Fibonacci number calculator in Elixir.

fib.ex

defmodule Fib do

  # Naive implementation
  
  def fib(0), do: 0
  def fib(1), do: 1
  
  def fib(n)
    when is_integer(n) and n > 1,
    do: fib(n-1) + fib(n-2)
    
  def fib(_), do: {:error, "positive integers only"}
  
  # Tail recursion version inspired by: http://stackoverflow.com/a/14294705
  # Not only can it take advantage of TCO, it also memoizes.

  def fib_tr(0), do: 0
  
  def fib_tr(n)
    when is_number(n) and n > 0,
    do: fib_tr(n, 1, 0, 1)
    
  def fib_tr(_), do: {:error, "positive integers only"}
  
  def fib_tr(n, m, _prev_fib, current_fib)
    when n == m,
    do: current_fib
    
  def fib_tr(n, m, prev_fib, current_fib),
    do: fib_tr(n, m+1, current_fib, prev_fib + current_fib)
    
end

defmodule Fib do
    def fib(a, b, n) do
        case n do
            0 ->
                a
            _ ->
                fib(b, a+b, n-1)
        end
    end
end
# Console Output
IO.puts Fib.fib(1,1,7)

Cash me outside howbow dah? 😄