How do I redifine a module that already exists in a codeblock

[dantheobserver]

When I have a code block that is a module, I can only define it once, it throws an erro that the module already exists when I attempt to redefine it. Is there a way to easily have a module in a block and be redefined without issue.

[jonatanklosko]

Hey @dantheobserver! We recently changed it such that a module can be defined in exactly one place, but you should be able to reevaluate. Do you get the error when evaluating the same cell for the second time? If so, can you provide a way to reproduce it? :)

[dantheobserver]

SO I did some more digging to reproduce it and this is what I found

1. Define a module with a function that would throw a runtime error (in this case wrong concat arguments
2. in another block, call that module, and error would be thrown
3. Fix module function and re-evaluate
4. Receive the following error
   ** (CompileError) projects/elixir/livebook/Learn_OTP.livemd#cell:cnobzqahs3c47n3vvblngzyateat5nsz:1: module AsyncQueue is already defined

To resolve I open the process panel and reconnect.

[jonatanklosko]

What is your Livebook and Elixir version?

[dantheobserver]

v0.8.0, elixir v1.14.2

Also I tried to do a simple example but it allowed me to re-evaluate, in my case it's a genserver class that it is happening with

defmodule AsyncQueue do
  use GenServer

  def init(state), do: {:ok, state}

  def handle_call(:dequeue, _from, [value | state]) do
    {:reply, value, state}
  end

  def handle_call(:dequeue, _from, []), do: {:reply, nil, []}

  def handle_call(:queue, _from, state), do: {:reply, state, state}

  def handle_cast({:enqueue, val}, state), do: {:noreply, val ++ state}

  @doc """
    Client API
  """
  def start_link(state \\ []) do
    GenServer.start_link(__MODULE__, state, name: __MODULE__)
  end

  def dequeue(), do: GenServer.call(__MODULE__, :dequeue)
  def queue(), do: GenServer.call(__MODULE__, :queue)
  def enqueue(val), do: GenServer.cast(__MODULE__, {:enqueue, val})
end

The handle_cast fails if you don't pass an array, and after it fails, if I try to fix and re-evaluate it it throws the error

[jonatanklosko]

Ah that makes sense, I created #1576.

FWIW in general you should be starting processes under a supervisor, in Livebook you'd install :kino and do Kino.start_child(AsyncQueue). But we should improve that case regardless.

[jonatanklosko]

Sidenote: you can restart the runtime with 00 shortcut :)

[dantheobserver]

Thanks for the help, I'll definitely keep those in mind. Haven't gotten to the supervisor section in my material yet so I'll look that up