In the previous article, we've set-up everything required for a basic event-sourced model.

We can:

• record the decisions taken by the game logic in the form of events
• rebuild the current game state from a list of past events
• handle a command that triggers game logic

So far, every command we've talked about was accepted by the game logic, processed, and events were produced. But sometimes, things are not going as expected, and the command is rejected.

One example, we don't accept several players with the same name. When a player tries to register, but another player is already there with the name selected, we need to refuse the command and obviously inform the player.

Rebuilding the state and our memory

First, as a hrefresher, let's see how the state is built.

Let's pretend that a player named Jack wants to register.

An AddPlayer command is issued by the UI:

history = //... list of past events
command = %AddPlayer{player_name: "Jack"}

action_result = GameState.handle_message(history, command)

The command is dispatched to the correct handler, the game logic produces a PlayerJoinedTeam event we would find in the action_result variable in the snippet above:

def handle(state, %AddPlayer{player_name: name}) do
  [
    PlayerJoinedTeam.with(player: name)
  ]
end

This event joins the others in history, the list of past events.

When we want to add a second player, we need to rebuild the state.

As seen in the previous article, GameState module contains an apply_event function with a clause matching against %PlayerJoinedTeam{} :

def apply_event(state, %PlayerJoinedTeam{} = event) do
    %{state | players: [event.data.player | state.players]}
  end

When a PlayerJoinedTeam event is applied, the player is inserted into the list of players.

Hi! I'm Jack!

Jack #1 is already in, some other player wants to join as well, and is named Jack too, Jack #2.

A business rule disallows two players to share the same name, so we'll need to reject Jack #2 attempt to register as Jack, and we'll ask him to pick another name.

The traditional way of dealing with errors in Elixir code is the OK/Error tuple.

When something is going OK, the function returns a tuple in the form

{:ok, result}

and when something failed

{:error, the_error}

This is the path I decided to go with, but I made a slight twist and introduced an ActionResult module.

defmodule DoctorP.Game.ActionResult do

  alias __MODULE__

  defstruct [
    events: [],
    error: nil
  ]

  def new(), do:
    %ActionResult{}

  def new(events), do:
    %ActionResult{events: events}

  def error(error), do:
    %ActionResult{error: error}

  def add(%ActionResult{error: error}, %ActionResult{}) when not is_nil(error), do:
    %ActionResult{error: error}

  def add(%ActionResult{}, %ActionResult{error: other_error}) when not is_nil(other_error), do:
    %ActionResult{error: other_error}

  def add(%ActionResult{events: events}, %ActionResult{events: other_events}), do:
    %ActionResult{events: events ++ other_events}

end

This module is a glorified OK\Error tuple but has functions that will come in handy later. They allow us to combine multiple ActionResult together, concatenating events list when everything is right, returning an error when something went wrong.

Armed with this module, let's see how we can implement the business rule.

First, we add a private has_player_named function that returns true when we already know a player with the name given in arguments and false otherwise.

defp has_player_named(state, player_name) do
  state.players
  |> Enum.any?(fn x -> x == player_name)
end

We can now change the handle function to

def handle(%{} = state, %AddPlayer{player_name: player_name}) do
  cond do
    has_player_named(state, player_name) -> ActionResult.error(:player_name_not_available)
    true -> ActionResult.new([PlayerJoinedTeam.with(player: player_name)])
  end
end

When a player tries to register with a name already taken by someone else, the function produces an ActionResult with an error :player_name_not_available. That same ActionResult is then returned by the GameState.handle_message function called with the command at the beginning of the process.

That's it!

And that's a lot... We'll keep for a following article how to deal with the ActionResult once it's returned from GameState.handle_message.