求教高手——一段perl代码

perlearner

求教各位高手，在《InterMediatePerl》中这段文字和代码怎样理解？
为什么不能在闭包中直接返回  "return $total_size += -s if -f;",而是要用if控制结构来判断是否有参数呢？
How would we get the total size of all found files from the callback? Earlier, we were able to do this by making $total_size visible. If we stick the definition of $total_size into the subroutine that returns the callback reference, we won’t have access to the variable. But we can cheat a bit. For one thing, we can determine that we’ll never call the callback subroutine with any parameters, so, if the subroutine receives a parameter, we make it return the total size:
use File::Find;
sub create_find_callback_that_sums_the_size {
  my $total_size = 0;
  return sub {
    if (@_) { # it’s our dummy invocation
      return $total_size;
    } else { # it’s a callback from File::Find:
      $total_size += -s if -f;
    }
  };
}
my $callback = create_find_callback_that_sums_the_size(  );
find($callback, ‘bin’);
my $total_size = $callback->(‘dummy’); # dummy parameter to get size
print "total size of bin is $total_size\n";

Kitaisky

Precisely because of File::Find, it's one of those weird balls of Perl (actually due to the underlying C programs, I'd guess)

find (\&wanted, "Directory") .... ' find() ' makes &wanted do all the work, and yet IGNORES all the return values from &wanted...

so the $callback can certainly have a return value, but just being ignored by ' find '

perlearner

回复 2# Kitaisky


    非常感谢，同时还有一个疑问
为什么If we stick the definition of $total_size into the subroutine that returns the callback reference, we won’t have access to the variable.？
是否应该按照Reference Counting引用计数理论来理解呢，如果是的话，具体应该怎样分析？
望不吝赐教。

perlearner

回复 3# perlearner


    同时，在File::Find的文档中也提到，find (\&wanted, "Directory")这种格式的用法中，\&wanted针对Directory中的每个条目进行递归搜索，每找到一个文件或目录，就对其采取\&wanted操作。但为什么直接返回$total_sizes += -s if -f，就无法在外部访问到$total_size变量了呢？