From 7f5f47042a6542eb35559f13a1d30fcd859b837f Mon Sep 17 00:00:00 2001
From: Braeden Sowinski <braeden@sowinski.dev>
Date: Tue, 29 Sep 2026 11:55:46 -0700
Subject: [PATCH] lecture 04

---
 lectures/04/counter/.formatter.exs            |  4 ++
 lectures/04/counter/.gitignore                | 26 +++++++++++
 lectures/04/counter/README.md                 | 21 +++++++++
 lectures/04/counter/counter.db                |  1 +
 lectures/04/counter/lib/counter.ex            | 18 ++++++++
 .../04/counter/lib/counter/application.ex     | 21 +++++++++
 lectures/04/counter/lib/counter/store.ex      | 38 ++++++++++++++++
 lectures/04/counter/lib/counter/worker.ex     | 45 +++++++++++++++++++
 lectures/04/counter/mix.exs                   | 29 ++++++++++++
 lectures/04/counter/test/counter_test.exs     |  8 ++++
 lectures/04/counter/test/test_helper.exs      |  1 +
 lectures/04/counter2/.formatter.exs           |  4 ++
 lectures/04/counter2/.gitignore               | 26 +++++++++++
 lectures/04/counter2/README.md                | 21 +++++++++
 .../04/counter2/lib/counter/application.ex    | 21 +++++++++
 lectures/04/counter2/lib/counter/worker.ex    | 38 ++++++++++++++++
 lectures/04/counter2/mix.exs                  | 29 ++++++++++++
 lectures/04/counter2/test/counter_test.exs    |  8 ++++
 lectures/04/counter2/test/test_helper.exs     |  1 +
 lectures/04/lec4.ex                           | 36 +++++++++++++++
 lectures/04/notes.ex                          | 31 +++++++++++++
 note.md                                       |  1 +
 22 files changed, 428 insertions(+)
 create mode 100644 lectures/04/counter/.formatter.exs
 create mode 100644 lectures/04/counter/.gitignore
 create mode 100644 lectures/04/counter/README.md
 create mode 100644 lectures/04/counter/counter.db
 create mode 100644 lectures/04/counter/lib/counter.ex
 create mode 100644 lectures/04/counter/lib/counter/application.ex
 create mode 100644 lectures/04/counter/lib/counter/store.ex
 create mode 100644 lectures/04/counter/lib/counter/worker.ex
 create mode 100644 lectures/04/counter/mix.exs
 create mode 100644 lectures/04/counter/test/counter_test.exs
 create mode 100644 lectures/04/counter/test/test_helper.exs
 create mode 100644 lectures/04/counter2/.formatter.exs
 create mode 100644 lectures/04/counter2/.gitignore
 create mode 100644 lectures/04/counter2/README.md
 create mode 100644 lectures/04/counter2/lib/counter/application.ex
 create mode 100644 lectures/04/counter2/lib/counter/worker.ex
 create mode 100644 lectures/04/counter2/mix.exs
 create mode 100644 lectures/04/counter2/test/counter_test.exs
 create mode 100644 lectures/04/counter2/test/test_helper.exs
 create mode 100644 lectures/04/lec4.ex
 create mode 100644 lectures/04/notes.ex

diff --git a/lectures/04/counter/.formatter.exs b/lectures/04/counter/.formatter.exs
new file mode 100644
index 0000000..d2cda26
--- /dev/null
+++ b/lectures/04/counter/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/lectures/04/counter/.gitignore b/lectures/04/counter/.gitignore
new file mode 100644
index 0000000..ccae38f
--- /dev/null
+++ b/lectures/04/counter/.gitignore
@@ -0,0 +1,26 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+counter-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/
diff --git a/lectures/04/counter/README.md b/lectures/04/counter/README.md
new file mode 100644
index 0000000..f15a4ad
--- /dev/null
+++ b/lectures/04/counter/README.md
@@ -0,0 +1,21 @@
+# Counter
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `counter` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:counter, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/counter>.
+
diff --git a/lectures/04/counter/counter.db b/lectures/04/counter/counter.db
new file mode 100644
index 0000000..ddf901f
--- /dev/null
+++ b/lectures/04/counter/counter.db
@@ -0,0 +1 @@
+ƒa
\ No newline at end of file
diff --git a/lectures/04/counter/lib/counter.ex b/lectures/04/counter/lib/counter.ex
new file mode 100644
index 0000000..2e364fe
--- /dev/null
+++ b/lectures/04/counter/lib/counter.ex
@@ -0,0 +1,18 @@
+defmodule Counter do
+  @moduledoc """
+  Documentation for `Counter`.
+  """
+
+  @doc """
+  Hello world.
+
+  ## Examples
+
+      iex> Counter.hello()
+      :world
+
+  """
+  def hello do
+    :world
+  end
+end
diff --git a/lectures/04/counter/lib/counter/application.ex b/lectures/04/counter/lib/counter/application.ex
new file mode 100644
index 0000000..a32efdd
--- /dev/null
+++ b/lectures/04/counter/lib/counter/application.ex
@@ -0,0 +1,21 @@
+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
diff --git a/lectures/04/counter/lib/counter/store.ex b/lectures/04/counter/lib/counter/store.ex
new file mode 100644
index 0000000..ba43dea
--- /dev/null
+++ b/lectures/04/counter/lib/counter/store.ex
@@ -0,0 +1,38 @@
+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
diff --git a/lectures/04/counter/lib/counter/worker.ex b/lectures/04/counter/lib/counter/worker.ex
new file mode 100644
index 0000000..e3f2201
--- /dev/null
+++ b/lectures/04/counter/lib/counter/worker.ex
@@ -0,0 +1,45 @@
+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
diff --git a/lectures/04/counter/mix.exs b/lectures/04/counter/mix.exs
new file mode 100644
index 0000000..a48c727
--- /dev/null
+++ b/lectures/04/counter/mix.exs
@@ -0,0 +1,29 @@
+defmodule Counter.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :counter,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Counter.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end
diff --git a/lectures/04/counter/test/counter_test.exs b/lectures/04/counter/test/counter_test.exs
new file mode 100644
index 0000000..99884d6
--- /dev/null
+++ b/lectures/04/counter/test/counter_test.exs
@@ -0,0 +1,8 @@
+defmodule CounterTest do
+  use ExUnit.Case
+  doctest Counter
+
+  test "greets the world" do
+    assert Counter.hello() == :world
+  end
+end
diff --git a/lectures/04/counter/test/test_helper.exs b/lectures/04/counter/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/lectures/04/counter/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()
diff --git a/lectures/04/counter2/.formatter.exs b/lectures/04/counter2/.formatter.exs
new file mode 100644
index 0000000..d2cda26
--- /dev/null
+++ b/lectures/04/counter2/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/lectures/04/counter2/.gitignore b/lectures/04/counter2/.gitignore
new file mode 100644
index 0000000..ccae38f
--- /dev/null
+++ b/lectures/04/counter2/.gitignore
@@ -0,0 +1,26 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+counter-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/
diff --git a/lectures/04/counter2/README.md b/lectures/04/counter2/README.md
new file mode 100644
index 0000000..f15a4ad
--- /dev/null
+++ b/lectures/04/counter2/README.md
@@ -0,0 +1,21 @@
+# Counter
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `counter` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:counter, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/counter>.
+
diff --git a/lectures/04/counter2/lib/counter/application.ex b/lectures/04/counter2/lib/counter/application.ex
new file mode 100644
index 0000000..da1b98f
--- /dev/null
+++ b/lectures/04/counter2/lib/counter/application.ex
@@ -0,0 +1,21 @@
+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)
+      Supervisor.child_spec({Counter.Worker, :c1}, id: :counter1),
+      Supervisor.child_spec({Counter.Worker, :c2}, id: :counter2)
+    ]
+
+    # 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
diff --git a/lectures/04/counter2/lib/counter/worker.ex b/lectures/04/counter2/lib/counter/worker.ex
new file mode 100644
index 0000000..5bd0920
--- /dev/null
+++ b/lectures/04/counter2/lib/counter/worker.ex
@@ -0,0 +1,38 @@
+defmodule Counter.Worker do
+  use GenServer
+
+  # client
+  def start_link(name) do
+      GenServer.start_link(__MODULE__, nil, name: name)
+  end
+
+  def inc(name, amt \\ 1) do
+      GenServer.cast(name, {:inc, amt})
+  end
+
+  def value(name) do
+    GenServer.call(name, :value)
+  end
+
+  # server impl
+  @impl true
+  def init(_arg) do
+    {:ok, 0}
+  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
+end
diff --git a/lectures/04/counter2/mix.exs b/lectures/04/counter2/mix.exs
new file mode 100644
index 0000000..a48c727
--- /dev/null
+++ b/lectures/04/counter2/mix.exs
@@ -0,0 +1,29 @@
+defmodule Counter.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :counter,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Counter.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end
diff --git a/lectures/04/counter2/test/counter_test.exs b/lectures/04/counter2/test/counter_test.exs
new file mode 100644
index 0000000..99884d6
--- /dev/null
+++ b/lectures/04/counter2/test/counter_test.exs
@@ -0,0 +1,8 @@
+defmodule CounterTest do
+  use ExUnit.Case
+  doctest Counter
+
+  test "greets the world" do
+    assert Counter.hello() == :world
+  end
+end
diff --git a/lectures/04/counter2/test/test_helper.exs b/lectures/04/counter2/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/lectures/04/counter2/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()
diff --git a/lectures/04/lec4.ex b/lectures/04/lec4.ex
new file mode 100644
index 0000000..8fe5c2f
--- /dev/null
+++ b/lectures/04/lec4.ex
@@ -0,0 +1,36 @@
+defmodule Lec4 do
+  def create(n \\ 4) do
+    Enum.map(1..n, fn _ -> parent = self(); spawn(fn -> loop(parent) end) end)
+  end
+
+  def info(pid) when is_pid(pid) do
+    case Process.info(pid, [:trap_exit, :links]) do
+      nil -> {pid, :dead}
+      x -> {pid, x}
+    end
+  end
+
+  def info(pids) when is_list(pids) do
+    Enum.map(pids, &info/1)
+  end
+
+  def trap_exit(pid) do
+    send(pid, :trap_exit)
+  end
+
+  def link(pid, other) do
+    send(pid, {:link, other})
+  end
+
+  defp loop(parent) do
+    receive do
+      :trap_exit ->
+        Process.flag(:trap_exit, true)
+      {:link, other} ->
+        Process.link(other)
+      x ->
+        send(parent, x)
+    end
+    loop(parent)
+  end
+end
diff --git a/lectures/04/notes.ex b/lectures/04/notes.ex
new file mode 100644
index 0000000..25b2d4b
--- /dev/null
+++ b/lectures/04/notes.ex
@@ -0,0 +1,31 @@
+# raise("hell") # runtime error
+# exit(:die)    # typtically exit with an atom
+# throw(:what)
+# throw(1)      # can throw anything
+
+try do
+    1/0
+catch type, value ->
+    IO.inspect({type, value})
+end
+
+# 3 types of runime error
+try do
+    raise("hell")
+catch type, value ->
+    IO.inspect({type, value})
+end
+
+try do
+    throw :up
+catch type, value ->
+    IO.inspect({type, value})
+end
+
+try do
+    exit(1)
+catch type, value ->
+    IO.inspect({type, value})
+after
+    IO.puts("done")
+end
diff --git a/note.md b/note.md
index 7fb0cac..55f8488 100644
--- a/note.md
+++ b/note.md
@@ -10,3 +10,4 @@
 # projects
 `mix new <name>` creates a prject dir with name
 `iex -S mix`     runs mix script
+`mix new <name> --sup`