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GOOPS: Customizing class instantiation

From: Ludovic Courtès
Subject: GOOPS: Customizing class instantiation
Date: Tue, 26 Sep 2006 10:24:58 +0200
User-agent: Gnus/5.110006 (No Gnus v0.6) Emacs/21.4 (gnu/linux)


I'm trying to "dynamically" create classes which I would then like to
instantiate.  The behavior I would like to achieve is that:

  guile> (define c (make-a-class 'some-name))
  guile> (define obj (make c))
  guile> obj
  #<<my-class> some-name deadbeef>

To that purpose, I have a metaclass named `<my-class>' and a procedure
`make-a-class' that simply returns instances of `<my-class>'.  Since
`compute-cpl' is customized for instances of `<my-class>', I can make
sure that instances of `<my-class>' have a proper CPL that includes
`<top>'.  Here's the code:

  (use-modules (oop goops))
  (read-set! keywords 'prefix)

  (define-class <my-class> (<class>)
    (the-slot :init-value #t))

  (define-method (compute-cpl (c <my-class>))
    (format (current-error-port) "CPL (~a)~%" c)
    (list c <top>))

  (define-method (make-instance (c <my-class>))
    (format (current-error-port) "make-instance (~a)~%" c)

  (define (make-a-class name)
    (let ((c (make <my-class>)))
      (slot-set! c 'name name)

The issue is that when instantiating an instance of `<my-class>' (read
that twice ;-)), I'm getting the following error:

  ;; Create a class (instance of `<my-class>').
  guile> (define c (make-a-class 'paf))
  CPL (#<<my-class> ??? 301f8930>)
  guile> c
  #<<my-class> paf 301f8930>

  ;; Instantiate it.
  guile> (make c)
  make-instance (#<<my-class> paf 301f8930>)

  <unnamed port>: In expression (let* (#) (format # "make-instance (~a)~%" ...) 
  <unnamed port>: No applicable method for #<<generic> initialize (10)> in call 
(initialize #<struct 301f8930:301f7320> ())
  ABORT: (goops-error)

This is quite unexpected since I would expect, at least, the
`initialize' class for instances of `<top>' to be applicable.  The other
surprising thing is that the instance to be initialized is still seen as
#<struct ...> instead of #<paf ...> or some such.

Any idea?


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