<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"><br></div><div dir="ltr"><blockquote type="cite">One more comment: This is a bad example on the part of AMOP but for
a different reason than Ron described. If you really want a
counted-class, using a class-allocated slot is a much better
approach. That way you don't need all this metaclass nonsense.</blockquote></div><blockquote type="cite"><div dir="ltr">
<br>
-SS<font face="monospace"><br></font></div></blockquote><div><br></div>That’s the approach I’ve used:<div><br></div><div>(cs1 :accessor :cs1 <font color="#e22400">:allocation :class</font>)</div><div><br></div><div>in the class definition for some class X. But since I want all inheriting classes of X, e.g., Y, to have its own class allocated slot, I have to repeat the class slot definitions in class Y as well, e.g.,</div><div><br></div><div>(cs1 :accessor :cs1 :allocation :class)</div><div><br></div><div>ad nauseam. Otherwise I’ll be talking to the unique cs1 slot in class X when dealing with Y.</div><div><br></div><div>I understand the behavior, I wish there would be a control mechanism that would allow creating unique class slots for inherited classes, e.g., by stating:</div><div><br></div><div>(cs1 :accessor :cs1 :allocation :class <font color="#0056d6">:shared-class-allocation nil</font>)</div><div><blockquote type="cite"><div dir="ltr"><font face="monospace">
<br>
<br>
</font>On 3/4/21 3:31 PM, Ron Garret wrote:<br>
<blockquote type="cite" cite="mid:AC81379F-82CE-4C90-995D-FF1729A8B2DE@flownet.com">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
The first thing to do is to see if a different CL implementation
gives the same error:
<div><br>
</div>
<div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">➔
sbcl</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">This
is SBCL 1.1.9, an implementation of ANSI Common Lisp.</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">More
information about SBCL is available at <<a href="http://www.sbcl.org/" moz-do-not-send="true">http://www.sbcl.org/</a>>.</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;
min-height: 14px;"><br>
</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">SBCL
is free software, provided as is, with absolutely no warranty.</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">It
is mostly in the public domain; some portions are provided
under</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">BSD-style
licenses. See the CREDITS and COPYING files in the</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">distribution
for more information.</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">(defclass
rectangle ()</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
((height :initform 0.0 :initarg :height)</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
(width :initform 0.0 :initarg :width)))</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;
min-height: 14px;"><br>
</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">#<STANDARD-CLASS
RECTANGLE></div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">(defclass
counted-class (standard-class)</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
((counter :initform 0)))</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;
min-height: 14px;"><br>
</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">#<STANDARD-CLASS
COUNTED-CLASS></div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">(make-instance
'counted-class</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
:name 'counted-rectangle</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
:direct-superclasses (list (find-class 'rectangle))</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
:direct-slots ())</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;
min-height: 14px;"><br>
</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">debugger
invoked on a SIMPLE-ERROR:</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
The class #<STANDARD-CLASS RECTANGLE> was specified as a
super-class of the</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
class #<COUNTED-CLASS COUNTED-RECTANGLE>, but the
meta-classes</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
#<STANDARD-CLASS STANDARD-CLASS> and #<STANDARD-CLASS
COUNTED-CLASS> are</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
incompatible. Define a method for SB-MOP:VALIDATE-SUPERCLASS
to avoid this</div>
<div style="margin: 0px; font-size: 10px; font-family: Monaco;">
error.</div>
</div>
<div><br>
</div>
<div>So this appears to be a bug in the MOP book. And, if you
think about it, the example in the book really doesn’t make
sense. COUNTED-CLASS is a metaclass, i.e. all of its instances
are classes. But RECTANGLE is not a meta-class, it is a class.
All of its instances are rectangles, which are not classes. So
it makes no sense to create an instance of COUNTED-CLASS which
inherits from RECTANGLE. The instances of such a class would
have to be both classes and rectangles, and that’s not possible.</div>
<div><br>
</div>
<div>rg</div>
<div><br>
<div>
<div>On Mar 4, 2021, at 2:07 PM, Paul Krueger <<a href="mailto:plkrueger@comcast.net" moz-do-not-send="true">plkrueger@comcast.net</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=UTF-8">
<div style="word-wrap: break-word; -webkit-nbsp-mode: space;
line-break: after-white-space;" class=""><span style="font-family: Monaco; font-size: 12px;" class="">I
was trying to do a little MOP hacking and when what I
was trying to do got errors I went back to “The Art of
the Metaobject Protocol” and ran an example from there
to see if it encountered similar errors, which it did
(most of this from p. 72 of the book):</span><br style="font-family: Monaco; font-size: 12px;" class="">
<br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">I’m running Clozure Common Lisp Version 1.11.6
(v1.11.6) DarwinX8664</span><br style="font-family:
Monaco; font-size: 12px;" class="">
<br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">? (defclass rectangle ()</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> ((height :initform 0.0 :initarg :height)</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> (width :initform 0.0 :initarg :width)))</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">#<STANDARD-CLASS RECTANGLE></span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">? (defclass counted-class (standard-class)</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> ((counter :initform 0)))</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">#<STANDARD-CLASS COUNTED-CLASS></span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">? (setf (find-class 'counted-rectangle)</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> (make-instance 'counted-class</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> :name 'counted-rectangle</span><br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> :direct-superclasses (list (find-class
'rectangle))</span><br style="font-family: Monaco;
font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class=""> :direct-slots ()))</span><br style="font-family: Monaco; font-size: 12px;" class="">
<blockquote type="cite" style="font-family: Monaco;
font-size: 12px;" class="">Error: The class
#<STANDARD-CLASS RECTANGLE> was specified as a<br class="">
super-class of the class #<COUNTED-CLASS
COUNTED-RECTANGLE>;<br class="">
but the meta-classes #<STANDARD-CLASS
STANDARD-CLASS> and<br class="">
#<STANDARD-CLASS COUNTED-CLASS> are
incompatible.<br class="">
While executing: #<a class="moz-txt-link-rfc2396E" href="CCL::STANDARD-KERNEL-METHODCCL::ENSURE-CLASS-INITIALIZED(CCL::SLOTS-CLASS)"><CCL::STANDARD-KERNEL-METHOD
CCL::ENSURE-CLASS-INITIALIZED (CCL::SLOTS-CLASS)></a>, in
process Listener(4).<br class="">
Type cmd-. to abort, cmd-\ for a list of available
restarts.<br class="">
Type :? for other options.<br class="">
</blockquote>
<span style="font-family: Monaco; font-size: 12px;" class="">1 > </span><br style="font-family: Monaco;
font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">?</span><br style="font-family: Monaco;
font-size: 12px;" class="">
<br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">My question is whether this is a problem with
CCL’s implementation or a spec change of some sort that
invalidates the example from the book. It’s not clear to
me how you could ever employ meta-classes without
getting this sort of error in CCL, so if this isn’t a
bug, what’s the work-around?</span><br style="font-family: Monaco; font-size: 12px;" class="">
<br style="font-family: Monaco; font-size: 12px;" class="">
<span style="font-family: Monaco; font-size: 12px;" class="">Thanks ...</span></div>
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</blockquote>
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