I’ve seen this question pop up more than a few times in #ruby-lang. For sooth, I hit this wall once myself: why is my case statement broken?

Here’s two seemingly equivalent pieces of code:

the_what = :sealab

if the_what.class == Symbol
    puts "The what is a symbol."
elsif the_what.class == String
    puts "The what is a string."
else
  puts "I don't know what the what is."
end

and

the_what = :sealab

case the_what.class
when Symbol
  puts "The what is a symbol."
when String
  puts "The what is a string."
else
  puts "I don't know what the what is."
end

Running the if statement gives The what is a symbol. Running the case statement gives I don’t know what the what is. So, is case broken?

No. We just made an faulty assumption. Listen: case != if.

We all know if conditions work by checking for the truthiness of a statement. Is it true or is it false? Simple.

case works differently. Rather than checking for true or false using ==, case uses === to test expressions. How is this different?

Unlike, say, PHP, Ruby’s === can be overridden by classes to provide “meaningful semantics in case statements.” So says the Ruby core documentation. In fact, the docs call === “Case Equality.”

Back to our examples and trusty irb:

>> the_what = :sealab
=> :sealab
>> the_what.class == Symbol
=> true
>> the_what.class === Symbol
=> false

We’re on the right track. Checking the === docs for Module, we see === returns true if the passed object is an instance of the receiver. Since the_what.class gives us Symbol and a Symbol is not an instance of Symbol, we get false. In other words:

>> the_what = :sealab
=> :sealab
>> Symbol === Symbol
=> false
>> Symbol === the_what
=> true

Since the_what is a Symbol, we get true. This explains why our previous case statement is falling through to the final else. Let’s rewrite it with our newfound Ruby powers:

the_what = :sealab

case the_what
when Symbol
  puts "The what is a symbol."
when String
  puts "The what is a string."
else
  puts "I don't know what the what is."
end

Running this new code gives us The what is a symbol. That feels much better.

For further case magic, check out _why’s Wonder of the When-Be-Splat.