# Do I have to write @impl true before each function? **URL:** \ **Category:** Questions\ **Tags:** phoenix\ **Created:** [November 22, 2021, 7:26pm UTC](https://forum.elixirforum.com/t/do-i-have-to-write-impl-true-before-each-function/44036 "2021-11-22T19:26:17Z")\ **Posts on this page:** 1\ **Showing post:** 5
**Author:** ![tomkonidas](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/tomkonidas/32/22982_2.png) [@tomkonidas](https://forum.elixirforum.com/u/tomkonidas)\ **Post date:** [November 22, 2021, 8:11pm UTC](https://forum.elixirforum.com/t/do-i-have-to-write-impl-true-before-each-function/44036/5 "2021-11-22T20:11:16Z")
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. --- _[View the full topic](https://forum.elixirforum.com/t/do-i-have-to-write-impl-true-before-each-function/44036)._