Subject: bug#58297: GOOPS slot accessor specialization and inheritance do not compose

From: "Thompson, David" <[email-masked]>
Date: Tue, 4 Oct 2022 20:21:51 -0400

In Guile, slot accessor specialization and inheritance do not compose.
For example, you can't specialize an accessor's setter for a parent
class and have it apply to a child class.  Every child class defines
new slot accessor methods. which means that the specialized parent
methods will not be called since the new methods take precedence.

The code below demonstrates the issue:

  (use-modules (oop goops))

  (define-class <person> ()
    (name #:init-keyword #:name #:accessor name))

  (define-method ((setter name) (person <person>) new-name)
    (display "renaming!\n")
    (slot-set! person 'name new-name))

  (define-class <child> (<person>))

  (define p1 (make <person> #:name "Alice"))
  (define p2 (make <child> #:name "Bob"))

  ;; Only the first set! call uses the specialized setter method defined
  ;; above.
  (set! (name p1) "Ada")
  (set! (name p2) "Ben")

I would have expected the specialized setter method to apply to both
<person> and <child> since <child> does not shadow the 'name' slot.

I compared this behavior with that of Common Lisp and found that CLOS
does not clobber the method from the parent class, as demonstrated by
this example program that I tested with SBCL:

  (defclass person ()
    ((name :initarg :name :accessor name)))

  (defmethod (setf name) (new-name (obj person))
    (format t "renaming!~&")
    (setf (slot-value obj 'name) new-name))

  (defclass child (person) ())

  (defvar p1 (make-instance 'person :name "Alice"))
  (defvar p2 (make-instance 'child :name "Bob"))

  ;; Both of these setf calls use the specialized setf method defined
  ;; above.
  (setf (name p1) "Ada")
  (setf (name p2) "Ben")

I find the Common Lisp behavior much more desirable.  Is this a bug or
intended behavior?

Thanks for reading,

- Dave