2015/12/19

# 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*-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))))
``````