SICP 問題 4.17

lambdaを評価すると新しくフレームが作られます．
これを防ぐためにletでunassignmentを束縛するのではなくdefineで内部定義します．
define-variable!はフレームに新たな変数を追加する手続きなので余計なフレームは作られません．

scheme
(define (scan-out-defines body)
  (define (split-def-body proc-body)
    (let iter ((proc-body proc-body)
               (def '())
               (body '()))
      (cond ((null? proc-body) (cons (reverse def) (reverse body)))
            ((definition? (car proc-body)) (iter (cdr proc-body)
                                                 (cons (car proc-body) def)
                                                 body))
            (else (iter (cdr proc-body)
                        def
                        (cons (car proc-body) body))))))
  (let* ((def-body-list (split-def-body body))
         (def-list (car def-body-list))
         (body-list (cdr def-body-list)))
    (if (null? def-list)
        body
        (append  (map (lambda (x) (make-definition (definition-variable x) ''*unassigned*))
                    def-list)
                 (map (lambda (x) (list 'set! (definition-variable x)
                                        (definition-value x)))
                      def-list)
                 body-list))))