The "complex interfaces" problem (don't know how to properly name it). Was trying something similar in a telegram bot app using the "local interfaces" approach.

app_handler.go

package app

type Handler interface {
  Handle(handler func(msg Message))
}

type Message interface {
  GetText() string
  SendResponse(text string) error
}

impl_bot.go

package bot

// separate package
// bot implements the Handler, or tries to, for that matter :)
type Bot struct {}

// tried local message, that is identical to the app.Message
type messageImpl struct {}
func (m messageImpl) GetText() string {}
func (m messageImpl) SendResponse(text string) error {}
// can have more methods that are not used in app.Message
func (m messageImpl) SelfDelete() error {}

// var _ Message = (app.Message)(nil)
// interfaces are equal and can be cast to each other
type Message interface {
  GetText() string
  SendResponse(text string) error
}

// there is not way to implement handler without importing app.Message
// doesn't work, Bot doesn't implement app.Handler
// even though Message is identical to app.Message :(
func (b Bot) Handle(handler func(msg Message)) {
  
}

// has to be
func (b Bot) Handle(handler func(msg app.Message)) {}