Re: #f = #none ?

[alms@cambridge.apple.com (Andrew LM Shalit)]

At 10:57 AM 11/18/92 -0500, Stavros Macrakis wrote:
> Imagine a function which takes an optional argument which is to be
> True, False, or Dont-care.  Presumably if the argument is omitted, the
> value should be Dont-care.  How will we encode these values?  We can't
> use #f for False, because it also means that the argument isn't there.
> We could use #t for true, but we'll have to be careful, because all
> boolean functions in Dylan allow any non-#f value to represent
> True....
> 
> So perhaps we'll need to define a new type which takes values
> (singletons) True, False, and #f...  Seems silly, and error-prone.
> 
>         -s

(bind ((no-value (cons #f #f)))
  (define-method true-false-dontcare (#key (arg no-value))
    (select arg id?
      ((#t) "sharp-t was passed")
      ((#f) "sharp-f wass passed")
      ((no-value) "no argument was passed")
      ;otherwise error
      )))