Perl常数在哪里被其值替换？ [英] Where are perl constants replaced by their values?

问题描述

我试图在perl中使用常量值，偶然发现以下奇怪的行为：

I tried to use constant values in perl and stumbled upon the following weird behaviour:

#!/usr/bin/perl
use strict;
use warnings;

use Data::Dumper;

use constant {
    a => "b"
};

my $c = { a => a };
my %d;
$d{a} = a;

print Dumper($c);
print Dumper(\%d);

将输出

$VAR1 = {
          'a' => 'b'
        };
$VAR1 = {
          'a' => 'b'
        };

常量 a 在右侧被替换表达式 $ d {a} = a 和 a => a ，但不在左侧。

The constant a was replaced on the right hand side of the expressions $d{a} = a and a => a, but not on the left side.

我知道常量是使用inlinable subs实现的（在此处记录），并且如果未预先声明，则子名称将根据其名称求值（在此处记录），但是我看不出在我的示例中 a 进行评估的原因在同一行代码中，一次是值，一次是名称，尤其是在散列分配中- a => a 可能是 => 将左位置解释为字符串（如果它以字母开头）的结果。

I know that constants are implemented using inlinable subs (documented here) and that sub names evaluate to their names if they aren't predeclared (documented here), but I can see no reason why in my example a evaluates once to the value and once to the name in the same line of code, especially in the assignment to the hash - The a => a might be a consequence of => interpreting the left site as a string if it begins with a letter.

边注：添加括号使子调用显式产生预期结果：

Sidenote: Adding parantheses to make the sub-call explicit yields the expected result:

# ...
my $c = { a() => a }; # or my $c = { a, a };
my %d;
$d{a()} = a;
# ....

输出：

$VAR1 = {
          'b' => 'b'
        };
$VAR1 = {
          'b' => 'b'
        };

（所有示例均经过perl 5.18测试）

(all examples tested with perl 5.18)

推荐答案

constant 页的结尾洞穴，答案

如果在自动引述裸词的上下文中使用常量，可能会遇到麻烦（对于任何子例程调用都是如此）。例如，您不能说 $ hash {CONSTANT} ，因为 CONSTANT 将被解释为字符串。

You can get into trouble if you use constants in a context which automatically quotes barewords (as is true for any subroutine call). For example, you can't say $hash{CONSTANT} because CONSTANT will be interpreted as a string.

然后继续使用找到的解决方案

It then proceeds with the solution you found

使用 $ hash {CONSTANT（）} 或 $ hash {+ CONSTANT} 来防止裸词引用机制

Use $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the bareword quoting mechanism from kicking in.

然后也将其拼写为散列

类似地，由于 => 运算符在其左侧立即引用了一个裸字，因此您必须说 CONSTANT（） => ‘value’（或只是用逗号代替大箭头），而不是 CONSTANT => ‘值’。

Similarly, since the => operator quotes a bareword immediately to its left, you have to say CONSTANT() => 'value' (or simply use a comma in place of the big arrow) instead of CONSTANT => 'value'.

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