Re: Why require dummy values?

["Scott G. Miller" <scgmille@xxxxxxxxxxxxxxxxxx>]

On Tue, Apr 02, 2002 at 10:55:50AM -0600, John David Stone wrote:
>         The sample implementation SRFI-28 appears to require that a dummy
> value be supplied whenever the ~% or ~~ escape sequence is used in a format
> string, so that, for instance
> 
>                             (format "~~~a" #\b)
> 
> crashes -- the format string contains two escape sequences but only one
> corresponding value.  I suspect that ~% and ~~ should not be counted as
> ``value-requiring escape sequences'' in the sense intended in the
> specification.

Yes, this is a bug in both the spec and the implementation.
> 
>         The code for the sample implementation also uses () rather than '()
> to denote the empty list, which is non-R5RS.
The consequence of writing it on a lax scheme system.

> 
>         I propose instead the following sample implementation of SRFI-28.
> It depends on SRFI-6 and SRFI-23.
> 
> -----------------------------------------------------------------------------
> (define format
>   (lambda (format-string . objects)
>     (let ((buffer (open-output-string)))
>       (let loop ((format-list (string->list format-string))
>                  (objects objects))
>         (cond ((null? format-list) (get-output-string buffer))
>               ((char=? (car format-list) #\~)
>                (if (null? (cdr format-list))
>                    (error 'format "Incomplete escape sequence")
>                    (case (cadr format-list)
>                      ((#\s)
>                       (if (null? objects)
>                           (error 'format "No value for escape sequence")
>                           (begin
>                             (write (car objects) buffer)
>                             (loop (cddr format-list) (cdr objects)))))
>                      ((#\a)
>                       (if (null? objects)
>                           (error 'format "No value for escape sequence")
>                           (begin
>                             (display (car objects) buffer)
>                             (loop (cddr format-list) (cdr objects)))))
>                      ((#\%)
>                       (display #\newline buffer)
>                       (loop (cddr format-list) objects))
>                      ((#\~)
>                       (display #\~ buffer)
>                       (loop (cddr format-list) objects))
>                      (else
>                       (error 'format "Unrecognized escape sequence")))))
>               (else (display (car format-list) buffer)
>                     (loop (cdr format-list) objects)))))))

I like this implementation.  The error checking is far superior to the
version I hastily wrote.  

I'll submit the changes to the spec today.

	Scott

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