blocks|key|1119538|text|Date类包括Comparable#==方法，但该方法调用接收器的<=>方法。在本例中是Date#<=>，它需要另一个Date对象。当它接收到nil时，它返回nil。这种行为看起来肯定不一致，我不知道背后的原因。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1119539|entityMap^0|7|D|X|3|18|8|1Z|3|27|3|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|G|$]]

The Date class includes the <code>Comparable#==</code> method, but that method invokes the <code>&lt;=&gt;</code> method of the receiver. In this case that's <code>Date#&lt;=&gt;</code>, which expects another Date object. When it receives <code>nil</code> it returns <code>nil</code>. This behavior certainly seems inconsistent, and I don't know the reasons behind it.

blocks|key|1334715|text|我检查了源码，下面是我发现的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1334716|由Comparable定义的比较运算符将函数rb_cmpint与<=>一起使用。当其中一个操作数为nil时，rb_cmpint将引发异常。|offset|length|style|CODE|1334717|因此，如果rhs与lhs不具有可比性，则可比较的运营商会提出例外。也就是说，5+<+2为false，但5+<+"la"会引发异常。他们这样做是为了区分<不为真的情况，因为rhs较小，以及不为真的情况，因为rhs不可比较。或者换句话说:当x+<+y为false时，表示x+>=+y为true。因此，在情况并非如此的情况下，它会抛出异常。|1334718|==引发异常将是不好的，因为==通常不(也不应该)要求其操作数是可比较的。但是，==使用与其他操作数相同的方法，这确实会引发异常。因此，整个函数被简单地包装在一个rb_rescue中。如果抛出异常，则返回nil。|1334719|请注意，这只适用于ruby+1.8。这在1.9中已经修复，现在==永远不会返回nil+(当然，除非您定义自己的==返回)。|1334720|entityMap^0|0|M|9|W|3|1I|9|0|12|5|1F|8|23|1|3A|5|3P|6|0|0|2|E|2|14|2|29|9|2U|3|0|V|2|13|3|1J|2|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Q|8|@$D|R|E|S|F|G]|$D|T|E|U|F|G]|$D|V|E|W|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|X|8|@$D|Y|E|Z|F|G]|$D|10|E|11|F|G]|$D|12|E|13|F|G]|$D|14|E|15|F|G]|$D|16|E|17|F|G]]|9|@]|A|$]]|$1|J|3|K|5|6|7|18|8|@$D|19|E|1A|F|G]|$D|1B|E|1C|F|G]|$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]|$D|1H|E|1I|F|G]]|9|@]|A|$]]|$1|L|3|M|5|6|7|1J|8|@$D|1K|E|1L|F|G]|$D|1M|E|1N|F|G]|$D|1O|E|1P|F|G]]|9|@]|A|$]]|$1|N|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|O|$]]

I checked the source and here's what I found out:

The comparison operators defined by Comparable all use the function <code>rb_cmpint</code> together with <code>&lt;=&gt;</code>. <code>rb_cmpint</code> raises an exception when one of the operands is nil.

So the operators of Comparable raise an exception if the rhs is not comparable to the lhs. I.e. <code>5 &lt; 2</code> is false, but <code>5 &lt; "la"</code> raises an exception. They do this to differentiate between cases where <code>&lt;</code> is not true because the rhs is smaller and cases where it's not true because the rhs is not comparable. Or in other words: When <code>x &lt; y</code> is false that implies that <code>x &gt;= y</code> is true. So in cases where that would not be the case, it throws an exception.

<code>==</code> raising an exception would be bad, because <code>==</code> usually does not (and should not) require its operands to be comparable. However <code>==</code> uses the same method as the other operands, which does raise an exception. So the whole function is simply wrapped in an <code>rb_rescue</code>. And that returns <code>nil</code> if an exception is thrown.

Note that this only applies to ruby 1.8. This has been fixed in 1.9 and now <code>==</code> never returns <code>nil</code> (except of course if you define your own <code>==</code> that does).

blocks|key|1119608|text|如果您的代码依赖于此，那么您可以始终使用.nil？任何Ruby对象都会响应的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1119609|>>+Date.new.nil?
=>+false|code-block|syntax|javascript|1119610|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you're depending on this for code, you can always use the .nil? method which any Ruby Object responds to. 

<pre><code>&gt;&gt; Date.new.nil?
=&gt; false
</code></pre>

blocks|key|1119650|text|它的发生是因为你不能比较未定义的东西。这是可取的，因为如果你的操作数中至少有一个没有定义，那么你就不能对结果得出任何结论，这与断言事实不同。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1119651|许多语言将nil和false等同起来，这纯粹是为了方便而怀疑。这在数学上肯定是不正确的。|1119652|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

It happens because you can't compare things that are not defined. It's desirable because if at least one of your operands is not defined then you can't draw any conclusion about the result, which is different from asserting truth.

A lot of languages treat nil and false the same, which is suspect is purely for convenience. It's certainly not mathematically correct.

When writing some rspec today, I came across some unexpected behavior with comparing Date (and Time) instances to nil. Here's a sample using raw ruby (no Rails or other libraries):

<pre><code>user@MacBook-Work ~ $ ruby -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0]
user@MacBook-Work ~ $ irb
&gt;&gt; 1 == nil
=&gt; false
&gt;&gt; "string" == nil
=&gt; false
&gt;&gt; :sym == nil
=&gt; false
&gt;&gt; false == nil
=&gt; false
&gt;&gt; [] == nil
=&gt; false
&gt;&gt; {} == nil
=&gt; false
&gt;&gt; Proc.new {} == nil
=&gt; false
</code></pre>

So far, so good, right?

<pre><code>&gt;&gt; Date.new == nil
=&gt; nil
&gt;&gt; Time.new == nil
=&gt; nil
</code></pre>

Date does implement its own ===, which works fine:

<pre><code>&gt;&gt; Date.new === nil
=&gt; false
</code></pre>

Is there any explanation as to why this happens or why this is desired behavior? == seems to be implemented from Comparable.==, however documentation on that doesn't given any indication that it would ever return nil. What's the design decision to this?

Update! This is not the case in 1.9.2:

<pre><code>$ irb
ruby-1.9.2-p136 :001 &gt; require 'date'
 =&gt; true 
ruby-1.9.2-p136 :002 &gt; Date.new == nil
 =&gt; false 
ruby-1.9.2-p136 :003 &gt; Time.new == nil
 =&gt; false 
</code></pre>

In Ruby, why does a equality with nil ("Date.new == nil") return nil?

今天在编写一些rspec时，我在将日期(和时间)实例与nil进行比较时遇到了一些意外的行为。这是一个使用原始ruby的示例(没有Rails或其他库)：user@MacBook-Work ~ $ ruby -vruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin1...

问在Ruby中，为什么与nil ("Date.new == nil")的相等返回nil？
EN

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问在Ruby中，为什么与nil ("Date.new == nil")的相等返回nil？
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