SICP Exercise 4.12
在這個練習中我們使用框架原有的表示,即把框架表示爲表的序對,而不是練習4.11中的方式。爲了把環境的遍歷和框架的遍歷分開,我增加了兩個過程,如下:
(define (scan var frame)
"在框架frame中查找var,如果找到返回相應的vals,如果沒有找到,返回#f。"
(define (iter vars vals)
(cond ((null? vars) #f)
((eq? var (car vars))
vals)
(else (iter (cdr vars) (cdr vals)))))
(iter (frame-variables frame)
(frame-values frame)))
(define (env-loop var env)
"在所有框架中查找var,如果找到返回相應的vals,如果沒有找到,報錯。"
(if (eq? env the-empty-environment)
(error "Unbound variable" var)
(let ((vals (scan var (first-frame env))))
(if vals
vals
(env-loop var (enclosing-environment env))))))
現在我們環境操作就變的簡單多了:
(define (lookup-variable-value var env)
(car (env-loop var env)))
(define (set-variable-value! var val env)
(set-car! (env-loop var env) val))
(define (define-variable! var val env)
(let ((frame (first-frame env)))
(let ((vals (scan var frame)))
(if vals
(set-car! vals val)
(add-binding-to-frame! var val frame)))))
下面,我給出了一個簡單的測試:
(define vars '(a b c d))
(define vals (list 1 2 3 4))
(define a (make-frame vars vals))
(define env (cons a '()))
(lookup-variable-value 'a env)
(set-variable-value! 'a 100 env)
(lookup-variable-value 'a env)
(define-variable! 'e 5 env)
(lookup-variable-value 'e env)
運行結果如下:1
100
5