Today I discovered Ruby doing something unexpected with the == operator. With a class like this:

class Value
  # ... initialize, other methods, etc. ...
  def ==(other)
    self.a_method == other.a_method
  end
end

and a test that effectively evaluated the expression:

  3 == Value.new(args)

I expected a message saying that Value couldn’t be coerced to Fixnum. Instead I got

undefined method `a_method' for 3:Fixnum (NoMethodError)

I thought that 3 == Value.new(args) was equivalent to 3.==(Value.new(args)), yet this message suggests Ruby is evaluating Value.new(args).==(3). It’s almost as if Ruby has decided that == is commutative. How can that happen? I was obviously missing something. Continue reading Commutative equality in Ruby? →