嵌套接口：X未实现Y（Z方法的类型错误）

Question

In one package I have an interface Repository that has a method GetReporter that returns an interface Reporter. This is used by a function Execute that takes a Repository and gets its Reporter via the GetReporter function.

In another package I have a struct GithubRepository that has a method GetReporter that returns a GithubReporter.

In a third package I want to call the Execute function out of package #1 with a GithubRepository instance.

I am trying to have package 1 and 2 independent of each other, without one importing something from the other. The 3rd package should combine the first two.

Golang returns:

cannot use githubRepository (type GithubRepository) as type Repository in argument to Execute:
    GithubRepository does not implement Repository (wrong type for GetReporter method)
        have GetReporter(string) GithubReporter
        want GetReporter(string) Reporter

Code:

package main

// Package #1

type Repository interface {
  GetReporter(string) Reporter
}

type Reporter interface {
  ChangeStatus(string) error
}

func Execute(r Repository) {
  // Do something with the repository
}

// Package #2

type GithubRepository struct {
}

type GithubReporter struct {
}

func (repo *GithubRepository) GetReporter(sha string) GithubReporter {
 return GithubReporter{}
}

func (reporter *GithubReporter) ChangeStatus(status string) error {
  // Change the status
  return nil
}

// Package #3

func main() {
  githubRepository := GithubRepository{}
  Execute(githubRepository)
}

Go Playground: https://play.golang.org/p/ph0sZnyAC5I

Answer 1

It was impossible to make two package independent in such a case. However, with go1.9 and its type alias, this can be done.

First, as go proverbs say, A little copy is better than a little dependency. You should copy the part of definition of Reporter to package B, and change the signature according to it: func (repo GithubRepository) GetReporter(sha string) Reporter.

Yet the compiler won't understand that the two interface is the samething. But with the help of type alias, it can be worked around. Change both definition type Reporter interface {...} to type Reporter = interface {...}. And it will compile now.