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23.6 The R5RS syntax-rules System

R5RS defines an alternative system for macro and syntax transformations using the keywords define-syntax, let-syntax, letrec-syntax and syntax-rules.

The main difference between the R5RS system and the traditional macros of the previous section is how the transformation is specified. In R5RS, rather than permitting a macro definition to return an arbitrary expression, the transformation is specified in a pattern language that

The last point is commonly referred to as being hygienic: the R5RS syntax-case system provides hygienic macros.

For example, the R5RS pattern language for the false-if-exception example of the previous section looks like this:

(syntax-rules ()
  ((_ expr)
   (catch #t
          (lambda () expr)
          (lambda args #f))))

In Guile, the syntax-rules system is provided by the (ice-9 syncase) module. To make these facilities available in your code, include the expression (use-syntax (ice-9 syncase)) (see Using Guile Modules) before the first usage of define-syntax etc. If you are writing a Scheme module, you can alternatively include the form #:use-syntax (ice-9 syncase) in your define-module declaration (see Creating Guile Modules).