英文:
Why defining a recursive-display procedure in Guile causes infinite loop?
问题
以下过程会导致无限循环：
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display (car args))
(recursive-display (cdr args)))))
我仍在学习Guile，所以对这种情况感到有些困惑。
英文:
The following procedure causes an infinite loop:
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display (car args))
(recursive-display (cdr args)))))
I'm still learning about Guile so I'm a bit confused why that's the case.
答案1
得分: 3
使用display来打印args的值并查看输出：
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display args)
(recursive-display (cdr args)))))
> (recursive-display 1 2 3)
(1 2 3)
((2 3))
(())
(())
(())
...
你的函数recursive-display具有可变数量的参数。当你像(recursive-display 1 2 3)这样调用它时，传递给这个函数的所有参数都被包装在列表中，所以变量args的值是'(1 2 3)。
接下来，你调用(recursive-display (cdr args))。同样的规则再次适用-所有参数都被包装在列表中，所以args现在的值是'((2 3))。
在所有后续迭代中，args的值都是'(())。
当你添加 apply 时，它将按预期工作：
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display (car args))
(apply recursive-display (cdr args)))))
> (recursive-display 1 2 3)
123#t
英文:
Use display to print the value of args and look at the output:
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display args)
(recursive-display (cdr args)))))
> (recursive-display 1 2 3)
(1 2 3)
((2 3))
(())
(())
(())
...
Your function recursive-display has a variable number of arguments. When you call something like (recursive-display 1 2 3), all arguments passed to this function are wrapped in the list, so variable args has a value '(1 2 3).
Next, you call (recursive-display (cdr args)). The same rule applies again- all arguments are wrapped in the list, so args has now value '((2 3)).
In all following iterations, args has a value '(()).
When you add apply, it will work as expected:
(define (recursive-display . args)
(if (null? args)
#t
(begin
(display (car args))
(apply recursive-display (cdr args)))))
> (recursive-display 1 2 3)
123#t
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