Supervised Stack

Mix.install([
  {:kino, github: "livebook-dev/kino", override: true},
  {:kino_lab, "~> 0.1.0-dev", github: "jonatanklosko/kino_lab"},
  {:vega_lite, "~> 0.1.4"},
  {:kino_vega_lite, "~> 0.1.1"},
  {:benchee, "~> 0.1"},
  {:ecto, "~> 3.7"},
  {:math, "~> 0.7.0"},
  {:faker, "~> 0.17.0"},
  {:utils, path: "#{__DIR__}/../utils"},
  {:tested_cell, git: "https://github.com/BrooklinJazz/tested_cell"}
])

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Supervised Stack

We’ve created a Stack process.

defmodule Stack do
  use GenServer

  def start_link(state, opts) do
    server_name = Keyword.get(opts, :name, __MODULE__)
    GenServer.start_link(__MODULE__, state)
  end

  def init(stack) do
    {:ok, stack}
  end

  def push(pid, item) do
    GenServer.cast(pid, {:push, item})
  end

  def pop(pid) do
    GenServer.call(pid, :pop)
  end

  def handle_call(:pop, _from, [head | tail]) do
    {:reply, head, tail}
  end

  def handle_cast({:push, item}, stack) do
    {:noreply, [item | stack]}
  end
end

The Stack process can store a list and push/2/pop/1 elements onto and off of the stack.

{:ok, pid} = Stack.start_link([], name: :example)
Stack.push(pid, 1)
Stack.push(pid, 2)
2 = Stack.pop(pid)
1 = Stack.pop(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.

Here we use GenServer.start/2 to avoid crashing the livebook.

{:ok, pid} = GenServer.start(Stack, [])
Stack.pop(pid)

In the Elixir cell below, start the Stack process under a supervisor so that it will restart with an empty stack when it crashes.

Commit Your Progress

Run the following in your command line from the project folder to track and save your progress in a Git commit.

$ git add .
$ git commit -m "finish supervised stack exercise"