Guard clauses

account.ex

defmodule Account do
  def display_name(%{first: first, last: last}) when not is_nil(first) do
    String.trim("#{first} #{last}")
  end
  def display_name(%{username: username}) when not is_nil(username) do
   "#{username}"
  end
  def display_name(_), do: "New User"
end