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))))