SICP 問題 4.07

let*をネストしたletで置き換える．

;; (let* ((x 3)
;;        (y (+ x 2))
;;        (z (+ x y 5)))
;;   (* x z))

;; (let ((x 3))
;;   (let ((y (+ x 2)))
;;     (let ((z (+ x y 5)))
;;       (* x z))))

;; let*
(define (let*? exp) (tagged-list? exp 'let*))
(define (let*-parameters exp) (cadr exp))
(define (let*-variables exp) (map car (let*-parameters exp)))
(define (let*-expressions exp) (map cadr (let*-parameters exp)))
(define (let*-body exp) (cddr exp))

(define (make-let parameters bodys)
  (cons 'let (cons parameters bodys)))

(define (let*->nested-lets exp)
  (expand-lets (let-parameters exp) (let-bodys exp)))

(define (expand-lets parameters bodys)
  (cond ((null? parameters) (error "EXPAND-LETS required pair, but " parameters))
        ((null? (cdr parameters))
         (make-let (list (car parameters))
                   bodys))
        (else (make-let (list (car parameters))
                        (list (expand-lets (cdr parameters) bodys))))))

(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->combination exp) env))
        ((let*? exp) (eval (let*->nested-lets exp) env)) ;;let*を追加
        ((begin? exp)
         (eval-sequence (begin-actions exp) env))
        ((cond? exp) (eval (cond->if exp) env))
        ((and? exp) (eval (and->if exp) env))
        ((or? exp) (eval (or->if exp) env))
        ((application? exp)
         (my-apply (eval (operator exp) env)
                   (list-of-values (operands exp) env)))
        (else
         (error "Unknown expression type: EVAL" exp))))

gosh> (let*->nested-lets '(let* ((x 1)
                                 (y (+ x x))
                                 (z (* x y)))
                            (+ x y z)
                            (* x y z)))
(let ((x 1))
  (let ((y (+ x x)))
    (let ((z (* x y)))
      (+ x y z)
      (* x y z))))