Example of cross-package interfaces in golang

test_a_sayer.go

package a

import "fmt"

type Sayer interface {
    Say() string
}

type Formal struct{}

func (p Formal) Greet(s interface{Sayer}) string {
    return fmt.Sprintf("Hello, %s", s.Say())
}

test_b_sayer.go

package b

import "fmt"

type Sayer interface {
    Say() string
}

type Casual struct{}

func (p Casual) Greet(s interface{Sayer}) string {
    return fmt.Sprintf("Hey %s!", s.Say())
}

test_greet_greet.go

package greet

type Sayer interface {
    Say() string
}

type Greeter interface {
    Greet(s interface{Sayer}) string
}

func Greet(g interface{Greeter}, i Sayer) string {
    return g.Greet(i)
}

test_main.go

package main

import (
    "log"
    "test/a"
    "test/b"
    "test/greet"
)

type Who struct {
    to string
}

func (w Who) Say() string {
    return w.to
}

type Bad struct {
    to string
}

func (w Bad) Say(exclaim bool) string {
    said := w.to
    if exclaim {
        said += "!!!"
    }
    return said
}

func main() {
    log.Println("Testing...")

    a := a.Formal{}
    b := b.Casual{}


    w := &Who{
        to:"world",
    }

    x := &Bad{
        to:"world",
    }

    log.Println(greet.Greet(a, w))
    log.Println(greet.Greet(b, w))
    //log.Println(greet.Greet(b, x))
}

Thanks. Good example

In your method signatures, just use the interface types themselves. As you have them now, they declare new anonymous types that embed the type you actually care about.

E.g., instead of Greet(s interface{Sayer}), you would just use Greet(s Sayer). And instead of Greet(g interface{Greeter}, i Sayer) you would just use Greet(g Greeter, i Sayer).

Thanks. It's very helpful