一例Scheme中的Continuation消除
ee.zsy
posted @ 2011年4月17日 01:03
in 未分类
, 1423 阅读
大家好，我是新人，这是我的第一篇的说。
试着写一篇,有相同爱好的回应一下看法吧,让我知道以后写点什么内容好。
这篇想说的是Continuation可以作为对程序执行的一种抽象（比如搜索树的回溯，比如协作式多任务），而不是先仅仅Scheme中特有的执行机制。
下面的例子是为了描述问题简单而举出来的，可能不容易让人联想到实际的问题上来。
考虑下面一段代码：
(define (for-each-with-continuation proc list1)
(if (null? list1)
'()
(begin
(call-with-current-continuation
(lambda (c) (proc c (car list1))))
(for-each-with-continuation
proc (cdr list1)))))
如果使用
(for-each-with-continuation
(lambda (c i) (display i) (c))
'(1 2 3 4 5))
将输出
12345
其中可以把call-with-current-continuation去掉
(define (for-each-with-continuation0 proc list1)
(if (null? list1)
'()
(proc
(lambda ()
(for-each-with-continuation0 proc (cdr list1)))
(car list1))))
这样变换的前提是延续作为了尾递归使用，事实上就是goto+apply的组合。
这样做的目的是在支持goto语句但不支持延续的语言中使用Continuation机制。
在Scheme中可以这样实现支持尾递归的return语句：
(define (myfun)
(call-with-values
(lambda ()
(call/cc
(lambda (return-apply)
...
(return-apply func2 . args)
...
)))
(lambda (fun . args)
(apply fun args))))
其实已经很多语言提供这种return了，如javascript，lua...
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