为什么 map() 掩盖了“未定义值"错误? [英] Why does map() mask the 'undefined value' error?

问题描述

我想知道为什么在第二个代码段中调用 map 会导致未定义值"错误消失?

I'm wondering why a call to map in the second snippet makes the 'undefined value' error gone?

use strict;
use warnings;
my $x;
my @a = @{ $x }; # error: Can't use an undefined value as an ARRAY reference

比较:

use strict;
use warnings;
my $x;
my @a = map $_, @{ $x }; # no error, @a is empty

推荐答案

这是由于 map() 做别名(它本质上是使用 for() 循环).正在发生的事情是 aref 正在左值上下文中使用，因此正在 auto-vivified 存在.

This is due to the way that map() does aliasing (it's essentially using a for() loop). What's happening is that the aref is being used in l-value context, and therefore is being auto-vivified into existence.

在您之前的示例中，您试图直接在 r 值上下文中使用 aref，这就是它生成错误的原因(因为没有发生自动激活).

In your former example, you're attempting to use the aref directly in r-value context, which is why it generates the error (because no auto-vivification happens).

您可以简化您的测试以使用 for()，并且您将获得与 map() 相同的结果:

You can simplify your test to use for(), and you'll get the same result as with map():

use warnings;
use strict;

my $x;

for (@{ $x }){
    print "$_\n";
}

...没有输出.

要直观地看到 $x 被自动激活为数组引用，您可以使用 ref() 函数:

To visually see that $x was auto-vivified as an array reference, you can use the ref() function:

my $x;
my @a = map $_, @{ $x };

print ref $x;

输出:

ARRAY

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