Flatteing

list.ex

defmodule List do
  def flatten([]),            do: []
  def flatten([head | tail]), do: flatten(head) ++ flatten(tail)
  def flatten(head),          do: [head] 
end

list_test.exs

defmodule ListTest do
  use ExUnit.Case, async: true

  describe "flatten" do
    test "should flatten an empty list" do
      assert [] = List.flatten([])
    end
    
    test "should flatten an one-element list" do
      list = [1]
      
      assert ^list = List.flatten(list)
    end
    
    test "should wrap a single element" do
      element = 1
      
      assert [^element] = List.flatten(element)
    end
    
    test "should flatten a list" do
      list = [[1, 2, [3]], 4]

      assert [1, 2, 3, 4] == List.flatten(list)
    end
  end
end