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From: [e-mail] (Mark Kantrowitz)
Newsgroups: comp.lang.lisp,comp.lang.clos,news.answers
Subject: FAQ: CLOS and PCL Questions 5/6 [Monthly posting]
Summary: Questions about CLOS, PCL and object-oriented programming in Lisp
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Archive-name: lisp-faq/part5
Last-Modified: Thu Nov  5 19:30:40 1992 by Mark Kantrowitz
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;;; ****************************************************************
;;; Answers to Frequently Asked Questions about Lisp ***************
;;; ****************************************************************
;;; Written by Mark Kantrowitz and Barry Margolin
;;; lisp-faq-5.text -- 11959 bytes

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?
  [5-1]   What documentation is available about object-oriented
          programming in Lisp?
  [5-2]   How 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?
  [5-5]   Given the name of a class, how can I get the names of its slots?

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.

----------------------------------------------------------------
[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 (13.1.64.94) in the
/pub/pcl/ directory. Also in the same directory are sources for CLX R5
and an inspecter.

The sources for the CLOS Metaobject Protocol specification are
also available from parcftp as /pub/pcl/mop/spec.tar.Z. See also the book
``The Art of the Metaobject Protocol'' below. The Closette files
related to this book are available from parcftp as /pub/pcl/mop/closette.lisp.

----------------------------------------------------------------
[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
           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.
----------------------------------------------------------------
[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 LCL:STRUCTURE-REF
in Lucid and EXCL:STRUCTURE-REF in Allegro.
----------------------------------------------------------------
[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.
----------------------------------------------------------------
[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?

There are two main techniques of doing this kind of persistent object
storage. 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.

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:/afs/cs.cmu.edu/user/mkant/Public/Lisp-Utilities/.
(Note: You must cd to this directory in one atomic operation, as some
of the superior directories on the path are protected from access by
an anonymous ftp.) 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.

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

(defun class-slot-names (class-name)
  (mapcar #'clos:slot-definition-name
          (clos:class-slots (find-class class-name))))

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