Try module patching helpers

README.md

@@ -1,4 +1,4 @@
 Welcome to the home for [my personal livebooks](https://github.com/christhekeele/livebooks/tree/latest/livebooks): Elixir experiments, guides, and accompanying source code.
 
 Their history can be accessed [on GitHub](https://github.com/christhekeele/livebooks) by the clicking the "view source"
-`</>` icon next to the title of every page. Corrections, proposals, and discussions can be made in the GitHub [issues](https://github.com/christhekeele/livebooks/issues), [pull requests](https://github.com/christhekeele/livebooks/pulls), and [discussions](https://github.com/christhekeele/livebooks/discussions) pages respectively.
+`</>` icon next to the title of every page. Corrections, proposals, and discussions can be made in the GitHub [pull requests](https://github.com/christhekeele/livebooks/pulls), [issues](https://github.com/christhekeele/livebooks/issues), and [discussions](https://github.com/christhekeele/livebooks/discussions) pages respectively.

lib/livebook.ex

@@ -0,0 +1,9 @@
+defmodule Livebook do
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    [Livebook.Module]
+    |> Supervisor.start_link(strategy: :one_for_one, name: Livebook.Supervisor)
+  end
+end

lib/livebook/helpers.ex

@@ -0,0 +1,18 @@
+defmodule Livebook.Helpers do
+  @moduledoc """
+  Common helpers for these livebooks.
+
+  > #### `use Livebook.Helpers` {: .info}
+  >
+  > When you `use Livebook.Helpers`, the following will be
+  > imported into your livebook:
+  >
+  > - `Livebook.Module.defmodule/3`: an iterable version of `defmodule/2`
+  """
+
+  defmacro __using__(_opts \\ []) do
+    quote do
+      import Livebook.Module, only: [defmodule: 3]
+    end
+  end
+end

lib/livebook/module.ex

@@ -0,0 +1,91 @@
+defmodule Livebook.Module do
+  require Matcha.Table.ETS
+
+  @table_name __MODULE__
+
+  @doc false
+  def child_spec(options) do
+    %{
+      id: __MODULE__,
+      start: {__MODULE__, :start_link, options}
+    }
+  end
+
+  @doc false
+  def start_link(_options \\ []) do
+    if :ets.whereis(@table_name) == :undefined do
+      :ets.new(@table_name, [:ordered_set, :named_table, :public])
+      # We don't actually need to start/supervise any process
+      :ignore
+    else
+      {:error, {:already_started, self()}}
+    end
+  end
+
+  @doc """
+  Defines a module that can be iterated upon in a livebook.
+
+  When defining a module, you can specify a `v: integer` version for it.
+  You can then later redefine it in your livebook with an incremented version number
+  without error.
+
+  Under the covers, the code provided to define each module is stored,
+  and retreived and prepended to the code in later versions.
+  This means that functions defined in previous versions can be patched without warning by
+  using `defoverridable/1` just before re-defining them, and invoked in the new definition
+  using `super/1`. Note that because `alias/2`, `require/2`, and `import/2` are lexically scoped,
+  they must be repeated every module definition. Module attributes (`@/1`), however, work as expected.
+
+  Each version of the module is compiled and available as `"\#{module_name}.__v\#{version}"`,
+  then `alias/2`'d to `module_name` so later code works as expected.
+
+  Inspired by [this macro](https://github.com/brettbeatty/experiments_elixir/blob/master/module_patching.livemd)
+  and [this forum post](https://elixirforum.com/t/how-do-i-redifine-modules-in-livebook/56442/2).
+  """
+  defmacro defmodule(
+             _module_name = {:__aliases__, _, [alias | []]},
+             _version_specifier = [v: version],
+             _code = [do: block]
+           )
+           when version >= 0 do
+    module = Macro.expand({:__aliases__, [alias: false], [:"Elixir", alias]}, __CALLER__)
+    version = Macro.expand(version, __CALLER__)
+    version_namespace = "__v#{version}" |> String.to_atom()
+    module_namespace = module |> Module.concat(version_namespace)
+    add_version(module, version, block)
+
+    code =
+      history(module, version)
+      |> Macro.prewalk(fn
+        aliases = {:__aliases__, meta, [^alias | rest]} ->
+          # IO.inspect(aliases, label: :FOUND)
+          {:__aliases__, meta, [:"Elixir", alias | rest]}
+
+        code ->
+          code
+      end)
+
+    :ok
+
+    quote do
+      defmodule unquote(module_namespace), do: unquote(code)
+      alias unquote(module_namespace), as: unquote(module)
+    end
+  end
+
+  @doc false
+  def add_version(module, version, block) do
+    :ets.insert(@table_name, {{module, version}, block})
+  end
+
+  @doc false
+  def history(module, since_version) do
+    Matcha.Table.ETS.select @table_name do
+      {{^module, version}, block} when version <= since_version -> block
+    end
+    |> Enum.flat_map(&unblock/1)
+  end
+
+  defp unblock({:__block__, _meta, block}), do: block
+  defp unblock(ast), do: [ast]
+end