shape.go

package main

import "fmt"

type Areaer interface {
  Area() float64
}
type Shape struct {
  Areaer
}
type Circle struct {
  radius float64
}
func (c Circle) Area() float64 {
  return 3.14 * (c.radius * c.radius)
}

type Rectangle struct {
  width, length float64
}
func (r Rectangle) Area() float64 {
  return r.width * r.length
}
func (s Shape) String() string {
  return fmt.Sprintf("[%T] area = %f", s.Areaer, s.Area())
}

func main() {
	r := Shape{Rectangle{3, 4}}
	c := Shape{Circle{2}}
	fmt.Println(r)
	fmt.Println(c)
}