2015/12/21

SICP 問題 4.11

``````;; フレームを束縛のリストとして表現
(define (make-frame variables values)
(map cons variables values))

(define (first-binding frame) (car frame))
(define (rest-bindings frame) (cdr frame))

(define (binding-variable binding) (car binding))
(define (binding-value binding) (cdr binding))
(define (make-binding var val) (cons var val))

(set! frame (cons (make-binding var val) frame)))

;; 変更無し
(define (extend-environment vars vals base-env)
(if (= (length vars) (length vals))
(cons (make-frame vars vals) base-env)
(if (< (length vars) (length vals))
(error "Too many arguments supplied" vars vals)
(error "Too few arguments supplied" vars vals))))

;; 変更無し
(define (lookup-variable-value var env)
(define (env-loop env)
(define (scan vars vals)
(cond ((null? vars)
(env-loop (enclosing-environment env)))
((eq? var (car vars)) (car vals))
(else (scan (cdr vars) (cdr vals)))))
(if (eq? env the-empty-environment)
(error "Unbound variable" var)
(let ((frame (first-frame env)))
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))

;; frame-variablesとframe-valuesを作ればset-variable-value!とdefine-variable!は変更なし
(define (frame-variables frame) (map car frame))
(define (frame-values frame) (map cdr frame))
(define (set-variable-value! var val env)
(define (env-loop env)
(define (scan vars vals)
(cond ((null? vars)
(env-loop (enclosing-environment env)))
((eq? var (car vars)) (set-car! vals val))
(else (scan (cdr vars) (cdr vals)))))
(if (eq? env the-empty-environment)
(error "Unbound variable: SET!" var)
(let ((frame (first-frame env)))
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))

(define (define-variable! var val env)
(let ((frame (first-frame env)))
(define (scan vars vals)
(cond ((null? vars)
((eq? var (car vars)) (set-car! vals val))
(else (scan (cdr vars) (cdr vals)))))
(scan (frame-variables frame) (frame-values frame))))

;; 作らない場合はassqで走査する
(define (set-variable-value var val env)
(define (env-loop env)
(let ((target (assq var (first-frame env))))
(if target
(set-cdr! target val)
(env-loop (enclosing-environment env))))))
(env-loop env)

(define (define-variable! var val env)
(let* ((frame (first-frame env))
(target (assq var frame)))
(if target
(set-cdr! target val)