Can a GenServer Have Its Own Struct in Elixir?

Question

Scenario:

• I have a simple GenServer for managing some state.
• Currently, I am using map to manage my state. But it is growing as I am adding more data to the state.

Problem:

• So, to have some compile-time guarantee, can I have a struct in my GenServer module?
• And, if yes, is it a right approach?
• If not, what are the alternatives?

Answer 1

Just declare a normal struct (optionally in a module nested in your GenServer namespace) and use it as an initial state:

defmodule Test do
  defmodule State do
    defstruct ~w|foo bar baz|a
  end

  use GenServer

  def start_link(opts \\ []) do
    GenServer.start_link(__MODULE__, %State{foo: 42, bar: opts}, name: __MODULE__)
  end

  @impl true
  def init(opts \\ []), do: {:ok, opts}

  def state, do: GenServer.call(__MODULE__, :state)

  @impl true
  def handle_call(:state, _from, %State{} = state) do
    {:reply, state, state}
  end
end

with {:ok, _} <- Test.start_link(pi: 3.14) do
  IO.inspect Test.state, label: "State"
end
#⇒ State: %Test.State{bar: [pi: 3.14], baz: nil, foo: 42}