计划可变功能 [英] SCHEME Mutable Functions
本文介绍了计划可变功能的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
在过去的几个月中,我一直在自我学习Scheme R5RS,并且刚刚开始学习可变功能.我做了几个这样的功能,但似乎发现了我的错误.
I've been self-teaching myself Scheme R5RS for the past few months and have just started learning about mutable functions. I've did a couple of functions like this, but seem to find my mistake for this one.
(define (lst-functions)
(let ((lst '()))
(define (sum lst)
(cond ((null? lst) 0)
(else
(+ (car lst) (sum (cdr lst))))))
(define (length? lst)
(cond ((null? lst) 0)
(else
(+ 1 (length? (cdr lst))))))
(define (average)
(/ (sum lst) (length? lst)))
(define (insert x)
(set! lst (cons x lst)))
(lambda (function)
(cond ((eq? function 'sum) sum)
((eq? function 'length) length?)
((eq? function 'average) average)
((eq? function 'insert) insert)
(else
'undefined)))))
(define func (lst-functions))
((func 'insert) 2)
((func 'average))
推荐答案
您没有在使用它的过程中声明lst
参数,而是在调用它们时传递了它.我标记了已修改的行,请尝试以下操作:
You're not declaring the lst
parameter in the procedures that use it, but you're passing it when invoking them. I marked the lines that were modified, try this:
(define (lst-functions)
(let ((lst '()))
(define (sum lst) ; modified
(cond ((null? lst) 0)
(else
(+ (car lst) (sum (cdr lst))))))
(define (length? lst) ; modified
(cond ((null? lst) 0)
(else
(+ 1 (length? (cdr lst))))))
(define (average)
(/ (sum lst) (length? lst)))
(define (insert x)
(set! lst (cons x lst)))
(lambda (function)
(cond ((eq? function 'sum) (lambda () (sum lst))) ; modified
((eq? function 'length) (lambda () (length? lst))) ; modified
((eq? function 'average) average)
((eq? function 'insert) insert)
(else
'undefined)))))
现在它可以正常工作了:
Now it works as expected:
(define func (lst-functions))
((func 'insert) 2)
((func 'average))
=> 2
((func 'sum))
=> 2
((func 'length))
=> 1
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