# Agent
An `Agent` is a specialized `GenServer` that provides a set of helper functions which make it easy to store and update state in-memory using a process. Agents come in handy if you want a simple GenServer to hold onto state.
```elixir
defmodule RunElixir.GameState do
use Agent
# Public
@initial_state %{
rounds_played: 0,
current_points: 0,
total_points: 0
}
# Starts a new Agent process with an initial state.
def start_link(_args) do
Agent.start_link(fn -> @initial_state end)
end
# Creates a fire-and-forget message to the Agent.
#
# Unlike in a GenServer, we don't have to write callbacks
# for our events like `handle_cast/2` or `handle_call/3`
# but can define the callback as an anonymous function.
#
# The anonymous function receives the current state
# and has to return the new state.
def add_points(pid, points) do
Agent.cast(pid, fn state ->
Map.update!(state, :current_points, &(&1 + points))
end)
end
# Returns the current state.
def get_state(pid), do: Agent.get(pid, fn state -> state end)
# Ends the round by updating and returning the state.
#
# `Agent.get_and_update/2` receives the current state in the
# anonymous function and has to return a tuple with the state
# that should be returned to the calling process and
# the new process state.
def end_round(pid) do
Agent.get_and_update(pid, fn state ->
%{
rounds_played: rounds_played,
total_points: total_points,
current_points: current_points
} = state
new_state = %{
rounds_played: rounds_played + 1,
total_points: total_points + current_points,
current_points: 0
}
{new_state, new_state}
end)
end
end
{:ok, pid} = RunElixir.GameState.start_link([])
RunElixir.GameState.get_state(pid) |> IO.inspect(label: 1)
RunElixir.GameState.add_points(pid, 100) |> IO.inspect(label: 2)
RunElixir.GameState.get_state(pid) |> IO.inspect(label: 3)
RunElixir.GameState.end_round(pid) |> IO.inspect(label: 4)
```
<!-- livebook:{"output":true} -->
```
1: %{current_points: 0, total_points: 0, rounds_played: 0}
2: :ok
3: %{current_points: 100, total_points: 0, rounds_played: 0}
4: %{current_points: 0, total_points: 100, rounds_played: 1}
```
