Comment by zero_shift

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

Instead of

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

You could have

   type saves func([]byte) error

Seems less bulky than an interface, more concise to mock too.

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