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.
+2
A:
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
krzysz00
2010-06-21 17:03:16
you should have a balanced external representation. Currently you have an opening character, but no corresponding closing character. For reading then see for example READ-DELIMITED-LIST. See the example at http://www.lispworks.com/documentation/lw50/CLHS/Body/f_rd_del.htm .
Rainer Joswig
2010-06-21 17:12:34
maybe you're right, but it serves well for my purposes as there **must** be **exactly** two objects after the {
krzysz00
2010-06-21 17:31:44
yes, but it will confuse all editors. s-expressions should be balanced. It's a simple change anyway.
Rainer Joswig
2010-06-21 18:00:40
format is meant to be dumped and then rammed back in with *no* editing
krzysz00
2010-06-21 19:24:31
Yeah, but you publish an ugly hack on Stackoverflow and others may copy it. If you keep it just to your software, dealing with it is only your problem. Publishing substandard code on Stackoverflow is not a good idea. Your code is simply not a good example. S-Expression based formats are balanced and Common Lisp already gives an example with the external format for structures.
Rainer Joswig
2010-06-21 20:11:04
Fixed, thanks for the idea
krzysz00
2010-06-21 22:19:05
Great. You can simplify the form a bit and get rid of two lets by replacing them with one DESTRUCTURING-BIND: (destructuring-bind (type . list) (read-delimited-list ... Is the variable CLASS not better named OBJECT or INSTANCE? See also the above mentioned example in the CLHS. You might want to set the syntax attribute for the } character.
Rainer Joswig
2010-06-24 19:51:17
something in my codebase snapped when I tried that. Thanks, though. P.S., the running incantation of the code is at http://motm.svn.sourceforge.net/viewvc/motm/trunk/src/saves.lisp?view=log and also while you're there feel free to check out the rest of the game (http://motm.sourceforge.net)
krzysz00
2010-06-25 20:05:47