为什么＆QUOT;！＆QUOT;认为不好的形式在Perl？ [英] Why is "!!" considered bad form in Perl?

问题描述

在最近的一次面试过程中，我提出了一些示例Perl code其中所用的所谓秘方 !! 的operator 。后来，在讨论code时，面试官一问我，为什么我选择使用的，并表示，它被认为是不好的形式。他没有详细说明，为什么。

我和我的团队一直在使用该运营商多年，从来没有意识到这一点被认为是不好的形式。

请问嘭嘭运营商有副作用或其他意外的行为？为什么，或有些人可能会说，它是，认为是坏表？是否有地道的选择吗？

下面是在那里我会考虑 !! 接受和/或可取的。

的几个例子

1. 实际code在编码演习，这是加入布尔值的例子：

    同时（我的$行=＆LT; $ F＆GT;）{
       ＃SNIP
       存在$ {计数线}和$ {计数}行+ = !!的Chomp $线;
   }
    




2. 使用一个布尔值作为哈希键（显然是一个简单的例子）：

    子富{
       我（$输入）= @_;
       我的答复％=（''=＆gt;中假，1 =＆GT;真）;
       返回$回应{!! $输入};
   }
    




3. 在按位操作使用一个布尔值，甚至包（）：

    子富{
       我（$ A，$ B，$ C）= @_;
       我的$结果= !! $ A +（$！B）＆LT;＆LT; 1 +（$ !!℃）下; 2;
       返回$结果;
   }
    




4. 您需要通过外部库/过程做使用的类型转换，如一个数据库，其中只考虑某些值是truthy：

    我的$某物= $ dbh-＆GT; prepare（INSERT INTO表（旗，值）VALUES（，）？）
   $ sth-＆GT;执行（i_haz_cheeseburger，!! $ cheeseburger_string）
    

解决方案

您 !! 充分利用Perl中两个歧件事：<$ C $的具体值C> <！/ code>回报，并且可能的值中的一个是dualvar（含有字符串和多个标量）。使用 !! 来加剧这些行为固然精辟。虽然它的简洁可以是一个巨大的加，它也可以是一个相当大的负数。不要使用导致项目强制要求功能使用​​Perl的是在这个项目上禁止的。

有很多的替代品，以 $计数+ = !! （some_ex pression）来算的前pression的truthy值的出现。一个是三元， $计数+ =（some_ex pression）？ 1：0 。这也有点模糊。有一个很好的紧凑的方式做你想要的，这是使用后，如果：

  $ X ++如果some_ex pression;
 

此说，你在做什么。

During a recent job interview process, I submitted some sample Perl code which used the so-called "secret" !! operator. Later, when discussing the code, one of the interviewers asked me why I chose to use that, and indicated that it was considered bad form. He didn't elaborate as to why.

My team and I have been using this operator for years, without ever realizing it was considered "bad form."

Does the "bang bang" operator have side-effects or other unexpected behavior? Why is it, or might it be, considered "bad form" by some? Is there an idiomatic alternative?

Below are a few examples where I would have considered !! acceptable and/or desirable.

1. The actual code in the coding exercise, which is an example of adding booleans:

   while (my $line = <$F>) {
       # snip
       exists $counts{lines} and $counts{lines} += !! chomp $line;
   }
   

2. Using a boolean value as a hash key (clearly a simplified example):

   sub foo {
       my ($input) = @_;
       my %responses = ( '' => "False", 1 => "True" );
       return $responses{ !! $input };
   }
   

3. Using a boolean in a bitwise operation, or even pack():

   sub foo {
       my ( $a, $b, $c ) = @_;
       my $result = !!$a + (!! $b)<<1 + (!! $c)<<2;
       return $result;
   }
   

4. You need to do a typecast for use by an external library/process, such as a database, which only considers certain values to be truthy:

   my $sth = $dbh->prepare("INSERT INTO table (flag,value) VALUES (?,?)")
   $sth->execute("i_haz_cheeseburger", !! $cheeseburger_string)
   

解决方案

Your !! takes advantage of two obscure things in Perl: The specific values that ! returns, and that one of the possible values is dualvar (a scalar containing both a string and a number). Using !! to compound these behaviors is admittedly pithy. While its pithiness can be a huge plus, it can also be a rather big minus. Don't use features that result in a project mandating that "the use of Perl is forbidden on this project."

There are lots of alternatives to $count += !! (some_expression) to count occurrences of truthy values of that expression. One is the ternary, $count += (some_expression) ? 1 : 0. That too is a bit obscure. There is a nice compact way to do what you want, which is to use the post-if:

$x++ if some_expression;

This says exactly what you are doing.

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