Interfaces are a special type that define a collection of function signatures. You can think of an interface as saying, "a type must implement function X and function Y to satisfy this interface." If you create any type and implement the functions needed to satisfy the interface, your type can be used anywhere that the interface is expected. You don't have to specify that you are trying to satisfy an interface, the compiler will determine if it satisfies the requirements.
You can add as many other functions as you want to your custom type. The interface defines the functions that are required, but it does not mean that your type is limited to implementing only those functions.
The most commonly used interface is the error interface. ...
