modules.exs

defmodule Math do
  def sum(a, b) do
    a + b
  end

  # Private function
  defp do_sum(a, b) do
    a + b
  end

  def zero?(0) do
    true
  end

  def zero?(x) when is_integer(x) do
    false
  end
end

IO.puts Math.sum(5, 1)

IO.puts Math.zero?(0)
IO.puts Math.zero?(9)