# === is not a symmetric operator? **URL:** \ **Category:** Ruby\ **Created:** [August 23, 2010, 3:52pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727 "2010-08-23T15:52:40Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 3:52pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/1 "2010-08-23T15:52:40Z")
Is my understanding correct that === is not a symmetric operator? That, in fact, X === x and x === X mean different things? * * * irb(main):001:0\> class X irb(main):002:1\> end =\> nil irb(main):003:0\> x = X.new =\> #\ irb(main):004:0\> x === X =\> false # this is surprising irb(main):005:0\> X === x =\> true # this is the test I want … but was surprised when # x === X didn’t work. irb(main):006:0\> * * * Does x === X ever mean anything useful? ---
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 4:20pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/2 "2010-08-23T16:20:46Z")
2010/8/23 Ralph S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > Is my understanding correct that === is not a symmetric operator? That, in fact, > X === x > and > x === X > > mean different things? Exactly. > ``` > # x === X didn't work. > > ``` > > irb(main):006:0\> > > * * * > > Does x === X ever mean anything useful? That depends on what objects x and X refer to. Remember that X is only a constant - and not a class or module automatically. Cheers robert ---
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 4:21pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/3 "2010-08-23T16:21:56Z")
Hi, In message “Re: === is not a symmetric operator?” on Mon, 23 Aug 2010 22:52:11 +0900, Ralph S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: |Is my understanding correct that === is not a symmetric operator? That, in fact, | X === x |and | x === X | |mean different things? No. ``` matz. ``` ---
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 5:08pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/4 "2010-08-23T17:08:47Z")
Matz, Monday, August 23, 2010, 8:17:30 AM, you wrote: YM\> Hi, YM\> In message “Re: === is not a symmetric operator?” YM\> on Mon, 23 Aug 2010 22:52:11 +0900, Ralph S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: YM\> |Is my understanding correct that === is not a symmetric operator? That, in fact, YM\> | X === x YM\> |and YM\> | x === X YM\> | YM\> |mean different things? YM\> No. No!!! … They do mean the same thing??? Or … “No, they do not mean the same thing.”? ---
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 5:26pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/5 "2010-08-23T17:26:57Z")
2010/8/23 Ralph S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > YM\> | X === x > YM\> |and > YM\> | x === X > YM\> | > YM\> |mean different things? > > YM\> No. > > No!!! … They do mean the same thing??? > > Or … “No, they do not mean the same thing.”? Well, in a way they mean the same thing, i.e. method :=== is invoked on the left instance and the right instance is passed as argument. How :=== is implemented completely depends on the class (or even instance) at hand. Conventionally there is an implementation for Class and Module which tests for “instance of” relation. And for regular expressions :=== implements regexp matching. Etc. Cheers robert ---
**Author:** ![ralphshnelvar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/r/e19adc/32.png) [@ralphshnelvar](https://www.ruby-forum.com/u/ralphshnelvar)\ **Post date:** [August 23, 2010, 5:30pm UTC](https://www.ruby-forum.com/t/is-not-a-symmetric-operator/193727/6 "2010-08-23T17:30:28Z")
On Mon, Aug 23, 2010 at 8:52 AM, Ralph S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > irb(main):006:0\> > > * * * > > Does x === X ever mean anything useful? Typically, you define X#=== to be whatever is meaningful for you. So the answer depends on how you define it. Typically, when considering how you want to define it, you should be thinking about how you want it used in a case statement. For example, classes typically are used in a case statement to check if an object is an instance of that class: X = Class.new x = X.new case x when String puts “Do stringy thing” when Regexp puts “Do regexy thing” when X puts “Do Xy thing” else puts “Don’t know what to do” end Is the same as X = Class.new x = X.new if String === x puts “Do stringy thing” elsif Regexp === x puts “Do regexy thing” elsif X === x puts “Do Xy thing” else puts “Don’t know what to do” end In your example, note that the reason X === x in this case works, is because X inherits its === method from Module where “Case Equality—Returns true if anObject is an instance of mod or one of mod‘s descendents. Of limited use for modules, but can be used in case statements to classify objects by class.” [class Module - RDoc Documentation](http://ruby-doc.org/core/classes/Module.html#M001666) If you want to think of it as an operator, think of it like the minus sign, where 2-1 is not equal to 1-2, but probably the best thing would be to not think of it as an operator, but rather as a method exclusively intended for the convenience of case statements. Notice that the docs even preceed the description with “Case Equality”.