5.3 Syntax definitions

Syntax definitions are valid only at the top level of a <program>.

They have the following form:

(define-syntax <keyword> <transformer spec>)

<Keyword> is an identifier, and the <transformer spec> should be an instance of syntax-rules. The top-level syntactic environment is extended by binding the <keyword> to the specified transformer.

There is no ‘define-syntax’ analogue of internal definitions.

Although macros may expand into definitions and syntax definitions in any context that permits them, it is an error for a definition or syntax definition to shadow a syntactic keyword whose meaning is needed to determine whether some form in the group of forms that contains the shadowing definition is in fact a definition, or, for internal definitions, is needed to determine the boundary between the group and the expressions that follow the group. For example, the following are errors:

(define define 3)

(begin (define begin list))

(let-syntax
  ((foo (syntax-rules ()
          ((foo (proc args ...) body ...)
           (define proc
             (lambda (args ...)
               body ...))))))
  (let ((x 3))
    (foo (plus x y) (+ x y))
    (define foo x)
    (plus foo x)))