Using case

msort.ex

defmodule MSort do
  def msort(list) do
    case list do
      [] -> []
      [e] -> [e]
      _ ->
        {left, right} = Enum.split(list, div(Enum.count(list), 2))
        merge(msort(left), msort(right))
    end
  end

  def merge(left, right) do
    case {left, right} do
      {left, []} -> left
      {[], right} -> right
      _ ->
        [left_head | left_rest] = left
        [right_head | right_rest] = right
        if left_head < right_head do
          [left_head | merge(left_rest, right)]
        else
          [right_head | merge(right_rest, left)]
        end
    end
  end
end

IO.puts(inspect MSort.msort([3,5,2,7]))