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)))