Elixir: if key exists: update, otherwise: assoc

Remember Clojure: if key exists: update, otherwise: assoc? Let's do that with Elixir.

  @doc """
  If `key` exists in `map`, updates its value (assumed to be a list) by concatenating `value`.
  If `key` does not exist in `map`, creates `{key => [value]}` in `map`.
  ## Examples

    iex(35)> Tap.Worker.update_conj!(%{a: [1]}, :a, 2)
    %{a: [1, 2]}

    iex(36)> Tap.Worker.update_conj!(%{a: [1]}, :b, 2)
    %{a: [1], b: [2]}

  """
  @spec update_conj!(map, any, any) :: map
  def update_conj!(map, key, value) when is_map(map) do
    Map.update(map, key, [value], fn
      existing_values when is_list(existing_values) ->
        existing_values ++ [value]

      existing_value ->
        raise(ArgumentError, "Value is not a list: #{inspect(existing_value)}")
    end)
  end