[ clisp-Bugs-3487523 ] print-unreadable-object recursion

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Bugs item #3487523, was opened at 2012-02-14 02:27
Message generated for change (Comment added) made by haible
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Category: clisp
>Group: lisp error
>Status: Closed
>Resolution: Invalid
Priority: 5
Private: No
Submitted By: Vladimir Tzankov (vtz)
Assigned to: Bruno Haible (haible)
Summary: print-unreadable-object recursion

Initial Comment:
Following code causes infinite recursion:

(defclass c1 ()
  ((name :initarg :name :accessor name-of)))

(defmethod print-object ((c c1) stream)
  (print-unreadable-object (c stream :type t :identity t)
    (princ (name-of c) stream)))

(make-instance 'c1)

The message:
*** - SLOT-VALUE: The slot NAME of XXXXXX has no value 
where XXXXXX is the newly created instance is to blame



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>Comment By: Bruno Haible (haible)
Date: 2012-02-14 04:53

Message:
This is not a bug. When writing a PRINT-OBJECT method you have to be more
careful
and more defensive than elsewhere, precisely because PRINT-OBJECT can be
used in
error messages.

Change your method like this:
(defmethod print-object ((c c1) stream)
  (print-unreadable-object (c stream :type t :identity t)
  (when (slot-boundp c 'name) (princ (name-of c) stream))))


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Comment By: Pascal J. Bourguignon (informatimago)
Date: 2012-02-14 04:42

Message:
This is not a bug.  If you don't bind a slot, it's normal to get an error
when you try to access it.
Either don't try to read unbound slot, or bind all your slots (with
:initform).

[pjb@kuiper :0 ~]$ clisp -ansi -norc -q
[1]> (defclass c1 ()
        ((name :initarg :name :accessor name-of)))
#<STANDARD-CLASS C1>
[2]> (defmethod print-object ((c c1) stream)
       (print-unreadable-object (c stream :type t :identity t)
          (princ (name-of c) stream)))
#<STANDARD-METHOD (#<STANDARD-CLASS C1> #<BUILT-IN-CLASS T>)>
[3]> (make-instance 'c1 :name "Hello")
#<C1 Hello #x000333D0E7A0>
[4]> (defmethod print-object ((c c1) stream)
       (print-unreadable-object (c stream :type t :identity t)
          (when (slot-boundp c 'name) (princ (name-of c) stream))))
WARNING: Replacing method
         #<STANDARD-METHOD (#<STANDARD-CLASS C1> #<BUILT-IN-CLASS T>)> in
         #<STANDARD-GENERIC-FUNCTION PRINT-OBJECT>
#<STANDARD-METHOD (#<STANDARD-CLASS C1> #<BUILT-IN-CLASS T>)>
[5]> (make-instance 'c1)
#<C1  #x000333D29580>
[6]> (quit)
[pjb@kuiper :0 ~]$ 

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