Hi everyone,

I’m struggling to understand behaviours and dynamic dispatch. The example in the getting started with elixir page doesn’t make any sense to me:

defmodule Parser do
  @callback parse(String.t) :: {:ok, term} | {:error, String.t}
  @callback extensions() :: [String.t]

  def parse!(implementation, contents) do
    case implementation.parse(contents) do
      {:ok, data} -> data
      {:error, error} -> raise ArgumentError, "parsing error: #{error}"
    end
  end
end

Where does the implementation come from?

I tried using this myself when writing a module and I am doing something wrong, just not clear what. I don’t understand how to use the implementation’s functions when writing the handle_call method:

defmodule Program do
  use GenServer

  @callback inputs() :: [atom]
  @callback init(term) :: any
  @callback handle_data(any, map, map) :: term
  @callback emit(term) :: map

  @impl GenServer
  def init(arg) do
    ## How do I use the implementation's inputs function? there's no implementation passed in here
    __MODULE__.inputs
    |> Enum.each(fn x -> Phoenix.PubSub.subscribe :inputs, x end)

    {:ok, {%{}, __MODULE__.init(arg)}}
  end

  @impl true
  def handle_call(%OSC.Message{address: address, arguments: arguments}, _, {inputs, state}) do
    data = get_latest_reading(address, arguments)
    newinputs = %{inputs | address => data}
    # Same question in this area
    if map_size(newinputs) === length __MODULE__.inputs do
      newstate = __MODULE__.handle_data(state, inputs, newinputs)
      __MODULE__.emit(newstate)
      |> Enum.map(fn {k, v} -> Phoenix.PubSub.broadcast(:outputs, k, v) end)
      {:noreply, {newinputs, newstate}}
    else
      {:noreply, {newinputs, state}}
    end
  end

  def get_latest_reading(address, arguments) do
    receive do
      %OSC.Message{address: ^address, arguments: args} ->
        get_latest_reading(address, args)
    after 0 -> arguments
    end
  end
end