defmodule ListMath do
  def sum_list([head | tail], accumulator) do
    sum_list(tail, head + accumulator)
  end

  def sum_list([], accumulator) do
    accumulator
  end
end

IO.puts ListMath.sum_list([4,5,6],0)
15

IO.puts ListMath.sum_list([4,5,6])
** (UndefinedFunctionError) function ListMath.sum_list/1 is undefined...

defmodule ListMath do
  def sum_list([head | tail], accumulator // 0) do
    sum_list(tail, head + accumulator)
  end

  def sum_list([], accumulator) do
    accumulator
  end
end