I have a feeling you are brand new to this language (apologies if I’m wrong) and perhaps these answers are overcomplicating things (no disrespect to anyone trying to help here!)

The short answer is that you can choose to annotate your callback implementations (in the case of LiveView: render/1, mount/3, handle_event/3, etc) with @impl true. Without worrying about anything else, the absolute simplest thing this accomplishes is letting anyone reading the file know that these are callbacks. While it is optional, I highly recommend it.

@tomkonidas provided a very good explanation of behaviours (which you are now well aware is what @impl true is tied to) but my one-line definition is: “a set of functions a module must implement”. As a starting point, you can think of behaviours like interfaces from OO even though it’s not strictly true.

Also, when defining multiple versions of the same function, you only need to annotate the first one:

@impl true
def handle_event("update_post", params, socket) do
  {:noreply, socket}
end

# not required here!
def handle_event("delete_post", params, socket) do
  {:noreply, socket}
end

So you can define a behaviour and implement it. For example, lets use payment provider.

If we know we will have multiple payment providers (ex: Stripe, Paypal, etc…), we might want a behaviour for payment providers. We can do this with a behaviour.
We can define the behaviour and say that every module that implements the MyApp.PaymentProvider behaviour needs to implement a credit/2 and debit/2 for example.

Now in our own Stripe module we can @behaviour MyApp.PaymentProvider and that is saying that we will need to implement the 2 callbacks we defined:

• debit/2
• credit/2

Now we have a way to make sure we are able to switch between Paypal and Stripe because they follow the same contract. Both Paypal.credit/2 and Stripe.credit/2 should take the same args and return the same type/structure.

With Phoenix, they have optional callbacks, You can define your own or use the default. That is why you do not need to implement Phoenix.LiveComponent.update/2, but you can impl it if you want.

In your module, if you put one @impl over a function, I believe the compiler will complain. Because you iplemented it for another callback but not that specific one, so it thinks you made a possible mistake.

This feature informs the terminal which functions are returned from a call like in GenServer. You don’t need to include them at all, just the first one.

To learn more, view the link below, I recommend that you first understand what GenServers are.

https://elixircasts.io/%40impl-attribute