ADT

queue_adt.erl

-module(queue_adt).
-export([empty_queue/0,
	 enqueue/2,
	 dequeue/1,
	 current/1]).

empty_queue() ->
  {[], []}.

enqueue({[Value], []}, Element) ->
  {[Element], [Value]};
enqueue({In, Out}, Element) ->
  {[Element | In], Out}.

dequeue({[], []}) ->
  {error, queue_empty};
dequeue({[_Head], []}) ->
  {[], []};
dequeue({In, [_Head | [Next | Rest]]}) ->
  {In, [Next | Rest]};
dequeue({In, [_Head | _Rest]}) ->
  {[], lists:reverse(In)}.

current({[], []}) ->
  {error, queue_empty};
current({[Current], []}) ->
  Current;
current({_In, [Current | _Rest]}) ->
  Current.

queue_adt.rb

module QueueADT
  def self.empty_queue()
    {in: [], out: []}
  end

  def self.enqueue(queue, element)
    {in: queue[:in] + [element], out: queue[:out]}
  end

  def self.dequeue(queue)
    out = queue[:out]
    if out.empty?
      out = queue[:in].reverse
    end
    [out.pop, {in: [], out: out}]
  end
end

signature.erl

-module(queue_adt).
-export([empty_queue/0,
	 enqueue/2,
	 dequeue/1,
	 current/1]).