是否有理由让if语句后跟另一个以ruby的'and'运算符(&&)结尾的行? [英] Is there a reason for an if statement followed by another to end the line with an 'and' operator (&&) in ruby?

问题描述

我正试图弄清楚为什么它起作用，以及这是否是一种不好的做法.

I'm trying to figure out why this works and whether it's a bad practice.

以下代码将引发 SyntaxError :

if true &&
  puts "ok"
end

尽管以下代码不会，但将按预期执行:

Though the following code will not, it will execute as expected:

if true &&
  if true
    puts "ok"
  end
end

我在编写带有 if 语句的红宝石脚本时遇到了这个问题，该语句包含很大的条件链，因此我将每个脚本放在单独的行中，然后注释掉最后一个并执行脚本.此刻，我意识到 if 语句以&& 运算符结尾.当我期望它抛出时，我感到惊讶的是，不是因为紧接着又出现了另一个 if 语句.

I came across this while writing a ruby script with an if statement containing a big chain of conditions, so I put each one in a separate line and then I commented out the last one and executed the script. At the moment, I realized that the if statement ended with the && operator. While I expected it to throw, I was amazed that it didn't because another if statement followed right after.

我的红宝石版本是ruby 2.2.6p396 (2016-11-15 revision 56800) [x86_64-darwin16].

推荐答案

我正试图弄清楚为什么它起作用，以及这是否是一种不好的做法.

I'm trying to figure out why this works and whether it's a bad practice.

后一个问题很难回答，因为您的代码是如此抽象.此外，不良做法"是基于观点的，因此是不合时宜的.

The latter question is hard to answer because your code is so abstract. Also, "bad practice" is opinion-based and thus off-topic.

以下代码将引发 SyntaxError :

if true &&
 puts "ok"
end

这是一条红鲱鱼.请阅读错误消息:

This is a red herring. Please, read the error message:

SyntaxError: unexpected string literal, expecting `do' or '{' or '('
  puts "ok"
       ^

它没有说明if.

我希望您的代码等同于

if true && puts "ok"
end

确实引起了相同的SyntaxError:

SyntaxError: unexpected string literal, expecting `do' or '{' or '('
if true && puts "ok"
                ^

我们可以进一步缩小到

true && puts "ok"

这是因为&&的优先级高于空格分隔的参数列表，因此该行被解析为

Which is because && has higher precedence than a whitespace-delimited argument list, and thus this line is parsed as

(true && puts) "ok"

因此，要消除歧义，我们需要在参数列表中添加括号:

So, to disambiguate, we need to add parentheses to the argument list:

if true &&
  puts("ok")
end

现在代码可以按预期工作了.

And now the code works as expected.

尽管以下代码不会，但将按预期执行:

Though the following code will not, it will execute as expected:

if true &&
 if true
   puts "ok"
 end
end

的确，这是完全合法的.该行末尾的二进制运算符将隐式地继续该行，即此代码等效于:

Indeed, this is perfectly legal. A binary operator at the end of the line will implicitly continue the line, i.e. this code is equivalent to:

if true && if true
    puts "ok"
  end
end

或

if true && if true then puts "ok" end
end

或

if true && if true then puts "ok" end then end

我在写带有 if 语句的红宝石脚本时遇到了这个问题

I came across this while writing a ruby script with an if statement

这是您困惑的根源:这不是声明，而是 .这是if 表达式. (从技术上讲，ISO Ruby语言规范将其称为条件表达式.)Ruby中没有语句，只有表达式.

And here is the source of your confusion: this is not an if statement. This is an if expression. (Technically, the ISO Ruby Language Specification calls it a conditional expression.) There are no statements in Ruby, only expressions.

我感到惊讶的是，那不是因为紧随其后的另一个 if 陈述.

不是声明.它是一个与其他任何表达式一样的表达式. foo && bar是完全合法的，而foo && if true then 23 end也是完全合法的，因为它是完全相同的东西:一个&&运算符，其两侧都有一个表达式.

It isn't a statement. It is an expression like any other expression. foo && bar is perfectly legal and foo && if true then 23 end is also perfectly legal because it is the exact same thing: an && operator with an expression on either side.

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