Hello,
I can hardly be considered to have an in depth knowledge of Cells, so I might be completely wrong, but the problem seems to be with the print-object for model class: from family.lisp: (defmethod print-object ((self model) s) #+shhh (format s "~a" (type-of self)) (format s "~a~a" (if (mdead self) "DEAD!" "") (or (md-name self) (type-of self))))
I have no idea why this would even be called when accessing class-allocated slots, but it is, and fails because slots needed to print the prototype object are unbound. This can be worked around by setting them, adding:
(setf (slot-value (sb-mop:class-prototype (find-class 'my-model)) 'cells::.md-state) :prototype (slot-value (sb-mop:class-prototype (find-class 'my-model)) 'cells::.md-name) :prototype)
To your file after finalize-inheritance makes the example work. I have no idea if this doesn't break something, I know about PCL even less than about Cells.
Regards, Jakub Higersberger