RE: Really odd problem in a :children-function

> From: owner-lisp-hug@lispworks.com 
> [mailto:owner-lisp-hug@lispworks.com] On Behalf Of Taylor, Joshua
> Sent: 18 March 2006 01:28
> To: Lispworks HUG
> Subject: Really odd problem in a :children-function
> 
> 
> Hello all,
> I'm trying to use a graph-pane to display the results of some
> tree code that's really more of a demonstration than anything else.
> The problem I'm running into is really odd though.
> 
> Inside of a (make-instance 'capi:graph-pane ...) I have the
following:
> 
>     :children-function #'(lambda (i)
>                            (unless (isLeaf? i)
>                              (let* ((l (kdtreenode-left  i))
>                                     (r (kdtreenode-right i))
>                                     (c (cond
>                                         ((and l r) (print 
> "case 1")(list r l))
>                                         (l         (print 
> "case 2")(list l))
>                                         (r         (print 
> "case 3")(list r))
>                                         (t         (print 
> "case 4")()))))
>                                (prog1 c
>                                  (unless (every #'identity c)
>                                    (format t "~&c is ~A~%" c))))))
> 
> A good portion of that code I've added trying to track down this
> bug. The problem I get is that c ends up being, at one point,
> a list of nil and and good value, i.e.
> (nil #s(kdtreenode ...))
> 
> My output buffer does indeed have
> 
> "case 1"
> "case 1"
> "case 1"
> "case 1"
> "case 1"
> "case 1"
> c is (Nil #S(Kdtreenode ...))
> 
> which indicates to me that something strange is going on 
> inside of that
> cond. I really hope I've just been staring at this for too 
> long, and that i'm
> missing some very obvious that I would normally see. 
> Unforunately, I can't
> seem to reproduce it outside of the code I've got (which I 
> can provide if
> anyone would like to try).
> 
> Any thoughts?

That Nil in your output looks odd. My guess is that it is not CL:NIL.
Could it be the string "Nil"?

To see if this or something similar is the case, try changing the ~A
to ~S in the format string.

Simon