how_singleton_module_works.rb

#
# A hand-rolled singleton behaviour 
#
class SnowFlake
  class << self
    private :new
  end
  
  def self.instance
    @instance ||= new
  end
end

p SnowFlake.instance    # => #<SnowFlake:0x1003adfd8>
p SnowFlake.instance    # => #<SnowFlake:0x1003adfd8>

p SnowFlake.new
# => NoMethodError: private method ‘new’ called for SnowFlake:Class


#
# Singleton module from Ruby's standard lib
#

require 'singleton'

class SnowFlake
  include Singleton
end

p SnowFlake.instance   # => #<SnowFlake:0x1003adff8>
p SnowFlake.instance   # => #<SnowFlake:0x1003adff8>

p SnowFlake.new
# => NoMethodError: private method ‘new’ called for SnowFlake:Class