETF A-DATUM) 422000958C>
If you want to call via funcall the writer, you need to call above function.
If you have a plain form (setf (a-datum foo) 'bar)) then this needs to be resolved at macro expansion time.
The error message says that #'(setf funcall) is undefined. Thus (setf (funcall ...) ...) does not exist.
How do you get the writer function in your case?
CL-USER 11 > (fdefinition '(setf a-datum))
#<STANDARD-GENERIC-FUNCTION (SETF A-DATUM) 422000958C>
CL-USER 12 > (let ((name 'a-datum)) (fdefinition `(setf ,name)))
#<STANDARD-GENERIC-FUNCTION (SETF A-DATUM) 422000958C>
Task for you: what are the correct arguments for above function?

Answer 2

Rainer Joswigs's answer addresses the issue of why you can't set with the code that you have now. However, it's also important to note that there's no reason that reader, writer, or accessor name has to be the same as the slot name, so if what you've actually got is the slot name, then you should use (setf slot-value) with it. E.g.,
(defclass foo ()
((bar :accessor getbar :initform 42)))
(defparameter *foo* (make-instance 'foo))
;; neither of these work
(setf (bar *foo*) 34)
(funcall #'(setf bar) 34 *foo*)
(slot-value *foo* 'bar)
;=> 42
(setf (slot-value *foo* 'bar) 36)
;=> 26
(slot-value *foo* 'bar)
;=> 36