Go Interfaces

func main() {
  var r Shape = Rectangle{Length: 3, Width: 4}
  fmt.Printf("Type of r: %T, Area: %v, Perimeter: %v.", r, r.Area(), r.Perimeter())
}

func (r Rectangle) Area() float64 {
  return r.Length * r.Width
}
func (r Rectangle) Perimeter() float64 {
  return 2 * (r.Length + r.Width)
}

The methods defined in Shape are implemented in Rectangle.

type Rectangle struct {
  Length, Width float64
}

Struct Rectangle implicitly implements interface Shape by implementing all of its methods.

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