Re: #:getter procedure returns unexpected value in GOOPS

[Mark H Weaver]

David Pirotte <address@hidden> writes:

> Hi Mark,
>
>> > 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
>
>> Indeed, CLOS behaves as you expected...
>
> Are you sure about that?

Read this:

  http://clhs.lisp.se/Body/07_ec.htm

In particular:

 "In general, more than one class among C and its superclasses can
  define a slot with a given name.  In such cases, only one slot with
  the given name is accessible in an instance of C, and the
  characteristics of that slot are a combination of the several slot
  specifiers, computed as follows: [...]"

Here's a transcript of an SBCL session I tried on Debian:

--8<---------------cut here---------------start------------->8---
This is SBCL 1.0.57.0.debian, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
* (defclass foo ()
           ((a :initform 'foo :reader foo-a)
            (b :initform 42)))

#<STANDARD-CLASS FOO>
* (defclass bar (foo)
           ((a :initform 'bar)))

#<STANDARD-CLASS BAR>
* (values (foo-a (make-instance 'foo)) (foo-a (make-instance 'bar)))

FOO
BAR
--8<---------------cut here---------------end--------------->8---

Unknown_lamer on #guile tried the same thing with a different
implementation of Common Lisp, and got the same answer.

     Mark