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
}

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

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


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

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