perl - `sub bar { +{$_[1] => $_[2]} }` 到底是做什么的？

Question

我不明白 +这个例子中的糖牌是在护目镜时在某处拍摄的:

sub bar { +{$_[1] => $_[2]} }

我写了这个，在这里我看不到任何区别:

use Data::Dumper;

# Not any differences here
my $foo =  {value => 55};
my $bar = +{value => 55};

print Dumper $foo;
print Dumper $bar;

# Oh ! Here there is something...
sub foo {  {$_[1] => $_[2]} };
sub bar { +{$_[1] => $_[2]} };

print Dumper foo('value', 55);    
print Dumper bar('value', 55);    

foo返回

$VAR1 = 55;
$VAR2 = undef;

bar返回

$VAR1 = {
          '55' => undef
        };

Answer 1

它帮助解析器区分匿名哈希和代码块。

引用 Learning Perl Objects, References & Modules

because blocks and anonymous hash constructors both use curly braces in roughly the same places in the syntax tree, the compiler has to make ad hoc determinations about which of the two you mean. If the compiler ever decides incorrectly, you might need to provide a hint to get what you want. To show the compiler that you want an anonymous hash constructor, put a plus sign before the opening curly brace: +{ ... }. To be sure to get a block of code, just put a semicolon (representing an empty statement) at the beginning of the block: {; ... }.

或来自 map 上的文档功能:

"{" starts both hash references and blocks, so "map { ..." could
be either the start of map BLOCK LIST or map EXPR, LIST. Because
Perl doesn't look ahead for the closing "}" it has to take a guess
at which it's dealing with based on what it finds just after the
"{". Usually it gets it right, but if it doesn't it won't realize
something is wrong until it gets to the "}" and encounters the
missing (or unexpected) comma. The syntax error will be reported
close to the "}", but you'll need to change something near the "{"
such as using a unary "+" or semicolon to give Perl some help:

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

    %hash = map  ( lc($_), 1 ),   @array # evaluates to (1, @array)

or to force an anon hash constructor use "+{":

    @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
                                           # comma at end

to get a list of anonymous hashes each with only one entry apiece.