How to use Interfaces in GO

type shape interface {
    area() float64
    perimeter() float64
}

package main
import "fmt"
//Interface declaration
type shape interface {
    area() float64
    perimeter() float64
}
//Struct declaration for rectangle
type rectangle struct{
  length, height float64
}
//Struct declaration for circle
type circle struct{
  radius float64
}
//Method declarations for rectangle
func (r rectangle) area() float64 {
    return r.length * r.height
}
func (r rectangle) perimeter() float64 {
    return 2 * r.length + 2 * r.height
}
//Method declarations for circle
func (c circle) area() float64 {
    return 3.142 * c.radius * c.radius
}
func (c circle) perimeter() float64 {
    return 2 * 3.142 * c.radius
}
func main() {
    r := rectangle{length: 10.0, height: 5.0}
    c := circle{radius: 5.0}
    fmt.Println("Area of rectangle is ", r.area())
    fmt.Println("Parameter of rectangle is ", r.perimeter())
    fmt.Println("Area of circle is ", c.area())
    fmt.Println("Perimeter of circle is ", c.perimeter())
}