a-a.go

package a

type Printer struct {
}

func NewPrinter() *Printer {
  return &Printer{}
}

func (p *Printer) Print(s string) {}

a2-a2.go

package a2

type TestPrinter struct {}

// printer for test.
func (p *TestPrinter) Print(s string) {}

b-b.go

package b

type Printer interface {
  Print(s string)
}

func DoWithPrinter(p Printer) {
  p.Print("Hello")
  p.Print("World")
}

main.go

package main

import (
  "a"
  "a2"
  "b"
)

func main() {
  b.DoWithPrinter(a.NewPrinter())
  // for test
  //b.DoWithPrinter(&a2.TestPrinter{})
}