lecture 04

lectures/04/counter/lib/counter.ex

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+defmodule Counter do
+  @moduledoc """
+  Documentation for `Counter`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Counter.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end

lectures/04/counter/lib/counter/application.ex

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+defmodule Counter.Application do
+  # See https://hexdocs.pm/elixir/Application.html
+  # for more information on OTP Applications
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      # Starts a worker by calling: Counter.Worker.start_link(arg)
+      {Counter.Store, "counter.db"}, # we must start the store before the worker
+      {Counter.Worker, 0}
+    ]
+
+    # See https://hexdocs.pm/elixir/Supervisor.html
+    # for other strategies and supported options
+    opts = [strategy: :one_for_one, name: Counter.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end

lectures/04/counter/lib/counter/store.ex

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+defmodule Counter.Store do
+  use GenServer
+
+  # client
+  def start_link(file) do
+    GenServer.start_link(__MODULE__, file, name: __MODULE__)
+  end
+
+  def put(value) do
+    GenServer.cast(__MODULE__, {:put, value})
+  end
+
+  def get() do
+    GenServer.call(__MODULE__, :get)
+  end
+
+  # server
+  @impl true
+  def init(file) do
+    {:ok, file}
+  end
+
+  @impl true
+  def handle_call(:get, _from, file) do
+    reply =
+      case File.read(file) do
+        {:ok, content} -> :erlang.binary_to_term(content)
+        _ -> nil
+      end
+    {:reply, reply, file}
+  end
+
+  @impl true
+  def handle_cast({:put, value}, file) do
+    File.write(file, :erlang.term_to_binary(value))
+    {:noreply, file}
+  end
+end

lectures/04/counter/lib/counter/worker.ex

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+defmodule Counter.Worker do
+  use GenServer
+
+  @store  Counter.Store
+
+  # client
+  def start_link(n \\ 0) do
+    GenServer.start_link(__MODULE__, n, name: __MODULE__)
+  end
+
+  def inc(amt \\ 1) do
+    GenServer.cast(__MODULE__, {:inc, amt})
+  end
+
+  def value() do
+    GenServer.call(__MODULE__, :value)
+  end
+
+  # server
+  @impl true
+  def init(arg) do
+    {:ok, @store.get() || arg}
+  end
+
+  @impl true
+  def handle_call(:value, _from, state) do
+    {:reply, state, state}
+  end
+
+  @impl true
+  def handle_cast({:inc, amt}, state) do
+    {:noreply, state + amt}
+  end
+
+  # for demonstration only
+  @impl true
+  def handle_info(:reset, _state) do
+    {:noreply, 0}
+  end
+
+  @impl true
+  def terminate(_reason, state) do
+    @store.put(state)
+  end
+end