Question

If you want to make CLOS objects in common lisp printable (print readably), how do you go about doing this without using anything but print and read.

Answer 1

There are two parts to doing this, at least in my solution, however you will need this function (thanks to the guys at cl-prevalence for this (warn LLGPL)

(defun get-slots (object)
  ;; thanks to cl-prevalence
  #+openmcl
  (mapcar #'ccl:slot-definition-name
      (#-openmcl-native-threads ccl:class-instance-slots
       #+openmcl-native-threads ccl:class-slots
       (class-of object)))
  #+cmu
  (mapcar #'pcl:slot-definition-name (pcl:class-slots (class-of object)))
  #+sbcl
  (mapcar #'sb-pcl:slot-definition-name (sb-pcl:class-slots (class-of object)))
  #+lispworks
  (mapcar #'hcl:slot-definition-name (hcl:class-slots (class-of object)))
  #+allegro
  (mapcar #'mop:slot-definition-name (mop:class-slots (class-of object)))
  #+sbcl
  (mapcar #'sb-mop:slot-definition-name (sb-mop:class-slots (class-of object)))
  #+clisp
  (mapcar #'clos:slot-definition-name (clos:class-slots (class-of object)))
  #-(or openmcl cmu lispworks allegro sbcl clisp)
  (error "not yet implemented"))

Then, for reading you will need to run this piece of code, which sets up 1/2 of the syntax which is { type-of-object ((slot-name . slot-value) (slot-name . slot-value) ...)

(set-macro-character 
     #\{
     #'(lambda (str char)
     (declare (ignore char))
     (let ((list (read-delimited-list #\} str t)))
       (let ((type (first list))
         (list (second list)))
         (let ((class (allocate-instance (find-class type))))
           (loop for i in list do
            (setf (slot-value class (car i)) (cdr i)))
           class)))))

For printing, use

(defmethod print-object ((object standard-object) stream)
  (format stream "{ ~s ~s}" (type-of object)
      (loop for i in (get-slots object)
    collect (cons i (slot-value object i)))))

A *print-readably* is highly recommended when using all these methods. Also, note that circular relationships are untested