Implementing Enumerable protocol for the Person record not doesn't seem to work. I'm running this setup which recompiles *.ex files to be made available in iex. http://cromwellhaus.com/2013/07/my-learning-elixir-erlang-setup/

Person.ex

defrecord Person, children: []

defimpl Enumerable, for: Person do
  def count(person) do
    Enum.count(person.children)
  end 

  def member?(person, value) do
    person.children.member(value)
  end

  def reduce(person, acc, fun) do
    person.children.reduce(acc, fun)
  end
end

results

** (ArgumentError) argument error
    :erlang.apply(["a","b"], :count, [])
    /Users/cromwellryan/projects/elixir_001/math.ex:7: Enumerable.Person.count/1
    erl_eval.erl:569: :erl_eval.do_apply/6
    src/elixir.erl:152: :elixir.eval_forms/3
    /private/tmp/elixir-KfjI/elixir-0.9.3/lib/iex/lib/iex/server.ex:81: IEx.Server.eval/4
    /private/tmp/elixir-KfjI/elixir-0.9.3/lib/iex/lib/iex/server.ex:37: IEx.Server.do_loop/1

test.exs

me = Person.new( children: ["Ryan Jr", "Ben", "Lana", "Liam"] )

# Should return 4... I think
Enum.count(me) 

Turns out you need to reload modules explicitly in iex. elixirc will compile the defimpl for a type into many unique .beam files. You don't load those explicitly. To get the compiled updates into iex run

r(ModuleName)

where ModuleName is 'Person'. So to reload Person record and it's protocol implementations

r(Person)