Chapter 14

Go does not explicitly say that a struct implements an interface. If a struct implements all the methods of an interface, it automatically becomes of the interface type. Thus, in Go, interfaces are implemented implicitly.

Therefore, in Go, an interface type is a set of method signatures. The best way to understand interfaces is to see it in action.

package main
import "fmt"
type Shape interface {
area() float64
perimeter() float64
// This is a right-angled triangle
type Triangle struct {
base, height, hypotenuse float64
type Square struct {
length float64
// Triangle struct implicitly implements the `shape` interfce
func (tri Triangle) area() float64 {
return 0.5 * tri.base * tri.height
func (tri Triangle) perimeter() float64 {
return tri.base + tri.height + tri.hypotenuse
// Square struct implicitly implements the `shape` interfce
func (sq Square) area() float64 {
return sq.length * sq.length
func (sq Square) perimeter() float64 {
return sq.length * 4.0
// Now this method would work for both Triangle and Square
func printProperties(s Shape) {
func main() {
s := Square{length: 5}
t := Triangle{base: 3, height: 4, hypotenuse: 5}
// we can call printProperties on both s and t

In the snippet above, we have defined Shape to be an interface that provides the methods area() and perimeter(). Any struct that implements these two methods will become a Shape.

We then define two structs - Square and Triangle and implement the methods area() and perimeter() for each of them, allowing them to be a Shape.

We then define a function called printProperties() which prints the area and perimeter of a Shape provided as argument.

As you would see if you run this program, the method printProperties() works on both s and t which are of type Square and Triangle respectively.

Side note: The above example actually depicts polymorphism - ability for instances to behave as other types. Check out more on polymorphism here.