=== vs. == in Ruby

Question

In Ruby, what is the difference between == and ===? The RDoc says

Case Equality—For class Object, effectively the same as calling #==, but typically overridden by descendents to provide meaningful semantics in case statements.

Is #== the same as ==? And could you provide an example of when/how this is used in case statements?

Answer 1

The two really have nothing to do with each other. In particular, #== is the equality operator and #=== has absolutely nothing to with equality. Personally, I find it rather unfortunate that #=== looks so similar to #==, uses the equals sign and is often called the case equality operator, triple equals operator or threequals operator when it really has nothing to do with equality.

I call #=== the case subsumption operator (it's the best I could come up with, I'm open to suggestions, especially from native English speakers).

The best way to describe a === b is "if I have a drawer labeled a, does it make sense to put b in it?"

So, for example, Module#=== tests whether b.is_a?(a). If you have Integer === 2, does it make sense to put 2 in a box labeled Integer? Yes, it does. What about Integer === 'hello'? Obviously not.

Another example is Regexp#===. It tests for a match. Does it make sense to put 'hello' in a box labeled /el+/? Yes, it does.

For collections such as ranges, Range#=== is defined as a membership test: it makes sense to put an element in a box labeled with a collection if that element is in the collection.

So, that's what #=== does: it tests whether the argument can be subsumed under the receiver.

What does that have to with case expressions? Simple:

case foo when bar   baz end 

is the same as

if bar === foo   baz end