Elixir: if key exists: update, otherwise: assoc

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

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@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