Re: #:getter procedure returns unexpected value in GOOPS

[David Pirotte]

Hello Andy,

Le Mon, 09 Mar 2015 22:58:20 +0100,
Andy Wingo <address@hidden> a écrit :

> Hi!
> 
> On Sat 26 Apr 2014 01:24, "Diogo F. S. Ramos" <address@hidden> writes:
> 
> > When using GOOPS, if a class has a second slot, the #:getter procedure
> > of the first slot returns the value of the second slot when applied to
> > an instance of a subclass.
> >
> > (use-modules (oop goops))
> >
> > (define-class <foo> ()
> >   (a #:init-form 'foo #:getter foo-a)
> >   (b #:init-form 42))
> >
> > (define-class <bar> (<foo>)
> >   (a #:init-form 'bar))
> >
> >   (foo-a (make <foo>)) => foo
> >   (foo-a (make <bar>)) => 42
> >
> > I expected:
> >
> >   (foo-a (make <bar>)) => bar
> 
> I realize this is really late :)  But since this thread isn't linked to
> a bug, note that this is now fixed in stable-2.0 and master, to match
> the behavior in Guile 1.8 and previous, which is actually that:
> 
>   (foo-a (make <bar>)) => error!
> 
> because <bar> doesn't just define a different init-value for the slot,
> it defines a different slot entirely.

I'm late too :)

I am afraid this was an unfortunate and quite terrible [design? I doubt] bug in
Guile-1.8 then. Indeed, even Stklos does correctly implement subclass slot
redefinition as specified by the clos protocol [*]

;;; subclass-slot-redefinition.scm starts here
(define-class <person> ()
  ((name :accessor name :init-keyword :name :init-form "")
   (age :accessor age :init-keyword :age :init-form -1)))

(define-class <teacher> (<person>)
  ((subject :accessor subject :init-keyword :subject :init-form "")))

(define-class <maths-teacher> (<teacher>)
  ((subject :init-form "Mathematics")))
;;; ends here

address@hidden:~/alto/projects/stklos 8 $ stklos
*   STklos version 1.10
 *  Copyright (C) 1999-2011 Erick Gallesio - Universite de Nice <address@hidden>
* * [Linux-3.16.0-4-amd64-x86_64/pthread/readline/utf8]
stklos> (load "subclass-slot-redefinition.scm")
stklos> (define p2 (make <maths-teacher> :name 'john :age 34))
;; p2
stklos> (describe p2)
#[<maths-teacher> b34420] is an an instance of class <maths-teacher>.
Slots are: 
     age = 34
     name = john
     subject = "Mathematics"
stklos> 

[*]

In summary, the clos protocol says:

        [ this is a copy/paste from a clos tutorial, also pointed by
        [ the Stklos reference manual:

        [       http://www.aiai.ed.ac.uk/~jeff/clos-guide.html#slots

        When there are superclasses, a subclass can specify a slot that has 
already
        been specified for a superclass. When this happens, the information in 
slot
        options has to be combined. For the slot options listed above, either 
the
        option in the subclass overrides the one in the superclass or there is a
        union:

           :ACCESSOR  -  union
           :INITARG   -  union
           :INITFORM  -  overrides

        This is what you should expect. The subclass can change the default 
initial
        value by overriding the :initform, and can add to the initargs and 
accessors.

        However, the union for :accessor is just a consequence of how generic
        functions work. If they can apply to instances of a class C, they can 
also
        apply to instances of subclasses of C. (Accessor functions are generic.)

Note that the last sentence, which applies to getters and setters of course, is 
of
prime importance wrt our previous conversation and my reported bug about 
setters,
which must be inherited:

        wrt setters not being inherited, the current situation not only creates
        technical problems [it forces users to a [bad imo] programming style to
        overcome the bug] but it introduced a dual semantic for define-method 
and
        breaks this fundamental [language designed] rule: "... If they can 
apply to
        instances of a class C, they can also apply to instances of subclasses 
of
        C..."

Cheers,
David

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