[pro] Final empty parameters to format directives

[Didier Verna]

  Hello,

I just fell upon a case where implementations seem to differ on what to
do. The question is how to interpret a format directive in which the
last parameter is followed by a comma. Consider this test case:

    (defun fmt (stream argument colonp atsignp &rest params)
      (declare (ignore stream argument colonp atsignp))
      (format t "~S~%" params))
    
    (format t "~1,2/fmt/~%" t)
    (format t "~1,2,/fmt/~%" t)
    (format t "~1,2:/fmt/~%" t)
    (format t "~1,2,:/fmt/~%" t)


Here's what you get from various implementations:

SBCL / ECL / ABCL:
(1 2)
(1 2 NIL)
(1 2)
(1 2)

CMU-CL / CCL / CLISP / Allegro / LispWorks:
(1 2)
(1 2)
(1 2)
(1 2)


Funnily, my instinct tells me that nobody's right, as I would have
expected to get this:

(1 2)
(1 2 NIL)
(1 2)
(1 2 NIL)

Indeed, the parameters list is implicitely terminated by the directive
character OR a colon / at-sign. So using a final comma seems to me like
adding another, empty, parameter at the end.

I'm willing to report those divergences to the respective
implementations, but the problem is that I don't really know what to
report :-) Who's right? No matter how I read the spec, I cannot make a
decision and it seems to me that this is an unspecified corner case.

I reckon it's probably not a big deal, but this could be a problem for
user-defined functions relying on the actual number of parameters that
they get.

Comments?

-- 
Resistance is futile. You will be jazzimilated.

Scientific site:   http://www.lrde.epita.fr/~didier
Music (Jazz) site: http://www.didierverna.com