The Type Switch

package main
import (
  "fmt"
  "math"
)
type Square struct {
  side float32
}
type Circle struct {
  radius float32
}
type Shaper interface {
  Area() float32
}
func main() {
  var areaIntf Shaper
  sq1 := new(Square)
  sq1.side = 5
  areaIntf = sq1
  switch t := areaIntf.(type) {
    case *Square:
      fmt.Printf("Type Square %T with value %v\n", t, t)
    case *Circle:
      fmt.Printf("Type Circle %T with value %v\n", t, t)
    default:
      fmt.Printf("Unexpected type %T", t)
    }
}
func (sq *Square) Area() float32 {
  return sq.side * sq.side
}
func (ci *Circle) Area() float32 {
  return ci.radius * ci.radius * math.Pi
}