SICP 問題 4.20

;; a
;; letrecをlet式に変換すし，導出された式として実装する．

;; eval
(define (eval exp env)
  (cond ((self-evaluating? exp) exp)
        ((variable? exp) (lookup-variable-value exp env))
        ((quoted? exp) (text-of-quotation exp))
        ((assignment? exp) (eval-assignment exp env))
        ((definition? exp) (eval-definition exp env))
        ((if? exp) (eval-if exp env))
        ((lambda? exp)
         (make-procedure (lambda-parameters exp)
                         (lambda-body exp)
                         env))
        ((let? exp) (eval (let->lambda exp) env))
        ((let*? exp) (eval (let*->nested-lets exp) env))
        ((letrec? exp) (eval (letrec->let exp) env)) ;;letrecを追加
        ((begin? exp)
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((and? exp) (eval-and exp env))
        ((or? exp) (eval-or exp env))
        ((application? exp)
         (my-apply (eval (operator exp) env)
                   (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type --EVAL" exp))))

;; 選択子
(define (letrec? exp) (tagged-list? exp 'letrec))
(define (letrec-parameters exp) (cadr exp))
(define (letrec-variables exp) (map car (letrec-parameters exp)))
(define (letrec-expressions exp) (map cadr (letrec-parameters exp)))
(define (letrec-body exp) (cddr exp))

(define (letrec->let exp)
  (make-let (map (lambda (x) (list x ''*unassigned*))
                 (letrec-variables exp))
            (append (map (lambda (x y) (list 'set! x y))
                         (letrec-variables exp)
                         (letrec-expressions exp))
                    (letrec-body exp))))

gosh> (driver-loop)


;;; M-Eval input:
      (letrec ((fact
                (lambda (n)
                  (if (= n 1)
                      1
                      (* n (fact (- n 1)))))))
        (fact 10))

;;; M-Eval value:
3628800

;; b
;; 環境図はパス

;; Louiの言うことを素直に書いてみると以下の通りになる．
(define (f x)
  (let ((even? (lambda (n)
                 (if (= n 0) true (odd? (- n 1)))))
        (odd? (lambda (n)
                (if (= n 0) false (even? (- n 1))))))
    body ...))

;; これだと相互再帰部分でeven?を評価するときにはまだodd?が評価されていないためエラーになる．