Interfaces

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 measure(g geometry) {
    fmt.Println(g)
    fmt.Println(g.area())
    fmt.Println(g.perim())
}

func main() {
    r := rect{width: 3, height: 4}
    c := circle{radius: 5}

    measure(r)
    measure(c)
}

// A type implements an interface by implementing its methods.
// There is no explicit declaration of intent, no "implements" keyword.

struct Dog {}
struct Cat {}

fn (d Dog) speak() string {
	return 'woof'
}

fn (c Cat) speak() string {
	return 'meow' 
}

interface Speaker {
	speak() string
}

fn perform(s Speaker) {
	// TODO    
    str := s.speak()
	println(str)
}

dog := Dog{}
cat := Cat{}
perform(dog) // "woof" 
perform(cat) // "meow"