user.exs

defmodule User do
  defstruct firstname: "Ryan", lastname: "Levick"

  def full_name(user) do
    user.firstname <> " " <> user.lastname
  end
end

user = %User{}
User.full_name(user) =>

** (UndefinedFunctionError) undefined function: User.__struct__/0
    User.__struct__()
    (elixir) src/elixir_map.erl:55: :elixir_map.translate_struct/4
    (elixir) src/elixir_translator.erl:17: :elixir_translator.translate/2
    (elixir) src/elixir_translator.erl:351: :elixir_translator.translate_block/4
    (elixir) src/elixir_translator.erl:39: :elixir_translator.translate/2
    (elixir) src/elixir.erl:151: :elixir.quoted_to_erl/3
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/2