<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">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/">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">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: #<CCL::STANDARD-KERNEL-METHOD CCL::ENSURE-CLASS-INITIALIZED (CCL::SLOTS-CLASS)>, 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>_______________________________________________<br>Openmcl-devel mailing list<br><a href="mailto:Openmcl-devel@clozure.com">Openmcl-devel@clozure.com</a><br>https://lists.clozure.com/mailman/listinfo/openmcl-devel<br></blockquote></div><br></div></body></html>