4.9.3 Syntax-Rules

SCM supports [R5RS] syntax-rules macros See Macros.

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