Discoveries in Ruby(and Rails): Case equality operator

Ruby provides mechanisms for controlling the control-flow of a program. They consist of the if-statement families, if, elsif, else, and unless. Along with case/when.

“The basic idea of the case/when structure is that you take an object and cascade through a series of tests for a match, taking action based on the test that succeeds.”. Black, D. A., & Leo, J. (2019). The well-grounded Rubyist. Manning.

A typical case statement looks like this

tool = "google"

case tool
when "google"
   puts "tool is google"
when "basecamp"
   puts "tool is basecamp"
else
  puts "unknown tool"
end

Every ruby object has a case equality method, called ===, triple equal sign, sometimes called as case subsumption or threequal operator

instead of doing when tool.=== "google". The when "google" keyword provides a syntactical sugar over the previous approach.

The threeequal operator is defined everywhere on every built-in class. You can also implement in classes that you define.

Let's define a Car class.

class Car
  attr_accessor :brand, :type
  def initialize(brand, type)
    self.brand = brand
    self.type = type
  end

  def ===(other_car)                               
    self.brand == other_car.brand
  end
end

Now, we can use them along with case/when.

ford = Car.new "Ford", "F-150"
jeep = Car.new "Jeep", "Wrangler"
tesla_model_x = Car.new "Tesla", "Model-X"
tesla_model_y = Car.new "Tesla", "Model-Y"

case tesla_model_x
when ford
    puts "It's a ford"
when jeep
    puts "It's a Jeep"
when tesla_model_y
    puts "Both are Tesla made"
else
    puts "unknown manufacturer"
end

running this through a ruby session, we get the following

Both are Tesla made
=> nil

Underneath, the ruby interpreter makes a call to the === method on the object, which we defined inside the Car class

  def ===(other_car)                               
    self.brand == other_car.brand
  end

With tripleequal sign operator or case subsumption operator you can allow your classes to work elegently with Ruby's case/when control-flow operators.

Thanks for reading, and Happy Coding.

Resources

• The Well-Grounded Rubyist