supervised_stack.livemd

Supervised Stack

Mix.install([
  {:youtube, github: "brooklinjazz/youtube"},
  {:hidden_cell, github: "brooklinjazz/hidden_cell"},
  {:tested_cell, github: "brooklinjazz/tested_cell"},
  {:utils, path: "#{__DIR__}/../utils"}
])

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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, name: server_name)
  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 beta_curriculum folder to track and save your progress in a Git commit.

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

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