Rather than defining all these one-method interfaces, why not specify a function type?

Instead of

    type Saver interface {
        Save(data []byte) error
    }

   type saves func([]byte) error

It's more effort when you need to "promote" the port / input type to a full interface, but I think that's a reasonable tradeoff to avoid callers of your function constantly creating structs just to hang methods off

With Go had something similar to Java's `@FunctionalInterface` annotation, where a functions signature (parameters and return type) is implicitly matched against the single abstract method of a functional interface and where existing matching methods of any object can also be used as implementations of functional interfaces.