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4.9.3 Syntax-Rules

SCM supports [R5RS] syntax-rules macros See Macros in Revised(5) Scheme.

The pattern language is extended by the syntax (... <obj>), which is identical to <obj> except that ellipses in <obj> are treated as ordinary identifiers in a template, or as literals in a pattern. In particular, (... ...) quotes the ellipsis token ... in a pattern or template.

For example:

(define-syntax check-tree
  (syntax-rules ()
    ((_ (?pattern (... ...)) ?obj)
     (let loop ((obj ?obj))
       (or (null? obj)
           (and (pair? obj)
                (check-tree ?pattern (car obj))
                (loop (cdr obj))))))
    ((_ (?first . ?rest) ?obj)
     (let ((obj ?obj))
       (and (pair? obj)
            (check-tree ?first (car obj))
            (check-tree ?rest (cdr obj)))))
    ((_ ?atom ?obj) #t)))

(check-tree ((a b) ...) '((1 2) (3 4) (5 6))) ⇒ #t

(check-tree ((a b) ...) '((1 2) (3 4) not-a-2list) ⇒ #f

Note that although the ellipsis is matched as a literal token in the defined macro it is not included in the literals list for syntax-rules.

The pattern language is also extended to support identifier macros. A reference to an identifier macro keyword that is not the first identifier in a form may expand into Scheme code, rather than raising a “keyword as variable” error. The pattern for expansion of such a bare macro keyword is a single identifier, as in other syntax rules the identifier is ignored.

For example:

(define-syntax eight
    (syntax-rules ()
      (_ 8)))

(+ 3 eight) ⇒ 11
(eight) ⇒ ERROR
(set! eight 9) ⇒ ERROR