Celestin Apprentice 2/Apprentice-Release2.iso/Information/Languages/Lisp/lisp-faq⁄part5

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From: [e-mail] (Mark Kantrowitz)
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Subject: FAQ: Object-oriented Programming in Lisp 5/7 [Monthly posting]
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Date: 13 Nov 1994 08:01:49 GMT
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Summary: Questions about CLOS, PCL and object-oriented programming in Lisp
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;;; ****************************************************************
;;; Answers to Frequently Asked Questions about Lisp ***************
;;; ****************************************************************
;;; Written by Mark Kantrowitz and Barry Margolin
;;; lisp_5.faq

This post contains Part 5 of the Lisp FAQ. It is cross-posted to the
newsgroup comp.lang.clos because it contains material of interest to
people concerned with CLOS, PCL and object-oriented programming in
Lisp. The other parts of the Lisp FAQ are posted only to the
newsgroups comp.lang.lisp and news.answers.

If you think of questions that are appropriate for this FAQ, or would
like to improve an answer, please send email to us at [e-mail].

CLOS/PCL Questions (Part 5):

  [5-0]   What is CLOS (PCL) and where can I get it?
          How do you pronounce CLOS? What is the Meta-Object Protocol (MOP)?
  [5-1]   What documentation is available about object-oriented
          programming in Lisp?
  [5-2]   How do I write a function that can access defstruct slots by
          name?  I would like to write something like
          (STRUCTURE-SLOT <object> '<slot-name>).
  [5-3]   How can I list all the CLOS instances in a class?
  [5-4]   How can I store data and CLOS instances (with possibly circular
          references) on disk so that they may be retrieved at some later
          time? (Persistent Object Storage)
  [5-5]   Given the name of a class, how can I get the names of its slots?
  [5-6]   Free CLOS software.
  [5-7]   Common CLOS Blunders

Search for \[#\] to get to question number # quickly.

In general, questions about object oriented programming in Lisp,
especially questions about using CLOS or compiling PCL, should be
directed to the newsgroup comp.lang.clos.

----------------------------------------------------------------
Subject: [5-0]  What is CLOS (PCL) and where can I get it?
                How do you pronounce CLOS?

CLOS (Common Lisp Object System) is the object-oriented programming
standard for Common Lisp. It is the successor to Symbolics FLAVORS and
Xerox LOOPS (Lisp Object Oriented Programming System). The acronym
CLOS is pronouned either as "See-Loss" or "Closs", depending on taste.
PCL (Portable Common Loops) is a portable CLOS implementation, and is
available by anonymous ftp from
   parcftp.xerox.com:/pub/pcl/ [13.1.64.94]
Also in the same directory are sources for CLX R5 and an inspecter.

Most Common Lisp implementations now include their own CLOS
implementations. Common Lisp implementations with native CLOS include:
MCL, {A}KCL, Allegro CL (including Allegro CL\PC), Ibuki, Lucid,
Medley, Symbolics Genera, CLOE, and Harlequin Lispworks. CMU CL uses a
customized version of PCL as their CLOS. However, not all native CLOS
implementations have as detailed a meta-object protocol as PCL. For
example, MCL 2.0 users sometimes use the july-1d version of PCL
instead of the native CLOS for precisely this reason.

The book ``The Art of the Metaobject Protocol'' (see below) includes
the CLOS Metaobject Protocol specification as chapters 5 and 6.  The
sources for the MOP spec itself are available from
   parcftp.xerox.com:/pub/pcl/mop/ [13.1.64.94]
as the file spec.tar.Z, but this is no substitute for buying the
book, since the book contains a lot of useful explanatory material
beyond the spec. The Closette files related to the book are also
available from parcftp as the file closette.lisp.

The CLOS code repository is available by anonymous ftp to
   nervous.cis.ohio-state.edu:/pub/lispusers/clos/ [128.146.61.200]
If you've got code you'd like to add to the repository, send mail to
Arun Welch, [e-mail].

----------------------------------------------------------------
Subject: [5-1] What documentation is available about object-oriented
               programming in Lisp?

Books about object-oriented programming in Lisp include:

   1. dpANS CL describes the entire Common Lisp language, which includes the
      CLOS standard.  Informally, CLtL2 can also be used to learn about CLOS,
      but please remember that CLtL2 is not an official X3J13 committee
      document. (The presentation of CLtL2 differs from that of the draft
      proposed standard, and some matters of fact have changed in the proposed
      standard since the publication of CLtL2.)

   2. Sonya E. Keene
      "Object-Oriented Programming in Common Lisp:
       A Programmer's Guide to CLOS"
      Addison-Wesley (Reading, MA), 1989. 266 pages. ISBN 0-201-17589-4.
           Tutorial introduction to CLOS with many examples and
           a lot of good advice for designing large programs using CLOS.

   3. Jo A. Lawless and Molly M. Miller.
      "Understanding CLOS: the Common Lisp Object System"
      Digital Press, 1991. 192 pages.

   4. Gregor Kiczales, Jim des Rivieres, and Daniel G. Bobrow.
      "The Art of the Metaobject Protocol"
      MIT Press, 1991. 335 pages. ISBN 0-262-61074-4, $34.95.
           The first part of the book presents a model CLOS implementation,
           introduces the basic principles of metaobject protocols, and
           works through the key elements of the CLOS Metaobject Protocol.
           The second half is the detailed specification of the CLOS
           Metaobject Protocol. A simple working interpreter suitable
           for experimentation is contained in an appendix.

   5. Robert R. Kessler and Amy R. Petajan.
      "LISP, Objects, and Symbolic Programming"
      Scott, Foresman and Company (Glenview, IL), 1988. 644 pages.
           Includes a small Lisp compiler.

   6. A short introduction to CLOS written by Jeff Dalton of the
      University of Edinburgh <[e-mail]> is available by
      anonymous ftp from
         aiai.ed.ac.uk:/lisp/random [192.41.104.6]
      as the file clos-guide.

   7. Andreas Paepcke, editor.
      "Object-Oriented Programming: The CLOS Perspective"
      MIT Press, 1993. 400 pages, ISBN 0-262-16136-2 ($40).
         This book is a collection of essays on the following topics:
           -  Description of CLOS and its design philosophy.
           -  The Metaobject Protocol and its use.
           -  Comparison of CLOS with Smalltalk, Eiffel, Sather, and C++.
           -  CLOS Uses and Methodology. Descriptions of two large CLOS
              applications (Sun's LispView and a hybrid knowledge
              representation tool) and an approach to documenting
              object-oriented protocols (similar to that of AMOP).
           -  Implementation details. Descriptions of TI CLOS for the
              Explorer and PCL's method dispatch mechanism.

----------------------------------------------------------------
Subject: [5-2] How can I write a function that can access defstruct slots
               by name?  I would like to write something like
               (STRUCTURE-SLOT <object> '<slot-name>).

There is currently no portable, built-in way to access structure slots
given only the name.  If your Common Lisp includes an implementation
of CLOS that supports the meta-object protocol specified in the
original X3J13 draft spec (document X3J13/88-003), then it probably will
allow (SLOT-VALUE <object> '<slot-name>); however, not all
implementations of CLOS currently provide this.  Lacking this, some
implementations may provide implementation-dependent functions that
allow access to structure slots by name; note that this may cause
saved images to be larger, as some implementations normally open-code
structure accessors and discard slot name information.

While it is not possible to write a fully general STRUCTURE-SLOT function,
it is not very difficult to write version that handles specific structure
types.  For instance, after defining:

   (defstruct spaceship name captain position velocity)

one may then define:

   (defun spaceship-slot (spaceship slot-name)
     (ecase slot-name
       (name (spaceship-name spaceship))
       (captain (spaceship-captain spaceship))
       (position (spaceship-position spaceship))
       (velocity (spaceship-velocity spaceship))))

or using CLOS (generic functions):

(defgeneric spaceship-slot (spaceship slot-name)
  (:method ((x spaceship) (slot (eql :name)))
    (spaceship-name x))
  (:method ((x spaceship) (slot (eql :captain)))
    (spaceship-captain x))
  (:method ((x spaceship) (slot (eql :position)))
    (spaceship-position x))
  (:method ((x spaceship) (slot (eql :velocity)))
    (spaceship-velocity x)))

Another popular way to define this is:

   (defun spaceship-slot (spaceship slot-name)
     (funcall (symbol-function
                (find-symbol (format nil "SPACESHIP-~A" slot-name)
                             #.(package-name *package*)))
              spaceship))

I personally recommend the first version.  It is likely to be much faster
and more memory efficient than the second version.  It's also easy to get
the second one wrong; many people forget to specify the package argument to
FIND-SYMBOL, which can cause incorrect results when the package at run time
is different from the one at compile time.  Even my version assumes that
SPACESHIP-SLOT is being defined in a file that is in the same package as
the one containing the structure definition; if this isn't the case,
#.(PACKAGE-NAME *PACKAGE*) should be replaced by a string naming the
correct package.

Another workaround is to define a MY-DEFSTRUCT macro that parses the
defstruct arguments and expands into a call to DEFSTRUCT along with a
definition of the runtime slot-accessor function.

Some non-portable techniques include the use of SYSTEM:STRUCTURE-REF
in Lucid (LCL:STRUCTURE-REF in earlier versions of Lucid) and
EXCL:STRUCTURE-REF in Allegro.
----------------------------------------------------------------
Subject: [5-3] How can I list all the CLOS instances in a class?

There is no built-in way to enumerate the instances of a class.  If you are
only interested in listing the instances of classes that you have defined,
it is not very difficult to implement it as part of your class definition.
Add a shared slot, e.g. ALL-INSTANCES, with an initial value of NIL, to the
class definition.  Then write an after-method on INITIALIZE-INSTANCE for
this class, which pushes the instance being initialized onto ALL-INSTANCES.
Note that this must be done separately for each class that wants to maintain
such a list; it can't be encapsulated in a mixin class, because all its
dependent classes would share the same ALL-INSTANCES slot.  A compromise
would be to use a mixin to define the INITIALIZE-INSTANCE after-method (and
any other general-purpose methods that use the slot), but not the shared
slot; it would be up to the descendant classes to define the slot at the
level of the class hierarchy that is appropriate. You could also try
defining the classes that need instance-recording as instances of a
metaclass that holds the instance registry on the class object. The
recording behavior could then be built-in to an after method on
initialize-instance for the root class of the metaclass, or even
allocate-instance. To allow for garbage collection of old instances,
you will also need to define a generic function to remove the recorded
instances from the list of instances.
----------------------------------------------------------------
Subject: [5-4]  How can I store data and CLOS instances (with possibly
                circular references) on disk so that they may be
                retrieved at some later time? (Persistent Object Storage)

There are two approaches to CLOS object persistence. The first uses
regular CLOS facilities in concert with a standard file system. The
second uses a more sophisticated storage facility, such as a database.
The first solution provides just the persistence, and usually cannot
retrieve only some of the objects from disc. The second goes beyond this
by providing some or all of the facilities typically associated with
databases: transaction management, concurrency control, queries,
selective object materialization, etc. Below, the two solutions are
addressed in turn.

There are two main techniques for the file system solution. The first
involves using #. to compile the data into a file.  The second
produces an ASCII representation which, when evaluated, will reproduce
an equivalent set of data.

If the data you wish to save is stored in the variable *hash-table*,
create a file containing just the lines
        (in-package "YOUR-PACKAGE")
        (setq *hash-table* '#.*hash-table*)
and compile it. The #. macro performs read-time evaluation of the
expression following the dot, and so this compiles the data into the
file. You may then load the file to restore the data. However, the
resulting binary file is not portable between Lisp implementations,
and sometimes not even for the same Lisp on different platforms. Also,
some Lisps will treat the data as constant, and place it on pages in
memory that are marked read-only (after it is loaded). If one tries to
later modify the data, these Lisps will signal an error. Lucid CL only
puts such constants in a read-only area when they appear inside
functions, so this should be safe. Allegro CL doesn't seem to complain
about modification if the data is a cons. DEC's VAXLisp, however, has
problems with #. circular structures in .fas files. MCL seems to work
well with using #. to save data (and even functions) to a file.

The other technique is to produce an ASCII representation of the Lisp
objects which may then be saved to a file. To reproduce the data, one
can load (or compile and load) the file. This technique is portable
between different Lisps and platforms. Unfortunately, the resulting
data is not necessarily EQ to the original. Kerry Koitzsch's
save-object.lisp package is included in the Lisp Utilities Repository,
   ftp.cs.cmu.edu:/user/ai/lang/lisp/code/ext/save_obj/
The Lisp Utilities Repository is described in detail in the answer to
question [6-1].

See also the discussion of MAKE-LOAD-FORM and MAKE-LOAD-FORM-SAVING-SLOTS
in CLtL2.

Here are some solutions to object persistence that use databases instead
of file systems.

Free Persistent Object Storage systems include:

   WOOD (William's Object Oriented Database) is a simple persistent
   object store for MCL 2.0, written by Bill St. Clair
   <[e-mail]>.  Its goal is to provide a way to
   save/restore Lisp objects to/from disk.  It is available by anonymous
   ftp from the MCL repository on cambridge.apple.com:/pub/MCL2/CONTRIB/.
   Send bug reports to [e-mail]. To be added to the
   mailing list, send mail to [e-mail].

   PCLOS is a persistent object store for CLOS that was implemented using
   CLOS's metaobject protocol. It allowed multiple different storage
   facilities to be used. The benefit of this approach is it lets you use
   different storage facilities depending on your needs, ranging from a
   full-blown database with concurrency control and recovery to a very
   fast store that trades speed for functionality. For example, an
   in-memory store that saves its state to disk periodically can be much
   faster than traditional databases, but risks loss of data due to
   crashes. Unfortunately, PCLOS was built on a very old version of CLOS,
   so it is unusable in its current form, and there are no plans to
   update it. The introductory MOP chapter in the book "Object-Oriented
   Programming: The CLOS Perspective" (see [5-1] above) explains how to
   do the rewrite in principle and shows how the CLOS MOP was used for
   object persistence in PCLOS. See also
      1. Andreas Paepcke, "PCLOS: A Flexible Implementation of CLOS
         Persistence", in Proceedings of ECOOP-88, S. Gjessing and K.
         Nygaard, editors, Lecture Notes in Computer Science, Springer
         Verlag, pages 374-389, 1988.  [Explains the PCLOS architecture.]
      2. Andreas Paepcke, "PCLOS: A Critical Review", in OOPSLA-89, 1989.
         [Uses PCLOS as a roadmap through the issues of object persistence.]
      3. Andreas Paepcke, "PCLOS Reference Manual", Hewlett-Packard
         Laboratories, December, 1991.
   For more information about PCLOS, write to Andreas Paepcke
   <[e-mail]>.

   GBB (see [6-3]) is a blackboard architecture with persistence of a
   sort. Every object in GBB is held in RAM, and there are dump and
   restore functions. It handles distributed object delivery and
   function-call triggering of receiving processes.

Commercial Persistent Object Storage systems include:

   AllegroStore is a high-performance object-oriented database management
   system from Franz. It offers Allegro CL users persistent object
   storage with very fast retrieval and update of object data.  It
   provides query processing and transaction-based operation. Built on a
   multi-client, multi-server architecture, it permits concurrent access
   over a network to objects by multiple independent processes.  It uses
   the ObjectStore ODBMS from Object Design Inc, and thus has both Lisp
   and C interfaces and is also very fast.  The CLOS interface is based
   on MOP, with dynamic class/schema modification and redefinition.
   AllegroStore can handle large-scale applications with minimal impact
   on performance. It uses page-faulting and page-locking mechanisms
   instead of object locking, providing high throughput and low overhead
   for concurrent access by multiple users.  Standard database features
   include deadlock detection, referential integrity, and inverse
   functions.  Exception handling is integrated into the Lisp condition
   system. AllegroStore runs on Sparc, SGI, HP, RS/6000, and
   MS-Windows/NT systems.  For more information, send email to
   [e-mail], write to Franz Inc., 1995 University Avenue, Berkeley,
   CA 94704, call 1-800-333-7260, 510-548-3600, fax [phone], or
   telex 340179 WUPUBTLXSFO.

   ITASCA ODBMS V2.2 is a distributed active object database management
   system. ITASCA allows clients to transparently access data that is
   distributed among multiple servers.  ITASCA supports full dynamic
   schema modification that can be performed during any phase of the
   software lifecycle.  Applications written in dissimilar and
   incompatible languages, such as C/C++ and CLOS, can share objects through
   ITASCA. ITASCA stores methods inside the database, promoting
   reusability and maintainability.  ITASCA is based on work at MCC's
   Object-Oriented and Distributed Systems Lab on the ORION system. For
   more information, write to Itasca Systems, Inc., 7850 Metro Parkway,
   Minneapolis, MN 55425, [e-mail], 612-851-3155, fax [phone].
   [Clint Hyde has written a MOP CLOS interface to Itasca, which has some
   features not present in their interface. For a free copy of his source
   code, send him mail to [e-mail].]

   Statice is a commercial product from Symbolics, that provides a
   powerful persistent ODBMS. It runs on Symbolics Lisp Machines and soon
   on Unix boxes under Lucid.

----------------------------------------------------------------
Subject: [5-5] Given the name of a class, how can I get the names
               of its slots?

(defun class-slot-names (class-name)
  "Given a CLASS-NAME, returns a list of the slots in the class."
  (mapcar #'clos:slot-definition-name
          (clos:class-slots (find-class class-name))))

(defmethod class-slot-names ((instance standard-object))
  "Given an INSTANCE, returns a list of the slots in the instance's class."
  (mapcar #'clos:slot-definition-name
          (clos:class-slots (class-of instance))))

You can use CLASS-DIRECT-SLOTS instead of CLASS-SLOTS if you don't
want inherited slots. Note that these functions are from the
meta-object protocol specified in the original X3J13 draft spec
(document X3J13/88-003), and may not be supported by all Lisps.

----------------------------------------------------------------
Subject: [5-6] Free CLOS software.

Software Repositories:

   The CLOS code repository is available by anonymous ftp to
   nervous.cis.ohio-state.edu:/pub/lispusers/clos/ [128.146.61.200]
   If you've got code you'd like to add to the repository, send
   mail to Arun Welch, [e-mail].
   The CLOS code repository includes dag.lisp.Z and 3DGeometry.lisp.

CORBA:

   The CORBA (Common Object Request Broker Architecture) specification's
   Dynamic Invocation Interfaces (DII) are a collection of interfaces for
   writing extensible, distributed object applications.  NEC Systems
   Laboratory Inc. has contributed both C and CLOS implementations of
   these specification to the Object Management Group (OMG). The CORBA
   specification (91-8-1) is available free to OMG members, and for $50
   to non-members. To order the document, send mail to [e-mail],
   or write to OMG, 492 Old Connecticut Path, Framingham, MA 01701, phone
   508-820-4300, fax [phone]. The DII implementation can be
   retrieved via anonymous ftp from omg.org:/pub/ [192.67.184.64] as the
   file NEC_DII/93-1-2.tar.Z. There are three dependencies in the
   contributed DII implementations.  First, the CLOS images require an
   Allegro CL license due to its use of the Lisp foreign function
   interface.  Second, the C executables require Sun SparcStations, but
   they can be re-compiled and re-linked for other architectures.  Third,
   the implementation of the `Invoke' DII interface is considered
   proprietary.  For further information, write to Don Vines, NEC Systems
   Laboratory, Inc., C&C Software Technology Center (CSTC), 1901 Gateway
   Drive, Irving, Texas 75038, call 214-518-3486, fax [phone], or
   send email to [e-mail].

----------------------------------------------------------------
Subject: [5-7] Common CLOS Blunders

This question is based on a document written by Marty Hall
<[e-mail]>. The full text of Marty's
"Introduction to CLOS" handout is available by anonymous ftp from
   ftp.cs.cmu.edu:/user/ai/lang/lisp/doc/intro/closintr.tgz

This question lists a variety of common errors that occur when writing
CLOS code. It is extremely useful to glance through this list when
debugging CLOS code.

[A] Omitting a set of parentheses around the arglist in DEFMETHOD.

    For example, writing
       (defmethod Area (Sq Square) ...)
    instead of
       (defmethod Area ((Sq Square)) ...)

    Lisp will accept the former, and think that you have two
    unspecialized arguments instead of one argument specialized as a
    Square.

[B] Missing parentheses around the slot definition list in DEFCLASS.

    For example, writing
       (defclass Rectangle (Polygon)
     (Width ...)
     (Height ...))
    instead of
       (defclass Rectangle (Polygon)
     ((Width ...)
      (Height ...)))

    Lisp will not accept the former, but the error message doesn't
    always clearly identify the problem.

[C] Forgetting to include an empty slot definition list if you don't
    define local slots in DEFCLASS.

    For example, writing
       (defclass Square (Rectangle))
    instead of
       (defclass Square (Rectangle) ())
    Lisp will not accept the former.

[D] Referring to the class name instead of the instance variable in
    DEFMETHOD.

    For example, writing
       (defmethod Area ((Sq Square))
     (* (Width Square) (Width Square)))
    instead of
       (defmethod Area ((Sq Square))
     (* (Width Sq) (Width Sq)))

    Lisp may give you a warning about an unknown free variable, but
    probably won't even do that if you type the defmethod directly into
    the Lisp Listener (Lucid doesn't). So you might not encounter a
    problem until run-time.

[E] Forgetting that accessors are functions and thus could conflict
    with built-in function names.

    For example, writing
       (defclass Graphical-Object ()
     ((Position :accessor Position)))
    will signal an error, since you cannot redefine the built-in
    POSITION function.

[F] Putting the new value last instead of first in the definition of a
    SETF method.

    For example, writing
       (defmethod (setf Area) ((Sq Square) (New-Area number))
     (setf (Width Sq) (sqrt New-Area)))
    instead of
       (defmethod (setf Area) ((New-Area number) (Sq Square))
     (setf (Width Sq) (sqrt New-Area)))

   Lisp will accept the former, causing users to be later puzzled as
   to why (setf (Area Square-1) 10) doesn't work.

[G] Putting the new value last instead of first in a call to a :writer
    method.

    For example, given
       (defclass Circle ()
     ((Radius :reader Radius :writer Set-Radius :initform 5)))
       (setq Circle-1 (make-instance 'Circle))
    and writing
       (Set-Radius Circle-1 10)
    instead of
       (Set-Radius 10 Circle-1)

[H] Confusion about documentation strings in DEFMETHOD.

    People often write code like
       (defmethod Area ((Rect Rectangle))
     "WIDTH times HEIGHT of the rectangle"
     (* (Width Rect) (Height Rect)))
    without clearly thinking about what this might mean.  Some people
    think it will make a documentation string on the generic function that
    can be retrieved by calling (DOCUMENTATION 'Area 'function) or the
    equivalent emacs keystrokes. Others vaguely expect it to make a doc
    string on each separate method, and that the call to DOCUMENTATION
    will somehow be able to automagically figure out which method it
    applies to.

    In fact, Lisp won't complain about this code, with the result that the
    documentation is added to the method *object*, which beginners
    probably know nothing about.

    Use the :documentation entry in defgeneric to add a documentation
    string to the generic function.

[I] Invalid :initargs are accepted by MAKE-INSTANCE.

    Many Lisp implementations will accept unknown keyword initargs without
    complaint, even at the highest safety settings.

    So the following code, which includes a typo of :SLOT1 instead of
    :SLOT-1 in the call to make-instance
       (defclass Foo ()
    ((Slot-1 :accessor Slot-1 :initarg :Slot-1 :initform 5)))

       (setq Test (make-instance 'Foo :Slot1 10))
    will not produce an error message, with the result that
    (Slot-1 Test) returns 5, not 10.

    This is a bug in the implementation; all implementations are
    supposed to flag this as an error.

[J] Forgetting the class must exist before any method can specialize upon it.

    Lisp programmers are used to being able to define functions in any
    order, where even if FOO calls BAR, FOO can be defined first. But doing
       (defmethod Area ((Rect Rectangle)) ...)
       (defclass Rectangle (Polygon) ...)
    is illegal. You have to define the class first.

    If while trying to debug the problem, you define the class by
    evaluating the definition without reordering the code to put the
    class first, you'll only run into problems later the next time you
    try to recompile the code from scratch.

    Many experienced programmers put each class definition in its own
    file at the top of the file, with methods for that class after it
    in the file. Others opt to put all the class definitions for a
    program in a single file that is loaded first, with methods in files
    that are loaded later.

[K] Changing a method to apply to a more general class does not
    supersede previous method.

    For example, suppose a user writes
       (defmethod Half-Area ((Rect Filled-Rectangle))
     (/ (Area Rect) 2))
    and then notices that this functionality could apply to all
    Rectangles, not just Filled-Rectangles. They might then change the
    class referenced in this method, under the specious intuition that by
    changing the old defintion, they are replacing it. In fact, they are
    actually adding a new, less-specific method.

    Now suppose that later on, they change this new method (in this
    example, by adding a call to FLOAT to avoid returning a ratio),
       (defmethod Half-Area ((Rect Rectangle))
     (float (/ (Area Rect) 2)))
    If they test it on an instance of Filled-Rectangle, they will be
    puzzled as to why their new definition appears to have not taken
    effect. But because the old definition specialized to Filled-Rectangle
    still exists, they're actually getting the old, more-specific definition.

    Moreover, the next time they restart a fresh Lisp and recompile
    the code, the problem will magicly disappear, since the old
    definition is no longer in the code.

----------------------------------------------------------------
;;; *EOF*