Singleton fields in Ruby

singleton.rb

def assert condition
  raise unless condition
end

#
# @fields at the class level are singleton fields.
# Singleton fields may be initialized but this is not required.
# Singleton fields can only be accessed by singleton methods.
# Singleton fields are not inherited by subclasses (although singleton methods *are* inherited).
#

class Singleton
  @field = 1
  
  def self.field
    @field
  end

  def self.field=(f)
    @field = f
  end
end

class DerivedSingleton < Singleton
  # does not inherit @field
end

assert Singleton.field == 1
Singleton.field = 2
assert Singleton.field == 2

assert DerivedSingleton.field == nil
DerivedSingleton.field = 1
assert Singleton.field == 2