leibniz.ex

defmodule Leibniz do
  # Finished - counter has reached the requested number of iterations
  def pi(iterations, iterations, _numerator, result), do: result

  def pi(iterations, counter, numerator, result) do
    pi(iterations, counter + 1, -numerator, result + (numerator / (3 + 2 * counter)))
  end

  # Entry point
  def pi(iterations), do: 4 * pi(iterations, 0, -1.0, 1.0)
end

Leibniz.pi(1000000)
# 3.141592663589326