perl - 如何停止在编译器阶段运行代码？

Question

这个问题需要了解编译器阶段，而不是BEGIN块。从Perl编程:第3版-第467页

It's also important to understand the distinction between compile phase and compile time, and between run phase and run time. A typical Perl program gets one compile phase, and then one run phase. A "phase" is a large-scale concept. But compile time and run time are small-scale concepts. A given compile phase does mostly compile-time stuff, but it also does some run-time stuff via BEGIN blocks. A given run phase does mostly run-time stuff, but it can do compile-time stuff through operators like eval STRING.

让我们举一个非常简单的例子

sub complex_sub {
  die 'code run';
}
sleep 5;
print 'good';
use constant FOO => complex_sub();

如果以上命令按原样运行，那么从用户角度来看， complex_sub将在编译器阶段运行。但是，只要稍加修改，我就可以拥有..

# Bar.pm
package Bar {
  use constant FOO => main::complex_sub();
}

# test.pl
package main {
  sub complex_sub {
    die 'code run';
  }
  sleep 5;
  print 'good';
  require Bar;
}

在上面的代码中， complex_sub在执行阶段运行。无论如何，从 complex_sub的角度来区分这两种情况以启用顶部语法，但禁止底部语法。

Answer 1

使用${^GLOBAL_PHASE}变量。在第一种情况下，它包含“START”，在第二种情况下，它包含“RUN”。

# RUN
perl -wE'say ${^GLOBAL_PHASE}'

# START
perl -wE'BEGIN {say ${^GLOBAL_PHASE}}'

# RUN
perl -wE'eval q{BEGIN {say ${^GLOBAL_PHASE}}}'

有关详细信息，请参见 perlvar。