iterate.exs

defmodule Foo do
  def iterate([]) do
    :done
  end

  def iterate([head | tail]) do
    {head, fn -> iterate(tail) end}
  end

  def reduce(coll, acc, fun) do
    case iterate(coll) do
      {val, next} -> _reduce(next, fun.(val, acc), fun)
      :done -> acc
    end
  end

  def _reduce(next, acc, fun) do
    case next.() do
      {val, next} -> _reduce(next, fun.(val, acc), fun)
      :done -> acc
    end
  end
end

IO.inspect Foo.iterate([3, 4])
IO.inspect Foo.reduce([3, 4], 0, &(&1 + &2))