Sum of elements on a list. "The process of taking a list and reducing it down to one value is known as a reduce algorithm and is central to functional programming."

list_sum.exs

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

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

IO.puts Math.sum_list([1, 2, 3], 0) #=> 6