为什么这个地图块含有一个显然没用的？ [英] Why does this map block contain an apparently useless +?

问题描述

浏览源代码时，我看到以下行：

While browsing the source code I saw the following lines:

my @files_to_keep = qw (file1 file2);
my %keep = map { + $_ => 1 } @files_to_keep;

在此代码中 + 做了什么片段？我使用 Data :: Dumper 查看是否取出加号做任何事情，但结果是一样的：

What does the + do in this code snippet? I used Data::Dumper to see whether taking out the plus sign does anything, but the results were the same:

  $ perl cleanme.pl
$VAR1 = {
          'file1' => 1,
          'file2' => 1
        };

推荐答案

这用于防止解析问题。加号表示解释器的行为就像一个正常的块，而不是一个表达式。

This is used to prevent a parsing problem. The plus symbol forces the interpreter to behave like a normal block and not an expression.

恐怕是你可能试图使用其他（表达式）创建一个散列引用，

The fear is that perhaps you are trying to create a hashreference using the other (expression) formulation of map like so.

@array_of_hashrefs = map {  "\L$_" => 1  }, @array

注意逗号。那么如果解析器猜测你在OP中给出了这个语句，那么会出现一个缺少逗号的语法错误！要看到区别，请引用$ _。无论什么原因，解析器都足以触发表达式行为。

Notice the comma. Then if the parser guesses that you are doing this given the statement in the OP there will a syntax error for missing the comma! To see the difference try quoting "$_". For whatever reason, the parser takes this as enough to trigger the expression behavior.

是的，它是一个奇怪的。因此，许多超级偏执的Perl程序员比需要多加多余的加号（我包括）。

Yes its an oddity. Therefore many extra-paranoid Perl programmers toss in the extra plus sign more often than needed (me included).

以下是 地图 文档。

Here are the examples from the map documentation.

%hash = map {  "\L$_" => 1  } @array  # perl guesses EXPR.  wrong
%hash = map { +"\L$_" => 1  } @array  # perl guesses BLOCK. right
%hash = map { ("\L$_" => 1) } @array  # this also works
%hash = map {  lc($_) => 1  } @array  # as does this.
%hash = map +( lc($_) => 1 ), @array  # this is EXPR and works!
%hash = map  ( lc($_), 1 ),   @array  # evaluates to (1, @array)

为了有趣的阅读（风格）和解析器发现错误的情况，请阅读以下内容： http://blogs.perl.org/users/tom_wyant/2012/01/the-case-of-the-overloaded-curlys.html

For a fun read (stylistically) and a case where the parser gets it wrong read this: http://blogs.perl.org/users/tom_wyant/2012/01/the-case-of-the-overloaded-curlys.html

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