2.11.4 Explicit Renaming

Explicit renaming is an alternative facility for defining macro transformers. In the MIT/GNU Scheme implementation, explicit-renaming transformers are implemented as an abstraction layer on top of syntactic closures. An explicit-renaming macro transformer is defined by an instance of the er-macro-transformer keyword:

special form: er-macro-transformer expression

The expression is expanded in the syntactic environment of the er-macro-transformer expression, and the expanded expression is evaluated in the transformer environment to yield a macro transformer as described below. This macro transformer is bound to a macro keyword by the special form in which the transformer expression appears (for example, let-syntax).

In the explicit-renaming facility, a macro transformer is a procedure that takes three arguments, a form, a renaming procedure, and a comparison predicate, and returns a new form. The first argument, the input form, is the form in which the macro keyword occurred.

The second argument to a transformation procedure is a renaming procedure that takes the representation of an identifier as its argument and returns the representation of a fresh identifier that occurs nowhere else in the program. For example, the transformation procedure for a simplified version of the let macro might be written as

(lambda (exp rename compare)
  (let ((vars (map car (cadr exp)))
        (inits (map cadr (cadr exp)))
        (body (cddr exp)))
    `((lambda ,vars ,@body)
      ,@inits)))

This would not be hygienic, however. A hygienic let macro must rename the identifier lambda to protect it from being captured by a local binding. The renaming effectively creates an fresh alias for lambda, one that cannot be captured by any subsequent binding:

(lambda (exp rename compare)
  (let ((vars (map car (cadr exp)))
        (inits (map cadr (cadr exp)))
        (body (cddr exp)))
    `((,(rename 'lambda) ,vars ,@body)
      ,@inits)))

The expression returned by the transformation procedure will be expanded in the syntactic environment obtained from the syntactic environment of the macro application by binding any fresh identifiers generated by the renaming procedure to the denotations of the original identifiers in the syntactic environment in which the macro was defined. This means that a renamed identifier will denote the same thing as the original identifier unless the transformation procedure that renamed the identifier placed an occurrence of it in a binding position.

The renaming procedure acts as a mathematical function in the sense that the identifiers obtained from any two calls with the same argument will be the same in the sense of eqv?. It is an error if the renaming procedure is called after the transformation procedure has returned.

The third argument to a transformation procedure is a comparison predicate that takes the representations of two identifiers as its arguments and returns true if and only if they denote the same thing in the syntactic environment that will be used to expand the transformed macro application. For example, the transformation procedure for a simplified version of the cond macro can be written as

(lambda (exp rename compare)
  (let ((clauses (cdr exp)))
    (if (null? clauses)
        `(,(rename 'quote) unspecified)
        (let* ((first (car clauses))
               (rest (cdr clauses))
               (test (car first)))
          (cond ((and (identifier? test)
                      (compare test (rename 'else)))
                 `(,(rename 'begin) ,@(cdr first)))
                (else `(,(rename 'if)
                        ,test
                         (,(rename 'begin) ,@(cdr first))
                         (cond ,@rest))))))))))

In this example the identifier else is renamed before being passed to the comparison predicate, so the comparison will be true if and only if the test expression is an identifier that denotes the same thing in the syntactic environment of the expression being transformed as else denotes in the syntactic environment in which the cond macro was defined. If else were not renamed before being passed to the comparison predicate, then it would match a local variable that happened to be named else, and the macro would not be hygienic.

Some macros are non-hygienic by design. For example, the following defines a loop macro that implicitly binds exit to an escape procedure. The binding of exit is intended to capture free references to exit in the body of the loop, so exit is not renamed.

(define-syntax loop
  (er-macro-transformer
   (lambda (x r c)
     (let ((body (cdr x)))
       `(,(r 'call-with-current-continuation)
         (,(r 'lambda) (exit)
          (,(r 'let) ,(r 'f) () ,@body (,(r 'f)))))))))

Suppose a while macro is implemented using loop, with the intent that exit may be used to escape from the while loop. The while macro cannot be written as

(define-syntax while
  (syntax-rules ()
    ((while test body ...)
     (loop (if (not test) (exit #f))
           body ...))))

because the reference to exit that is inserted by the while macro is intended to be captured by the binding of exit that will be inserted by the loop macro. In other words, this while macro is not hygienic. Like loop, it must be written using the er-macro-transformer syntax:

(define-syntax while
  (er-macro-transformer
   (lambda (x r c)
     (let ((test (cadr x))
           (body (cddr x)))
       `(,(r 'loop)
         (,(r 'if) (,(r 'not) ,test) (exit #f))
         ,@body)))))