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6.10.2.2 Hygiene

syntax-rules macros have a magical property: they preserve referential transparency. When you read a macro definition, any free bindings in that macro are resolved relative to the macro definition; and when you read a macro instantiation, all free bindings in that expression are resolved relative to the expression.

This property is sometimes known as hygiene, and it does aid in code cleanliness. In your macro definitions, you can feel free to introduce temporary variables, without worrying about inadvertently introducing bindings into the macro expansion.

Consider the definition of my-or from the previous section:

(define-syntax my-or
  (syntax-rules ()
    ((my-or)
     #t)
    ((my-or exp)
     exp)
    ((my-or exp rest ...)
     (let ((t exp))
       (if exp
           exp
           (my-or rest ...))))))

A naive expansion of (let ((t #t)) (my-or #f t)) would yield:

(let ((t #t))
  (let ((t #f))
    (if t t t)))
⇒ #f

Which clearly is not what we want. Somehow the t in the definition is distinct from the t at the site of use; and it is indeed this distinction that is maintained by the syntax expander, when expanding hygienic macros.

This discussion is mostly relevant in the context of traditional Lisp macros (see section Lisp-style Macro Definitions), which do not preserve referential transparency. Hygiene adds to the expressive power of Scheme.


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