Opaque interfaces in Golang

cocreteType.go

package opaque

type internalBasicType int

// 
// Prevents the interface to be implemented
// *outside* the package, since is lowerCase (thus private).
// 
// This is best suited for API and things we don't
// wanna people to override.
// 
func (m *internalBasicType) implementsOpaque() {}

func (m *internalBasicType) GetNumber() int {
    return int(*m)
}

func NewBasic(number int) Opaque {
    // This is a pointer to an int
    o := internalBasicType(number)
    // Derefence the pointer
    return &o
}

opaqueInterface.go

// http://www.onebigfluke.com/2014/04/gos-power-is-in-emergent-behavior.html?utm_content=buffer5ee21&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

package opaque

import "fmt"

type Opaque interface {     // Public
    GetNumber() int         // Public
    implementsOpaque()      // Private
}

func DoSomethingWithOpaque(o Opaque) string {
    return fmt.Sprintf("Hello opaque #%d", o.GetNumber())
}

opaqueMain.go

package main

import (
    "fmt"
    "opaque"
)

func main() {
    o := opaque.NewBasic(15)
    s := opaque.DoSomethingWithOpaque(o)
    fmt.Println("Doing something with an opaque i get:", s)
}