SICP問題4.7
2010/03/17 evalでlookup-variable-valueのトコが間違ってたので修正
2010/03/16 4.9のmake-named-letに合わせてmake-letを修正
問題のlet*式の例
(let* ((x 3) (y (+ x 2)) (z (+ x y 5))) (* x z)) ; 39
これは以下のようなletに書き換えることが可能である。
(let ((x 3)) (let ((y (+ x 2))) (let ((z (+ x y 5))) (* x z)))) ; 39
ということで以下の要領でlet*->nested-letsを実装する。
let*関連の手続き
(define (let*? exp) (tagged-list? exp 'let*)) (define (let*-parameters exp) (cadr exp)) (define (let*-body exp) (caddr exp)) (define (make-let parameters body) (cons 'let (list parameters body))) (define (let*->nested-lets exp) (define (iter reverse-parameters result) (if (null? reverse-parameters) result (iter (cdr reverse-parameters) (make-let (list (car reverse-parameters)) result)))) (iter (reverse (let*-parameters exp)) (let*-body exp)))
(データ主導型でない)evalへの組み込み
(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)) ; 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) (apply (eval (operator exp) env) (list-of-values (operands exp) env))) (else (error "Unknown expression type -- EVAL" exp))))