test.go

package main

import (
    "fmt"
    "math"
)

type geometry interface {
    area() float64
    perim() float64
}

type rect struct {
    width, height float64
}

type circle struct {
    radius float64
}

func(r rect) area() float64 {
    return r.width * r.height
}

func(r rect) perim() float64 {
    return 2*r.width + 2*r.height
}

func(c circle) area() float64 {
    return math.Pi * (c.radius * c.radius)
}

func(c circle) perim() float64 {
    return 2 * math.Pi * c.radius
}

func printStuff(g geometry) {
    fmt.Println(g)
    fmt.Println("Area:", g.area())
    fmt.Println("Perim:", g.perim())
}

func main() {
    r := rect{width: 5, height: 10}
    c := circle{radius: 6}

    printStuff(r)
    printStuff(c)

    fmt.Println("Press ENTER to exit the program.")
    fmt.Scanln()
}