Add Elixir examples for recursion

03_recursion/elixir/01_countdown.exs

@@ -0,0 +1,12 @@
+defmodule Countdown do
+  defguardp non_positive?(x) when x <= 0
+
+  def from(i) when non_positive?(i), do: nil
+
+  def from(i) do
+    IO.puts(i)
+    from(i - 1)
+  end
+end
+
+Countdown.from(5)

03_recursion/elixir/02_greet.exs

@@ -0,0 +1,18 @@
+defmodule Greeting do
+  def greet2(name) do
+    IO.puts("how are you, #{name}?")
+  end
+
+  def bye do
+    IO.puts("ok bye!")
+  end
+
+  def greet(name) do
+    IO.puts("hello, #{name}!")
+    greet2(name)
+    IO.puts("getting ready to say bye...")
+    bye()
+  end
+end
+
+Greeting.greet("adit")

03_recursion/elixir/03_factorial.exs

@@ -0,0 +1,6 @@
+defmodule Factorial do
+  def of(1), do: 1
+  def of(n), do: n * of(n - 1)
+end
+
+IO.puts(Factorial.of(5))