为什么使用大括号初始化一些哈希，还有一些使用括号？ [英] Why are some hashes initialized using curly braces, and some with parentheses?

问题描述

  my％HoH =（
 flintstones = > {
丈夫=>fred，
 pal =>barney，
}，
 jetsons => {
 husband => george，
 wife =>jane，
他的男孩=>elroy，＃需要的钥匙引号
}，
 simpsons => {
丈夫=>本垒打，
妻子=>marge，
 kid =>bart，
}，
）; 
  

为什么最上面的散列（起始行1）是用括号初始化的，而使用大括号初始化子哈希？

从Python背景来看，我必须说Perl很奇怪：）。

使用圆括号初始化一个散列（或数组）。哈希是一组字符串和一组标量值之间的映射。

 ％foo =（key1， value1，key2，value2，...）; ＃％表示散列
％foo =（key1 =>value1，key2 =>value2，...）; ＃相同的东西
  

大括号用于定义哈希参考。所有引用都是标量值。

  $ foo = {key1 => value1，key2 => value2，...}; ＃$表示标量
  

哈希是不是标量值。由于hash 中的值必须是标量，因此不可能使用散列作为另一个散列的值。

 ％bar =（key3 =>％foo）; ＃并不意味着你认为它的意思是
  

但是我们可以使用散列引用作为另一个散列值，因为哈希引用是标量。

  $ foo = {key1 => value1，key2 => value2}; 
％bar =（key3 => $ foo）; 
％baz =（key4 => {key5 =>value5，key6 =>value6}）; 
  

这就是为什么你会看到带有花括号的列表中的括号。

I'm looking at the following code demonstrating nested hashes:

my %HoH = (
    flintstones => {
        husband   => "fred",
        pal       => "barney",
    },
    jetsons => {
        husband   => "george",
        wife      => "jane",
        "his boy" => "elroy",  # Key quotes needed.
    },
    simpsons => {
        husband   => "homer",
        wife      => "marge",
        kid       => "bart",
    },
);

Why is it that the upper-most hash (starting line 1) is initialized using parentheses, whereas the sub-hashes are initialized using curly braces?

Coming from a python background I must say Perl is quite odd :).

解决方案

Coming from a Perl background I find Perl quite odd, too.

Use parentheses to initialize a hash (or an array). A hash is a map between a set of strings and a set of scalar values.

%foo = ( "key1", "value1",  "key2", "value2", ... );   #  % means hash
%foo = ( key1 => "value1",  key2 => "value2", ... );   # same thing

Braces are used to define a hash reference. All references are scalar values.

$foo = { key1 => "value1", key2 => "value2", ... };    #  $ means scalar

Hashes are not scalar values. Since the values in a hash must be scalars, it is therefore not possible to use a hash as a value of another hash.

%bar = ( key3 => %foo );     # doesn't mean what you think it means

But we can use hash references as values of another hash, because hash references are scalars.

$foo = { key1 => "value1", key2 => "value2" };
%bar = ( key3 => $foo );
%baz = ( key4 => { key5 => "value5", key6 => "value6" } );

And that is why you see parentheses surrounding a list of lists with braces.

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