Do I have to write @impl true before each function?

That is because he is implementing a Behaviour.

No, you do not need it for every function. But if you put on one function in the module,
then it will complain to put on the others.

I have no idea what you are talking about but thank you anyway.

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

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.

It is good practice.
Even better, to use the name of the behaviour module that you are implementing, instead of just true. This is necessary when a module implement more than one behaviour.

It will definitely improve the readability and maintainability of your code.

Read more about @impl and behaviours in:

• Module — Elixir v1.20.2
• Behaviour — Elixir v1.20.2
• https://elixir-lang.org/getting-started/typespecs-and-behaviours.html#behaviours

You need to understand the concept of behaviours first in order to understand what @impl is for.

his module is deprecated. Use @callback and @macrocallback attributes instead.

So, behaviour is deprecated?

The functionality of behaviours is still existing, though the old api is deperectated in favor of the newer one.

Can you give a real world example about this behavior? Have you ever used it? How? And what problem did it solve? I can’t wrap my head around it. I have never needed something like that.