A Tour of Go Interfaces

package main

import (
    "fmt"
    "math"
)
 type Abser interface {
     Abs() float64
 }

 func main() {
     var a Abser
     f := MyFloat(-math.Sqrt2)
     v := Vertex{3, 4}

     a = f // a MyFloat implements Abser
     a = &v  // a *Vertex implements Abser

     // In the following line, v is a Vertex(not *Vertex)
     // and does Not Implement Abser.
     //a = v 
     fmt.Println(a.Abs())
 }

 type MyFloat float64

 func (f MyFloat) Abs() float64 {
     if f < 0 {
         return float64(-f)
     }
     return float64(f)
 }

 type Vertex struct {
     X,Y float64
 }

 func (v *Vertex) Abs() float64 {
     return math.Sqrt(v.X*v.X + v.Y*v.Y)
 }