Supervised Stack

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.8.0", override: true},
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
])

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Supervised Stack

Previously we created a Stack GenServer process.

We’ve made a slight modification to the Stack process by adding a start_link/1 function so this named Stack can be started under a Supervisor.

defmodule Stack do
  use GenServer

  def start_link(opts) do
    GenServer.start_link(__MODULE__, [], name: __MODULE__)
  end

  @impl true
  def init(state) do
    {:ok, state}
  end

  @impl true
  def handle_call(:pop, _from, state) do
    [head | tail] = state
    {:reply, head, tail}
  end

  @impl true
  def handle_call({:push, element}, _from, state) do
    new_state = [element | state]
    {:reply, new_state, new_state}
  end
end

We’re able to push and pop elements off of the stack.

{:ok, stack_pid} = GenServer.start_link(Stack, [])

GenServer.call(stack_pid, {:push, 1}) |> IO.inspect(label: "push")
GenServer.call(stack_pid, {:push, 2}) |> IO.inspect(label: "push")
GenServer.call(stack_pid, {:push, 3}) |> IO.inspect(label: "push")

GenServer.call(stack_pid, :pop) |> IO.inspect(label: "pop")
GenServer.call(stack_pid, :pop) |> IO.inspect(label: "pop")
GenServer.call(stack_pid, :pop) |> IO.inspect(label: "pop")

# the final state is empty.
:sys.get_state(stack_pid)

However, there’s a bug. If we try to pop/1 an item off of an empty stack, the process will crash due to a function clause error because the handle_call/2 function expects a list with one or more elements.

Uncomment the following code to watch the Stack crash.

# {:ok, pid} = GenServer.start_link(Stack, [])
# GenServer.call(stack_pid, :pop)

Normally, we could handle this by using control flow. For example, we could make another handle_call/3 function clause for when the stack is empty.

# return `nil` when the stack is empty.
@impl true
def handle_call(:pop, _from, []) do
  {:reply, nil, []}
end

Instead, you’re going to start the Stack process under a supervisor so that it will be restarted when it crashes. In the Elixir cell below, start the Stack process under a supervisor so that it will restart with an empty stack when it crashes.

Example Solution

children = [
  {Stack, []}
]

{:ok, supervisor_pid} = Supervisor.start_link(children, strategy: :one_for_one)

Keep in mind, if you have already started a supervisor with the Stack process, your livebook may crash. You can resolve this issue by simply re-running the cell below to start the supervisor again.

You should be able to send a :pop message to the Stack process and the Supervisor will restart the Stack process.

Uncomment and evaluate the code below to test your supervisor.

# GenServer.call(Stack, :pop)

Mark As Completed

file_name = Path.basename(Regex.replace(~r/#.+/, __ENV__.file, ""), ".livemd")

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "supervised_stack_reading"
    "exercises" -> "supervised_stack_exercise"
  end

progress_path = __DIR__ <> "/../progress.json"
existing_progress = File.read!(progress_path) |> Jason.decode!()

default = Map.get(existing_progress, save_name, false)

form =
  Kino.Control.form(
    [
      completed: input = Kino.Input.checkbox("Mark As Completed", default: default)
    ],
    report_changes: true
  )

Task.async(fn ->
  for %{data: %{completed: completed}} <- Kino.Control.stream(form) do
    File.write!(
      progress_path,
      Jason.encode!(Map.put(existing_progress, save_name, completed), pretty: true)
    )
  end
end)

form

Commit Your Progress

Run the following in your command line from the curriculum folder to track and save your progress in a Git commit. Ensure that you do not already have undesired or unrelated changes by running git status or by checking the source control tab in Visual Studio Code.

$ git checkout -b supervised-stack-exercise
$ git add .
$ git commit -m "finish supervised stack exercise"
$ git push origin supervised-stack-exercise

Create a pull request from your supervised-stack-exercise branch to your solutions branch. Please do not create a pull request to the DockYard Academy repository as this will spam our PR tracker.

DockYard Academy Students Only:

Notify your teacher by including @BrooklinJazz in your PR description to get feedback. You (or your teacher) may merge your PR into your solutions branch after review.

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