Ruby Triple Equal

What does the === operator do in Ruby?

Just like with every other method in Ruby (or actually pretty much any object-oriented language),

a === b

means whatever the author of a's class wants it to mean.

However, if you don't want to confuse the heck out of your colleagues, the convention is that === is the case subsumption operator. Basically, it's a boolean operator which asks the question "If I have a drawer labelled a would it make sense to put b in that drawer?"

An alternative formulation is "If a described a set, would b be a member of that set?"

For example:

 (1..5) === 3           # => true
 (1..5) === 6           # => false

Integer === 42          # => true
Integer === 'fourtytwo' # => false

  /ell/ === 'Hello'     # => true
  /ell/ === 'Foobar'    # => false

The main usage for the === operator is in case expressions, since

case foo
when bar
  baz
when quux
  flurb
else
  blarf
end

gets translated to something (roughly) like

_temp = foo

if bar === _temp
  baz
elsif quux === _temp
  flurb
else
  blarf
end

Note that if you want to search for this operator, it is usually called the triple equals operator or threequals operator or case equality operator. I really dislike those names, because this operator has absolutely nothing whatsoever to do with equality.

In particular, one would expect equality to be symmetric: if a is equal to b, then b better be also equal to a. Also, one would expect equality to be transitive: if a == b and b == c, then a == c. While there is no way to actually guarantee that in a single-dispatch language like Ruby, you should at least make an effort to preserve this property (for example, by following the coerce protocol).

However, for === there is no expectation of either symmetry or transitivity. In fact, it is very much by design not symmetric. That's why I don't like calling it anything that even remotely resembles equality. It's also why I think, it should have been called something else like ~~~ or whatever.

ruby triple equal

We can't tell whether you truly get === or not from such a limited example. But here's a break down of what's really happening when you use ===, either explicitly or implicitly as part of a case/when statement such as the one used in the example..

The triple equal(===) has many different implementations that depend on the class of the left part. It's really just an infix notation for the .=== method. Meaning that the following statements are identical:

a.=== b
a === b

The difference doesn't look like much, but what it means is that the left hand side's === method is being invoked instead of some magical operator defined on the language level, that's like == but not quite. Instead === is defined in each class that uses it, maybe in an inherited class or Mixin.

The general definition of the triple equals is that it will return true if both parts are identical or if the right part is contained within the range of the left.

In the case of Class.===, the operation will return true if the argument is an instance of the class (or subclass). In the case where the left side is a regular expression, it returns true when the right side matches the regular expression.

The when of case is an implied === which compares the case variable to the when clause using === so that the following two statements produce the same result.

case a
when String
  puts "This is a String"
when (1..3)
  puts "A number between 1 and 3"
else
  puts "Unknown"
end

if String === a 
  puts "This is a String"
elsif (1..3) === a
  puts "A number between 1 and 3"
else
  puts "Unknown"
end

Check the documentation for the types you use on the left hand of a === or in a when statement to be sure exactly how things work out.

=== vs. == in Ruby

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

Why doesn't Array override the triple equal sign method in Ruby?

Because it's in the specification.

it "treats a literal array as its own when argument, rather than a list of arguments"

The spec was added February 3, 2009 by Charles Nutter (@headius). Since this answer's probably not what you're looking for, why don't you ask him?

To take a wild and completely uninformed stab in the dark, it seems to me that you might have hit upon the answer by using a splat in your question. Since the functionality is available by design, why duplicate when doing so would remove the ability to test Array equality? As Jordan points out above, situations exist where this is useful.

---

Future readers should note that unless the array in question is already instantiated, using an array at all is not necessary to match on multiple expressions:

x = 2

case x
  when 1, 2, 3 then "match"
  else "no match"
end # => "match"

3 Equals or Case Equality operator

The simple answer is: because it doesn't make sense. The relationship the operator describes is not commutative, why should the operator be?

Just look at your own examples: 5 is an Integer. But is Integer a 5? What does that even mean?

=== is the case subsumption operator, and subsumption doesn't commute.

The fact that the case subsumption operator uses equals signs, and that it is usually called the triple equals, threequals or case equality operator is terribly unfortunate since it not only has absolutely nothing to do with equality, but it also does not conform to many of the laws that equality conforms to, such as transitivity and as you mentioned commutativity.

For more of my ranting about === see

• What does the === operator do in Ruby?
• === vs. == in Ruby
• How does Integer === 3 work?

Triple equals versions of Enumerable functions

From the comment of mine -

Looking at your need, I can suggest you one way to meet the need, which you are looking for:

[MyClass, MyOtherClass] & some_object.class.ancestors

Although it is still an wrokaround using Array Intersection.

What's the difference between equal?, eql?, ===, and ==?

I'm going to heavily quote the Object documentation here, because I think it has some great explanations. I encourage you to read it, and also the documentation for these methods as they're overridden in other classes, like String.

Side note: if you want to try these out for yourself on different objects, use something like this:

class Object
  def all_equals(o)
    ops = [:==, :===, :eql?, :equal?]
    Hash[ops.map(&:to_s).zip(ops.map {|s| send(s, o) })]
  end
end

"a".all_equals "a" # => {"=="=>true, "==="=>true, "eql?"=>true, "equal?"=>false}

---

== — generic "equality"

At the Object level, == returns true only if obj and other are the same object. Typically, this method is overridden in descendant classes to provide class-specific meaning.

This is the most common comparison, and thus the most fundamental place where you (as the author of a class) get to decide if two objects are "equal" or not.

=== — case equality

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

This is incredibly useful. Examples of things which have interesting === implementations:

• Range
• Regex
• Proc (in Ruby 1.9)

So you can do things like:

case some_object
when /a regex/
  # The regex matches
when 2..4
  # some_object is in the range 2..4
when lambda {|x| some_crazy_custom_predicate }
  # the lambda returned true
end

See my answer here for a neat example of how case+Regex can make code a lot cleaner. And of course, by providing your own === implementation, you can get custom case semantics.

eql? — Hash equality

The eql? method returns true if obj and other refer to the same hash key. This is used by Hash to test members for equality. For objects of class Object, eql? is synonymous with ==. Subclasses normally continue this tradition by aliasing eql? to their overridden == method, but there are exceptions. Numeric types, for example, perform type conversion across ==, but not across eql?, so:

1 == 1.0     #=> true
1.eql? 1.0   #=> false

So you're free to override this for your own uses, or you can override == and use alias :eql? :== so the two methods behave the same way.

equal? — identity comparison

Unlike ==, the equal? method should never be overridden by subclasses: it is used to determine object identity (that is, a.equal?(b) iff a is the same object as b).

This is effectively pointer comparison.

Does the triple equal sign (===) behave differently in AssemblyScript?

Yes. In AssemblyScript tripple equal ("===") compare raw references and skip overloading operator ("=="). See docs.

There are have proposal avoid this non-standard for TypeScript behaviour. You could check and upvote this issue

triple equal sign R

You can define an infix operator (with the e in %e% for "equal";-):

`%e%` <- function(a, b) identical(a, b)
3 %e% 3
#[1] TRUE

Or if you want the triple-equal sign as

`%===%` <- function(a, b) identical(a, b)
3 %===% 3
#[1] TRUE

Or an example with vectors

1:3 %===% 1:3
#[1] TRUE

These infix operations (where the operator is used between the operands) can also be written as

`%===%`(1:3, 1:3) 

in the same way that you can write

`==`(3, 3)

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