gistfile1.rb

module MyMod
  
  # store the variable on the module itself
  @var = "initial value"  
  class << self
    attr_accessor :var
  end
  
  # first method that accesses the var
  def get_var
    MyMod.var
  end
  
  # second method that accesses the var
  def set_var value
    MyMod.var = value
  end
  
end


class MyClass
  include MyMod
end

# show that we can use the methods
mc = MyClass.new
mc.get_var                    # => "initial value"
mc.set_var "altered value"
mc.get_var                    # => "altered value"

# so where is the variable stored?
mc.instance_variables         # => []
MyClass.instance_variables    # => []
MyMod.instance_variables      # => [:@var]