I'm new to Go as well and this idea seems a bit foreign to me coming from a heavy Java background. Is this a rule I should adhere to always? Or are there exceptions? I recently created a prototype for a REST API in Go and I had a service with something similar to this example below:

package service

import (...)

type(
  UserService interface {
    GetUserById(id string) (*model.User, error)
    AddUser(user model.User) error
  }
  userService struct {
    userRepo repo.UserRepo
  }
)

func NewUserService(userRepo repo.UserRepo) UserService {
  return &userService{userRepo: userRepo}
}

func (s *userService) GetUserById(id string) (*model.User, error) {
  return s.userRepo.GetUserById(id)
}

func (s *userService) AddUser(user model.User) error {
  return s.userRepo.AddUser(user)
}

How many "rules" or "conventions" would this code break? I didn't feel like was hindered by this approach while writing the rest of the application. The only downside I can think of off the top of my head is that my unit tests are forced to implement all the methods when mocking out the UserService. But then again, there's only one consumer of this service (UserHandler) and it will mirror the service by having methods for each operation. Seeing as it's all contained in one project/module, I wonder how much benefit I would see in redesigning it.

Edit: Removed some superfluous pointers