Stack Server

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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Stack

A stack is a data structure which can push elements onto the top of the stack and pop them off of the top of the stack. We’re going to create a Stack process which stores a stack as a list of elements in it’s state. We should be able to send a {:push, element} message and a :pop message to add and remove elements from the beginning of the list.

The {:push, element} message should add an element onto the stack, and return the current state.

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

GenServer.call(stack_pid, {:push, 1})
[1]

GenServer.call(stack_pid, {:push, 2})
[2, 1]

The :pop message should remove an element from the stack and return the removed element.

2 = GenServer.call(stack_pid, :pop)

[1] = :sys.get_state(stack_pid)

1 = GenServer.call(stack_pid, :pop)

[] = :sys.get_state(stack_pid)

Example Solution

defmodule Stack do
  use GenServer

  def start_link(_opts) do
    GenServer.start_link(__MODULE__, [])
  end

  def push(stack_pid, element) do
    GenServer.call(stack_pid, {:push, element})
  end

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

  def init(_opts) do
    {:ok, []}
  end

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

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

Implement the Stack module below. You do not need to handle :pop-ing an element from an empty stack.

Consider starting with the handle_call/3 callback functions, then implement the client API functions Stack.push/2 and Stack.pop/1.

defmodule Stack do
  use GenServer

  @doc """
  Start the `Stack` process.

  ## Examples

      iex> {:ok, pid} = Stack.start_link([])
  """
  def start_link(_opts) do
  end

  @doc """
  Synchronously push an element onto the top of the `Stack`. Return the current stack.

  ## Examples

      iex> {:ok, pid} = Stack.start_link([])
      iex> Stack.push(pid, 1)
      [1]
      iex> Stack.push(pid, 2)
      [2, 1]
      iex> Stack.push(pid, 3)
      [3, 2, 1]
  """
  def push(stack_pid, element) do
  end

  @doc """
  Pop an element from the top of the `Stack`.

  ## Examples

      iex> {:ok, pid} = Stack.start_link([])
      iex> Stack.push(pid, 1)
      iex> Stack.push(pid, 2)
      iex> Stack.push(pid, 3)
      iex> Stack.pop(pid)
      3
      iex> Stack.pop(pid)
      2
      iex> Stack.pop(pid)
      1
  """
  def pop(stack_pid) do
  end

  @doc """
  Necessary callback function to start the `Stack` process.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Stack, []) 
  """
  def init(_opts) do
  end

  @doc """
  Callback function to add an element onto the top of the `Stack`.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Stack, [])
      iex> GenServer.call(pid, {:push, 1})
      [1]
      iex> GenServer.call(pid, {:push, 2})
      [2, 1]
      iex> GenServer.call(pid, {:push, 3})
      [3, 2, 1]
  """
  def handle_call({:push, element}, _from, state) do
  end

  @doc """
  Callback function to pop an element off of the top of the `Stack`.
  You do not need to handle popping when a stack is empty.

  ## Examples

      iex> {:ok, pid} = GenServer.start_link(Stack, [])
      iex> GenServer.call(pid, {:push, 1})
      iex> GenServer.call(pid, {:push, 2})
      iex> GenServer.call(pid, {:push, 3})
      iex> GenServer.call(pid, :pop)
      3
      iex> GenServer.call(pid, :pop)
      2
      iex> GenServer.call(pid, :pop)
      1
  """
  def handle_call(:pop, _from, state) do
  end
end

Mark As Completed

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

save_name =
  case Path.basename(__DIR__) do
    "reading" -> "stack_server_reading"
    "exercises" -> "stack_server_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 stack-server-exercise
$ git add .
$ git commit -m "finish stack server exercise"
$ git push origin stack-server-exercise

Create a pull request from your stack-server-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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