Examples of a module, class and singleton class in Ruby

multiple_instances_of_state.rb

class MultipleInstancesOfState
  attr_accessor :state

  def initialize(state:)
    @state = state
  end

  def foo
    @state.reverse!
  end
end

an_instance = MultipleInstancesOfState.new(state: 'wombat')
an_instance.foo
#=> "tabmow"

an_instance.foo
#=> "wombat"

another_instance = MultipleInstancesOfState.new(state: 'corge')
another_instance.foo
#=> "egroc"

no_state.rb

module NoState
  module_function

  def foo(stateless:)
    stateless.reverse
  end
end

NoState.foo(stateless: 'wombat')
#=> "tabmow"

NoState.foo(stateless: 'tabmow')
#=> "wombat"

single_instance_of_state.rb

require 'singleton'

class SingleInstanceOfState
  include Singleton

  attr_accessor :state

  def initialize
    @state = nil
  end

  def foo
    @state.reverse!
  end
end

singleton_instance = SingleInstanceOfState.instance
singleton_instance.state = 'wombat'
singleton_instance.foo
#=> "tabmow"
singleton_instance.foo
#=> "wombat"