Case Statements and Strict Equality

- 2 mins

I was playing around with an implementation of categories in Immedialist that involved a type-checking case statement in a module. I know, I know, probably not the greatest idea; but that’s not the point of this post.

First Pass

In order to see if I could get my Movie#category spec to pass with this new implementation, I wrote the case statement like this:

def category
  case self.class
  when Movie
    :film
  end
end

The method checks the class of the media type instance that is calling it (Movie, Book, Actor, etc.) and returns the correct category symbol. Pretty straightforward, right? This should totally pass.

But it doesn’t!

Failing Movie Spect

What I discovered upon debugging is that Ruby case statements use === to determine equality, and self.class === Movie returns false even when self is a Movie instance. Thus, the case statement returned nil.

Classes and the === Operator

I had never really looked too closely at === until now; I had just assumed that, like in JavaScript, it was a stricter version of ==, and I stayed away from it. But it turns out that Ruby’s use of === is different from JavaScript’s in that it’s more flexible and can be provided meaningful semantics depending on the class employing it.

This is what Dave Thomas has to say in Programming Ruby 1.9 on === in Class:

Ruby classes are instances of class Class. The === operator is defined in Class to test whether the argument is an instance of the receiver or one of its superclasses. So (abandoning the benefits of polymorphism and bringing the gods of refactoring down around your ears), you can test the class of objects. (131)

The Solution

This helpful quote, along with recalling single-line case statement syntax, led to this refactoring:

def category
  case self
  when Movie then :film
  end
end

This time, I’m taking advantage of the implementation of Class#===, which tests whether self is an instance of Movie. Cleaner code, and it passes:

Passing Movie Spec

I’m still deciding whether I’m going to ignore programming best practice and move forward with the case statement or try a different approach, but at least I learned more about Ruby’s case statement and === in the meantime!

Alex Wilkinson

Alex Wilkinson

software engineer, writer, board game designer

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