Re: identity

[Keisuke Nishida]

At 14 Apr 2001 11:03:38 +0200,
Marius Vollmer wrote:
> 
> > (define (identity x) x)
> > 
> > (define (id x)
> >   (display ";;; `id' is deprecated.  Use `identity' instead.\n")
> >   x)
> 
> Isn't that a bit much?  You'll get the warning on every use of `id',
> which might be an enourmous number of times.  In any case, the message
> should go to (current-error-port).

Right.  What about this?

  (define id
    (let ((warned #f))
      (lambda args
        (cond ((not warned)
               (display ";;; `id' is deprecated.  Use `identity' instead.\n"
                        (current-error-port))
               (set! warned #t)))
        (apply identity args))))

> Maybe this is a good opportunity to intriduce explicit support for
> deprecating into the module system?  We could for example have a way
> to emit a warning once when a deprecated binding is looked up.

We could write a macro `define-duplicate' or something that
produces the above code.  Should we do that?

  (define-macro (define-duplicate old new)
    `(define ,old
       (let ((warned #f))
         (lambda args
           (cond ((not warned)
                  (format (current-error-port)
                          ";;; `~A' is deprecated.  Use `~A' instead.\n"
                          ',old ',new)
                  (set! warned #t)))
           (apply ,new args)))))

  guile> (define-duplicate id identity)
  guile> (id 1)
  ;;; `id' is deprecated.  Use `identity' instead.
  1
  guile> (id 1)
  1