Ruby constants vs class methods

constants_vs_class_methods.rb

class Example
  NAME = "this is constant"
  SECRET_NAME = "this is a secret name"
  private_constant :SECRET_NAME

  def self.foo
    @foo ||= "this is foo"
  end

  def self.bar
    @@bar ||= "this is bar"
  end

  def self.baz
    @foo
  end

  def self.qux
    SECRET_NAME
  end

  def class_foo
    @@foo
  end

  def instance_foo
    @foo
  end

  def class_bar
    @@bar
  end

  def instance_bar
    @bar
  end
end

puts Example::NAME.inspect # => "this is constant"
# puts Example::SECRET_NAME.inspect # => private constant Example::SECRET_NAME referenced (NameError)
puts Example.foo.inspect # => "this is foo"
puts Example.bar.inspect # => "this is bar"
puts Example.baz.inspect # => "this is foo"
puts Example.qux.inspect # => "this is a secret name"
example = Example.new
# puts example.class_foo.inspect # => uninitialized class variable @@foo in Example (NameError)
puts example.instance_foo.inspect # => nil
puts example.class_bar.inspect # => "this is bar"
puts example.instance_bar.inspect # => nil


# To conclude, it is possible to define a class method like this:
class SenderEmailAddress
  DOMAIN_NAME = "yourcustomdomain.com"
  private_constant :DOMAIN_NAME

  def self.no_reply
    @email_no_reply ||= "Example <no-reply@#{DOMAIN_NAME}>"
  end
end

puts SenderEmailAddress.no_reply # => Example <no-reply@yourcustomdomain.com>