TIL: Private Ruby Constants

Sometimes I want to have a constant in my modules or classes that I don't want to be part of my API – I want a private constant.

The Module class contains the method private_constant which we can use to do just that. And since Module is the ancestor class of Class, it's available for classes too.

Let assume I have a module that looks like this:

module MyModule
  MY_CONSTANT = 'billy_ruffian'.freeze
    
  private_constant :MY_CONSTANT
    
  def self.my_constant
    MY_CONSTANT
  end
end

The constant, MY_CONSTANT, is hidden by the private_constant statement. I try and access it from outside the module, I get:

MyModule::MY_CONSTANT
private constant MyModule::MY_CONSTANT referenced (NameError)

But the constant remains accessible within the module:

MyModule.my_constant
 => "billy_ruffian"