How to execute a function on setf place

I have a list which contains some symbols and values. The goal is to setf the class slot with the accessor, whose symbol is provided by the list :

(defclass my-class ()
 ((attr :accessor attr)))

(let ((to-call '(attr "some-value"))
      (obj (make-instance 'my-class)))
 (setf `(,(car to-call) obj) (cadr to-call)))

I have tried via a macro :

(defmacro call-accessor (to-call)
 `(setf (,(car to-call) obj) "some-value"))

(let ((to-call '(attr "some-value"))
      (obj (make-instance 'my-class)))
 (call-accessor to-call))

Which fails too, since to-call is a symbol and not a list.

eval does not work since to-call is a lexical variable;
It is not possible to do a let over the macro to give it the list;
I have tried with with-slots and with-accessors but the problem remains the same, because they are macros too.
I have considered macros which declares other macro, and symbol-macrolet too.

How can I setf a slot via an accessor corresponding to a symbol in my list ?

Thank you.

Solution

Calling the accessor function

The goal is to setf the class slot with the accessor

An accessor is a pair of functions. You can get the part which sets the value via FDEFINITION. The name of the function is a list (SETF accessor-name ). This is unusual: Common Lisp has in this case function names which are not symbols, but lists.

CL-USER 14 > (let ((to-call '(attr "some-value"))
                   (obj (make-instance 'my-class)))
               (funcall (fdefinition `(setf ,(first to-call)))
                        (second to-call)
                        obj)
               (describe obj))

#<MY-CLASS 40200614FB> is a MY-CLASS
ATTR      "some-value"

Using a function call-accessor:

CL-USER 25 > (let ((to-call '(attr "some-value"))
                   (obj (make-instance 'my-class)))
               (flet ((call-accessor (obj to-call)
                        (funcall (fdefinition `(setf ,(first to-call)))
                                 (second to-call)
                                 obj)))
                 (call-accessor obj to-call)
                 (describe obj)))

#<MY-CLASS 402000220B> is a MY-CLASS
ATTR      "some-value"

using SETF with APPLY to hide the FDEFINITION call

To use setf with a computed accessor, one might need to use an apply form and a custom function.

Something like call-accessor would naturally be a function, because it does runtime lookup and takes values. Trying to use a macro would be more useful if the accessor would be known at compile time.

CL-USER 23 > (let ((to-call '(attr "some-value"))
                   (obj (make-instance 'my-class)))
               (flet (((setf my-setter) (new-value object accessor) 
                        (funcall (fdefinition `(setf ,accessor))
                                 new-value
                                 obj)))
                 (flet ((call-accessor (obj to-call)
                          (setf (apply #'my-setter obj (list (first to-call)))
                                (second to-call))))
                   (call-accessor obj to-call)
                   (describe obj))))

#<MY-CLASS 40200009AB> is a MY-CLASS
ATTR      "some-value"

Choice of data structure

I think it's okay to compute accessor functions and similar. There may be use cases for that. CLOS was designed to be dynamic and reflective to allow these things.