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On Fri, 12 Jul 2013 10:10:43 -0400, John Cowan <cowan@xxxxxxxxxxxxxxxx> wrote: > I think the EMPTY tag works > better. However, I think the habit of using conses as unique tags > just makes debugging output hard to decipher. I much prefer this style: > > (define foo-tag (string-copy "foo")) > > This is guaranteed to return a unique object. Interesting. I've tended to use conses as unique tags because that ports to essentially every Lisp in existence, past and present. Kind of weird to use a string this way, really. But we'll only use "eq?" on it, so that should be fine. Okay, I'll give that a whirl. > In Common Lisp, this > is spelled: > > (defconstant foo-tag (copy-seq "foo")) In Common Lisp I think I'd use this instead: (make-symbol "empty-values") Mentally I think the empty tag is more like a special symbol than a string. Of course, I can't use Scheme's string->symbol the same way, because that would *reuse* a symbol of the same name. --- David A. Wheeler