sub.rb

require 'alias_dci' 

class Player
  include AliasDCI::DataObject
  def foo(str)
    str + "b"
  end
  
  def to_string
    "a"
  end
end

class Context
  include AliasDCI::Context
  
  role :a do
    contract :to_string
    def foo
      b.frob(a.to_string)
    end
  end
  
  role :b do
    contract :foo
    def frob(str)
      r_b.foo(str)  # force role b's object's instance method #foo to be called.
    end
  end
  
  def initialize(a_player, another_player)
    assign_named_roles(:a => a_player, :b => another_player)
  end
  
  def call
    in_context do
      a.foo
    end
  end
end
player = Player.new

puts Context.new(player, player).call


# >> ab