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Why would the if statement work, but the case statement not work?

class A end # => nil a= A.new # => #<A:0x00007fd9d6134770>

If statement works

a.class == A # => true

Case statement does not work

case a.class # => A when A puts "done" else puts "else" # => nil end # => nil # >> else

I read that case statement use === operators instead of ==, how do I use == for the case statement?

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  • Try the if statement with case equality a.class === A. Commented Sep 25, 2019 at 2:59
  • Trying to do the reverse, a case statement with == Commented Sep 25, 2019 at 3:00
  • You can't use ==, it is always ===. Commented Sep 25, 2019 at 9:55
  • 1
    Possible duplicate of Ruby Case class check Commented Sep 25, 2019 at 9:56

1 Answer 1

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The form case specific when archetype... will always use the archetype.===(specific) subsumption check. To use ==, you would need to be explicit, and use the other form: case when boolean:

case when a.class == A puts "done" else puts "else" end

However, in this specific case (checking if an object belongs in a class), if you don't care whether a is an instance of A or an instance of a subclass of A (which is almost always the case), you can still use the subsumption check, like this:

case a # Note: not a.class when A puts "done" else puts "else" end

because A === a when A is a class is pretty much the same as a.is_a?(A). Note that this is different from a.class == A, which will not be true if a is an instance of a subclass of A. For example for class Dog < Animal, and fido = Dog.new, both fido.instance_of?(Animal) and fido.class == Animal are false, but both fido.is_a?(Animal) and Animal === fido are true.)

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