Interface type return

What is the difference between declare the address (&) or not once the return type is an interface?

func NewService(rep repository.Repository) Service {
    // or without the "&"
    return &myService{
        repository: rep
    }   
}

The difference is that if a method needed to implement the interface has a pointer receiver, you must return a pointer. If the method instead has a value receiver, you can use either. For example:

https://play.golang.org/p/ToGKyIjIJNQ

package main
import (
  "fmt"
)
type Hello interface {
  Hello() string
}
type World interface {
  World() string
}
type HelloWorlder interface {
  Hello() string
  World() string
}
type test struct{}
func (t *test) Hello() string {
  return "Hello"
}
func (t test) World() string {
  return "World"
}
func HelloWorld1() HelloWorlder {
  return &test{}
}
// if you uncomment this, it won't compile
//func HelloWorld2() HelloWorlder {
//    return test{}
//}
func main() {
  greeter1 := HelloWorld1()
  //greeter2 := HelloWorld2()
  fmt.Println(greeter1.Hello(), greeter1.World())
  //fmt.Println(greeter2.Hello(), greeter2.World())
}

this question boils down to assignability of an interface.

https://golang.org/ref/spec#Assignability

> A value x is assignable to a variable of type T ("x is assignable to
> T") if one of the following conditions applies: ...
> - T is an interface type and x implements T. ...

https://golang.org/ref/spec#Interface_types

> An interface type specifies a method set called its interface.

https://golang.org/ref/spec#Method_sets

> The method set of a type T consists of all methods declared with
> receiver type T. The method set of the corresponding pointer type *T
> is the set of all methods declared with receiver *T or T (that is, it
> also contains the method set of T)
>
> The method set of a type determines the interfaces that the type implements and the methods that can be called using a receiver of that type.

> What is the difference between declare the address (&) or not once the return type is an interface?

A build error.

The Effective Go has a chapter about the differences between pointer and value receivers.

https://golang.org/doc/effective_go#methods