perl - 为什么//在perl中优先级比平等低？

Question

为什么(至少)在perl 5.010中，//的优先级低于==？

例如这个

use 5.010;
my $may_be_undefined = 1;
my $is_equal_to_two = ($may_be_undefined//0 == 2);
say $is_equal_to_two;

(对我来说)打印出非常意外的结果。

Answer 1

这是因为//以及==属于运算符的类别。
==是“相等运算符”，尽管//属于“C样式逻辑运算符”类别。

举个例子; &&与//处于同一“类别”中，当涉及到运算符优先级时，以下两个语句是等效的。这样可能更容易理解？

  print "hello world" if $may_be_undefined && 0 == 2;
  print "hello world" if $may_be_undefined // 0 == 2;

Documentation of C-style Logical Defined-Or ( // )

Although it has no direct equivalent in C, Perl's // operator is related to its C-style or. In fact, it's exactly the same as ||, except that it tests the left hand side's definedness instead of its truth.

Thus, $a // $b is similar to defined($a) || $b (except that it returns the value of $a rather than the value of defined($a)) and yields the same result as defined($a) ? $a : $b (except that the ternary-operator form can be used as a lvalue, while $a // $b cannot).

This is very useful for providing default values for variables. If you actually want to test if at least one of $a and $b is defined, use defined($a // $b) .

The ||, // and && operators return the last value evaluated (unlike C's || and &&, which return 0 or 1).

Documentation of Operator Precedence and Associativity