Interface in Golang

PROGRAM:

package main

import (
    "fmt"
    "math"
)

type Geometry interface {
    Area() float64
    Perimeter() float64
}

type Rectangle struct {
    length float64
    width  float64
}

type Circle struct {
    radius float64
}

/* Rectangle type implementing Geometry interface */
func (r Rectangle) Area() float64 {
    return r.length * r.width
}

func (r Rectangle) Perimeter() float64 {
    return 2 * (r.length + r.width)
}

/* Circle type implementing Geometry interface */
func (c Circle) Area() float64 {
    return math.Pi * c.radius * c.radius
}

//Circumference
func (c Circle) Perimeter() float64 {
    return 2 * math.Pi * c.radius
}

func Calculate(g Geometry) {
    area := g.Area()
    perimeter := g.Perimeter()
    fmt.Println("Area-", area, "Perimeter-", perimeter, "\n")
}

func main() {
    /* create a variable of type Rectangle and initialize it */
    rect := Rectangle{15.50, 30.25}
    circ := Circle{4.2}

    Calculate(rect)
    Calculate(circ)
}

OUTPUT:
Area- 468.875 Perimeter- 91.5 

Area- 55.41769440932395 Perimeter- 26.389378290154262