I sometimes see Perl code like this:
我有时会看到像这样的Perl代码：
my ( $variable ) = blah....
What is the point of putting parentheses around a single variable? I thought parentheses were only used when declaring multiple variables, like:
将括号括在单个变量周围有什么意义？我认为括号仅在声明多个变量时使用，例如：
my ( $var1, $var2, $var3 ) = blah...
5 个解决方案
#1
36
There are several scenarios when there is a difference:
有几种情况存在差异：
-
When array is on right side
当数组在右侧时
my @array = ('a', 'b', 'c'); my $variable = @array; # 3 size of @array my ($variable) = @array; # 'a' $array[0] -
When list is on right side
当列表在右侧时
my $variable = qw/ a b c d /; # 'd' last item of the list my ($variable) = qw/ a b c d /; # 'a' first item of the list -
Subroutine with variable (array/scalar) return value
具有变量（数组/标量）返回值的子例程
sub myFunction { ... return (wantarray() ? @array : $scalar); } my $variable = myFunction(...); # $scalar from the subroutine my ($variable) = myFunction(...); # $array[0] from the subroutine
#2
19
The parentheses create a list context which affects how the right hand side of the assignment is evaluated.
括号创建一个列表上下文，该上下文会影响如何评估赋值的右侧。
Compare
比较
my $x = grep { /s/ } qw(apples bananas cherries);
print $x;
with
同
my ($x) = grep { /s/ } qw(apples bananas cherries);
print $x;
You will often use this construction when you just want to grab the first element of a list and discard the rest.
当你只想抓住列表的第一个元素并丢弃其余元素时，你经常会使用这种结构。
#3
7
I'm not a Perl pro (by all means, I'm not), but AFAIK it has to do with lists. Perl has different contexts (scalar, list). Using ($var) switches to list context, $var is scalar context.
我不是Perl pro（无论如何，我不是），但AFAIK与列表有关。 Perl有不同的上下文（标量，列表）。使用（$ var）切换到列表上下文，$ var是标量上下文。
my $var = (1, 2, 4); # $var = 4 (last element)
my ($var) = (1, 2, 4); # $var = 1
Please downvote this answer, if it is totally wrong :)
如果它是完全错误的话，请低估这个答案:)
#4
7
You are confusing two different things. First off, when using my to declare several variables, you need to use parentheses:
你混淆了两件不同的事情。首先，当使用my声明几个变量时，需要使用括号：
my $foo, $bar;
Does not work, as it is considered to be two different statements:
不起作用，因为它被认为是两种不同的陈述：
my $foo;
$bar;
So you need parentheses to group together the argument into an argument list to the function my:
所以你需要使用括号将参数组合在一个参数列表中，并将其放入函数my：
my($foo, $bar);
Secondly, you have explicit grouping in order to invoke list context:
其次，为了调用列表上下文，您有明确的分组：
$foo, $bar = "a", "b"; # wrong!
Will be considered three separate statements:
将被视为三个单独的陈述：
$foo;
$bar = "a";
"b";
But if you use parentheses to group $foo and $bar into a list, the assignment operator will use a list context:
但是如果使用括号将$ foo和$ bar分组到列表中，赋值运算符将使用列表上下文：
($foo, $bar) = ("a", "b");
Curiously, if you remove the RHS parentheses, you will also experience a hickup:
奇怪的是，如果你删除RHS括号，你也会遇到一个hickup：
($foo, $bar) = "a", "b"; # Useless use of a constant (b) in void context
But that is because the = operator has higher precedence than comma ,, which you can see in perlop. If you try:
但那是因为=运算符的优先级高于逗号，你可以在perlop中看到它。如果你试试：
my @array = ("a", "b");
($foo, $bar) = @array;
You will get the desired behaviour without parentheses.
您将获得没有括号的所需行为。
Now to complete the circle, lets remove the list context in the above and see what happens:
现在要完成圆圈，让我们删除上面的列表上下文，看看会发生什么：
my @array = ("a", "b");
$foo = @array;
print $foo;
This prints 2, because the array is evaluated in scalar context, and arrays in scalar context return the number of elements they contain. In this case, it is 2.
这打印2，因为数组在标量上下文中计算，标量上下文中的数组返回它们包含的元素数。在这种情况下，它是2。
Hence, statements such as these use list context:
因此，这些语句使用列表上下文：
my ($foo) = @array; # $foo is set to $array[0], first array element
my ($bar) = ("a", "b", "c"); # $bar is set to "a", first list element
It is a way of overriding the scalar context which is implied in scalar assignment. For comparison, these assignments are in scalar context:
它是一种覆盖标量赋值中隐含的标量上下文的方法。为了比较，这些分配在标量上下文中：
my $foo = @array; # $foo is set to the number of elements in the array
my $bar = ("a", "b", "c"); # $bar is set to "c", last list element
#5
2
I have posted an in-depth explanation on the subject on another site.
我已经在另一个网站上发布了关于这个主题的深入解释。
[I can't post it here. * doesn't support tables. Not that anyone usually has a problem with offsite linking to documentation.]
[我不能在这里张贴。 *不支持表。并非任何人通常都有异地链接到文档的问题。]
#1
36
There are several scenarios when there is a difference:
有几种情况存在差异：
-
When array is on right side
当数组在右侧时
my @array = ('a', 'b', 'c'); my $variable = @array; # 3 size of @array my ($variable) = @array; # 'a' $array[0] -
When list is on right side
当列表在右侧时
my $variable = qw/ a b c d /; # 'd' last item of the list my ($variable) = qw/ a b c d /; # 'a' first item of the list -
Subroutine with variable (array/scalar) return value
具有变量（数组/标量）返回值的子例程
sub myFunction { ... return (wantarray() ? @array : $scalar); } my $variable = myFunction(...); # $scalar from the subroutine my ($variable) = myFunction(...); # $array[0] from the subroutine
#2
19
The parentheses create a list context which affects how the right hand side of the assignment is evaluated.
括号创建一个列表上下文，该上下文会影响如何评估赋值的右侧。
Compare
比较
my $x = grep { /s/ } qw(apples bananas cherries);
print $x;
with
同
my ($x) = grep { /s/ } qw(apples bananas cherries);
print $x;
You will often use this construction when you just want to grab the first element of a list and discard the rest.
当你只想抓住列表的第一个元素并丢弃其余元素时，你经常会使用这种结构。
#3
7
I'm not a Perl pro (by all means, I'm not), but AFAIK it has to do with lists. Perl has different contexts (scalar, list). Using ($var) switches to list context, $var is scalar context.
我不是Perl pro（无论如何，我不是），但AFAIK与列表有关。 Perl有不同的上下文（标量，列表）。使用（$ var）切换到列表上下文，$ var是标量上下文。
my $var = (1, 2, 4); # $var = 4 (last element)
my ($var) = (1, 2, 4); # $var = 1
Please downvote this answer, if it is totally wrong :)
如果它是完全错误的话，请低估这个答案:)
#4
7
You are confusing two different things. First off, when using my to declare several variables, you need to use parentheses:
你混淆了两件不同的事情。首先，当使用my声明几个变量时，需要使用括号：
my $foo, $bar;
Does not work, as it is considered to be two different statements:
不起作用，因为它被认为是两种不同的陈述：
my $foo;
$bar;
So you need parentheses to group together the argument into an argument list to the function my:
所以你需要使用括号将参数组合在一个参数列表中，并将其放入函数my：
my($foo, $bar);
Secondly, you have explicit grouping in order to invoke list context:
其次，为了调用列表上下文，您有明确的分组：
$foo, $bar = "a", "b"; # wrong!
Will be considered three separate statements:
将被视为三个单独的陈述：
$foo;
$bar = "a";
"b";
But if you use parentheses to group $foo and $bar into a list, the assignment operator will use a list context:
但是如果使用括号将$ foo和$ bar分组到列表中，赋值运算符将使用列表上下文：
($foo, $bar) = ("a", "b");
Curiously, if you remove the RHS parentheses, you will also experience a hickup:
奇怪的是，如果你删除RHS括号，你也会遇到一个hickup：
($foo, $bar) = "a", "b"; # Useless use of a constant (b) in void context
But that is because the = operator has higher precedence than comma ,, which you can see in perlop. If you try:
但那是因为=运算符的优先级高于逗号，你可以在perlop中看到它。如果你试试：
my @array = ("a", "b");
($foo, $bar) = @array;
You will get the desired behaviour without parentheses.
您将获得没有括号的所需行为。
Now to complete the circle, lets remove the list context in the above and see what happens:
现在要完成圆圈，让我们删除上面的列表上下文，看看会发生什么：
my @array = ("a", "b");
$foo = @array;
print $foo;
This prints 2, because the array is evaluated in scalar context, and arrays in scalar context return the number of elements they contain. In this case, it is 2.
这打印2，因为数组在标量上下文中计算，标量上下文中的数组返回它们包含的元素数。在这种情况下，它是2。
Hence, statements such as these use list context:
因此，这些语句使用列表上下文：
my ($foo) = @array; # $foo is set to $array[0], first array element
my ($bar) = ("a", "b", "c"); # $bar is set to "a", first list element
It is a way of overriding the scalar context which is implied in scalar assignment. For comparison, these assignments are in scalar context:
它是一种覆盖标量赋值中隐含的标量上下文的方法。为了比较，这些分配在标量上下文中：
my $foo = @array; # $foo is set to the number of elements in the array
my $bar = ("a", "b", "c"); # $bar is set to "c", last list element
#5
2
I have posted an in-depth explanation on the subject on another site.
我已经在另一个网站上发布了关于这个主题的深入解释。
[I can't post it here. * doesn't support tables. Not that anyone usually has a problem with offsite linking to documentation.]
[我不能在这里张贴。 *不支持表。并非任何人通常都有异地链接到文档的问题。]