map_and_reduce.exs

defmodule MyEnum do
  def reduce([], accumulator, _function) do
    accumulator
  end

  def reduce([head | tail], accumulator, function) do
    reduce tail, function.(accumulator, head), function
  end

  def map(list, function) do
    reduce(list, [], fn(mapped_list, e) ->
      [function.(e) | mapped_list]
    end) |> Enum.reverse
  end
end

IO.puts MyEnum.reduce([1, 2, 3, 4], 0, &+/2)
IO.puts MyEnum.reduce([1, 2, 3, 4], 1, &*/2)

IO.inspect MyEnum.map([1, 2, 3, 4], &(&1 * &1))