Purpose of @callback transition_changeset is not clear

[vovayartsev]

Hey 👋

Thanks for a nice & clean library.
One question (pardon if a silly one) is about this callback definitions:

defmodule Fsmx.Fsm do
  ...
  if Code.ensure_loaded?(Ecto) do
    @callback transition_changeset(struct, Fsmx.state_t, Fsmx.state_t) :: Ecto.Changeset.t()
    @callback after_transition_multi(struct, Fsmx.state_t, Fsmx.state_t) ::
                {:ok, struct} | {:error, any}
  end
  ...

I thought the idea was to use @impl when overriding transition_changeset, like here:

defmodule App.StateMachineSchema do
  # ...

  @impl Fsmx.Fsm
  def transition_changeset(changeset, "one", "two", params) do
    # changeset already includes a :state field change
    changeset
    |> cast(params, [:data])
    |> validate_required([:data])
  end

but transition_changeset above accepts 4 params, while callback defines only 3 params.
Also there's no @behaviour in defmacro __using__.

This is not a bug report - just curious what was the original intent around @callback 🙏

[zamith]

The purpose of this callback is indeed to override as you've done in your example. The callback definition was wrong and is fixed as part of other changes in #16 . But yes, by not having @behavior the callback definitions are merely informative and not being enforced.

If you'd like to open a pull request changing this, it could be a good first contribution, if you are interested.