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From: jonl%kuwait@LUCID.COM (Jon L White)
Newsgroups: comp.lang.clos
Subject: Creating metaclasses
Message-ID: <9105020636.AA28415@kuwait>
Date: 2 May 91 06:36:43 GMT
References: <9105020128.AA28067@cs.rpi.edu>
Sender: welch@tut.cis.ohio-state.edu
Distribution: inet
Organization: CommonLoops
Lines: 87

re: [make a metaclass MY-CLASS that is its own metaclass]
    The following excerpt from Chapter 6 seems to suggest that
    what I want is impossible.

I hope it doesn't really make it "impossible", but simply leaves it as
unspecified.

The intent for Lucid's CLOS is to be able to do this, but after the 4.0
release went out, we discovered a glitch that prevented doing it in a
straightforward way.   Thanks to Kim Barrett of Chestnut Software for
pointing this out last September; it is filed as bug report #05645
"RFE: want to be able to make circular CLASS-OF link", and with this
patched, your example is quite feasible.  [see the "dribble" below.]

I say "feasible" rather than "straightforward", given all the constraints;
for example, note the need for doing the defmethod for VALIDATE-SUPERCLASS
at exactly the right point in time.   [Of course, you could retract that
method at a later time if you wanted to; but you might need it if you are
expecting MY-OBJECT to serve as a kind of "maximal" class within the MY-CLASS
metaclas hierarchy.  For other variations on this theme, see my email msg:
    Date: Tue, 12 Feb 91 14:48:41 PST
    From: Jon L White <jonl>
    To: kanderso@BBN.COM
    Cc: john@linus.mitre.org, barmar@think.com, commonloops@cis.ohio-state.edu
    Subject: accessing clos objects
]



. . .

> (load "/pods/central/patches/bug-5645")
;;; Loading binary file "/pods/central/patches/bug-5645.sbin"
#P"/tmp_mnt/net/ss/patches/bug-5645.sbin"
> (setq mo (defclass my-object (standard-object) ((file))))
#<Standard-Class MY-OBJECT>
> (setq mc (defclass my-class (standard-class my-object) ((my-class-info))))
#<Standard-Class MY-CLASS>
> (defmethod clos-sys:validate-superclass ((x my-class) (Y standard-class)) t)
#<Standard-Method VALIDATE-SUPERCLASS (MY-CLASS STANDARD-CLASS)>
> (change-class mc mc)
#<My-Class MY-CLASS>
> (change-class mo mc)
#<My-Class MY-OBJECT>
> (describe mo)
Class Object:
Name:                  MY-OBJECT
MetaClass:             MY-CLASS
Class-Precedence-List: (MY-OBJECT STANDARD-OBJECT T)
Direct-Superclasses:   (STANDARD-OBJECT)
Direct-Subclasses:     (MY-CLASS)
DirectSlots:
  SlotName                    Allocation  TypeSpec
    FILE
> (describe mc)
Class Object:
Name:                  MY-CLASS
MetaClass:             MY-CLASS
Class-Precedence-List: (MY-CLASS STANDARD-CLASS CLOS::SLOTTED-CLASS CLASS MY-OBJECT STANDARD-OBJECT T)
Direct-Superclasses:   (STANDARD-CLASS MY-OBJECT)
Direct-Subclasses:
DirectSlots:
  SlotName                    Allocation  TypeSpec
    MY-CLASS-INFO
InheritedSlots:
  SlotName                    Allocation  TypeSpec
    FILE
    CLOS::NAME
    CLOS::DIRECT-SUPERCLASSES
    CLOS::DIRECT-SUBCLASSES
    CLOS::DIRECT-METHODS
    CLOS::DIRECT-FORWARD-REFERENCED-SUPERCLASSES
    CLOS::PRECEDENCE-LIST
    CLOS::FULLY-DEFINED-P
    CLOS::WRAPPER-AND-ISLOT-COUNT
    CLOS::PLIST
    CLOS::DIRECT-SLOTS
    CLOS::SLOTS
    CLOS::DEFAULT-INITARGS
    CLOS-SYSTEM:SLOTD-INITARGS
>





-- JonL --