Redefining Elixir modules

I want to talk about one of the most AWESOME things that Elixir features. It is the ability to redefine module on the fly from iex shell.

For TLDR folks - watch the asciicast.

So, what does that mean?

Imagine that we have a system running simple GenServer that prints pre-compiled @time variable every one second.
Unrelated btw System.monotonic_time/0 is the way to track durations in Elixir, see System for more info.

Something like this:

defmodule ModuleRedefining.Worker do
  @moduledoc false

  use GenServer

  @time System.monotonic_time(:second)

  def start_link(args) do
    GenServer.start_link(__MODULE__, args, name: __MODULE__)
  end

  def init(_) do
    send_after()
    {:ok, %{}}
  end

  def handle_info(:print, state) do
    IO.inspect(@time)
    send_after()
    {:noreply, state}
  end

  defp send_after do
    Process.send_after(self(), :print, 1_000)
  end
end

So we launch our app, fire up our GenServer (in supervision or manually) and execute to our iex shell.
We should see the output something like:

-576460752
-576460752
-576460752
-576460752
-576460752
(...)

Then we copy (in clipboard) our module and paste it in the running iex shell.
Elixir will return warning that we are indeed redefining the module, and its new code representation (btw. if interested in Elixir's AST, check this article out).

warning: redefining module ModuleRedefining.Worker (current version loaded from _build/dev/lib/module_redefining/ebin/Elixir.ModuleRedefining.Worker.beam)
{:module, ModuleRedefining.Worker,
 <<70, 79, 82, 49, 0, 0, 15, 124, 66, 69, 65, 77, 65, 116, 85, 56, 0, 0, 1, 188, 0, 0, 0, 43, 30, 69, 108, 105, 120, 105, 114 46, 77, 111, 100, 117, 108, 101, 82, 101, 100, 101, 102, 105, 110, 105, 110, ...>>, {:send_after, 0}}

Then we would see changed output to something like:

-576460745
-576460745
-576460745
-576460745
(...)

So what just happened?

When we pasted the module code, elixir recompiled it and redefined the existing module. By compiling the code, @time compile-time constant has changed so our output changed as well.

With great power comes great responsibility, so use this cautiously. I find this very useful when debugging some weird problems on our staging environment, so I can quickly tryout new code variations and patches. For development environment, i just change the source and recompile in iex shell.