main.go

package main

import (
	"fmt"
)

type Small interface {
	Foo(int) int
}

type Big interface {
	Foo(int) int
	Boo(string) string
}

type Qoo struct {
	Name string
	Age  int
}

func (q *Qoo) Foo(i int) int {
	q.Age += i
	return q.Age
}

func (q *Qoo) Boo(s string) string {
	q.Name += s
	return q.Name
}

func NewSmall(age int) Small {
	return &Qoo{
		Age:  age,
		Name: "Hello ",
	}
}

func main() {
	s := NewSmall(10)
	fmt.Println("Small")
	if b, ok := s.(Big); ok {
		fmt.Println("Big")
		fmt.Println(b.Boo("World"))
	}
}