効率的に平均を計算するマクロ

average.sls

#!r6rs
(library (average)
  (export average)
  (import (rnrs))

  (define (%average . args)
    (/ (apply + args)
       (length args)))

  (define-syntax average
    (make-variable-transformer
     (lambda(ctx)
       (syntax-case ctx ()
         ((_ args ...)
          #'(letrec-syntax
                ((% (lambda(ctx2)
                      (syntax-case ctx2 ()
                        ((_ sum ())
                         #`(/ sum #,(+ (begin 'args 1) ...)))
                        ((_ sum (rt (... ...)))
                         #`(/ (+ sum rt (... ...))
                              #,(+ (begin 'args 1) ...)))
                        ((_ sum (rt (... ...)) r0 r1 (... ...))
                         (number? (syntax->datum #'r0))
                         #`(% #,(+ (syntax->datum #'r0)
                                   (syntax->datum #'sum))
                              (rt (... ...)) r1 (... ...)))
                        ((_ sum (rt (... ...)) r0 r1 (... ...))
                         #'(% sum (rt (... ...) r0) r1 (... ...)))))))
              (% 0 () args ...)))
         (_ #'%average)))))
  )