`syntax-rules` does not support patterns beginning with ellipsis, like `(... ...)`

[jpellegrini]

Hi @egallesio - happy holidays!

In page 24 of R7RS, the specification of patterns for syntax-rules contains this:

A template of the form (<ellipsis> <template>) is identical to <template>, except that ellipses within the template have no special meaning. That is, any ellipses contained within <template> are treated as ordinary identifiers. In particular, the template (<ellipsis> <ellipsis>) produces a single <ellipsis>. This allows syntactic abstractions to expand into code containing ellipses.

(define-syntax be-like-begin
  (syntax-rules ()
    ((be-like-begin name)
      (define-syntax name
        (syntax-rules ()
          ((name expr (... ...))
            (begin expr (... ...))))))))

(be-like-begin sequence)
(sequence 1 2 3 4)      ;; => result should be 4

But in STklos this raises an error when using the macro:

stklos> (be-like-begin sequence)
**** Error:
map: malformed list '#f'
	(type ",help" for more information)

I'll take a look into this, probably before the beginning of 2025 -- but I wanted to report the issue already.

[jpellegrini]

However... Most Schemes don't support that. And those who do don't seem to agree:

$ chibi-scheme
> (define-syntax f
  (syntax-rules ()
    ((f) '(a b (... c ... d) e))))
> (f)
(a b (c ... d) e)

gosh$ (define-syntax f
......  (syntax-rules ()
......    ((f) '(a b (... c ... d) e))))
#<undef>
gosh$ (f)
(a b c e)