[r6rs-discuss] Why can ... and _ not be literals?

[AndrevanTonder]

On Sun, 24 Jun 2007, David Van Horn wrote:

> It is compatible in the case of ellipsis; R5RS (4.3.2) states "It is an
> error for ... to appear in <literals>."  As far as I can tell, underscore
> is not distinguished in any way (so this is an incompatible change and
> legacy code will break in this case).

My error.  Even more embarrassingly, trying to use ... as a literal in my 
own expander does not work either.  There are advantages in doing so, though. 
Here is the germ of a simple pattern matcher (not tested):

    (define-syntax matches?
      (syntax-rules (...)
        ((_ x ())
         (null? x))
        ((_ x (y ...))
         (and (list? x)
              (forall? (lambda (z) (matches? z y))
                       x)))
        ((_ x (y . z))
         (and (pair? x)
              (matches? (car x) y)
              (matches? (cdr x) z)))
        ((_ x id) #t)))

    (matches? '((1 2) (3 4 5))
       ((x ...) ...))     ==> #t

It seems a pity to force someone to use syntax-case for this kind of thing
when syntax-rules would otherwise be adequate.

Andre