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copied!<p><code>Integer</code> is a class, which (at least in Ruby) means that it is just a boring old normal object like any other object, which just happens to be an instance of the <code>Class</code> class (instead of, say, <code>Object</code> or <code>String</code> or <code>MyWhateverFoo</code>).</p> <p><code>Class</code> in turn is a subclass of <code>Module</code> (although arguably it shouldn't be, because it violates the Liskov Substition Principle, but that is a discussion for another forum, and is also a dead horse that has already been beaten many many times). And in <code>Module#===</code> you will find the definition you are looking for, which <code>Class</code> inherits from <code>Module</code> and instances of <code>Class</code> (like <code>Integer</code>) understand.</p> <p><code>Module#===</code> is basically defined symmetric to <code>Object#kind_of?</code>, it returns <code>true</code> if its argument is an instance of itself. So, <code>3</code> is an instance of <code>Integer</code>, therefore <code>Integer === 3</code> returns <code>true</code>, just as <code>3.kind_of?(Integer)</code> would.</p> <blockquote> <p>So as I understand it, the <code>===</code> operator tests to see if the RHS object is a member of the LHS object.</p> </blockquote> <p>Not necessarily. <code>===</code> is a method, just like any other method. It does whatever I want it to do. And in some cases the "is member of" analogy breaks down. In this case it is already pretty hard to swallow. If you are a hardcore type theory freak, then viewing a type as a set and instances of that type as members of a set is totally natural. And of course for <code>Array</code> and <code>Hash</code> the definition of "member" is also obvious.</p> <p>But what about <code>Regexp</code>? Again, if you are formal languages buff and know your Chomsky backwards, then interpreting a <code>Regexp</code> as an infinite set of words and <code>String</code>s as members of that set feels completely natural, but if not, then it sounds kind of weird.</p> <p>So far, I have failed to come up with a concise description of <em>precisely</em> what <code>===</code> means. In fact, I haven't even come up with a good name for it. It is usually called the <em>triple equals operator</em>, <em>threequals operator</em> or <em>case equality operator</em>, but I strongly dislike those names, because it has absolutely nothing to do with equality.</p> <p>So, what does it do? The best I have come up with is: imagine you are making a table, and one of the column headers is <code>Integer</code>. Would it make sense to write <code>3</code> in that column? If one of the column headers is <code>/ab*a/</code>, would it make sense to write <code>'abbbba'</code> in that column?</p> <p>Based on that definition, it could be called the <em>subsumption operator</em>, but that's even worse than the other examples ...</p>

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