package main

import "fmt"

// Shape of thing (e.g. square)
type Shape interface {
	// Signature of functions that implement this interface
	Area() float64
	Perimeter() float64
}

// Different types of shapes:
type square struct {
	X float64
}

// Area() implementation (see the name of the function) for the Shape interface of the square type (as a receiver)
// receiver.field accesses the fields of the struct
func (s square) Area() float64 {
	return s.X * s.X
}

// Perimeter() implementation (see the name) for the Shape interface of the square type (as a receiver)
// receiver.field accesses the fields of the struct
func (s square) Perimeter() float64 {
	return 4 * s.X
}

// Calculate the shape of a thing (area and perimeter)
// Function using the interface Shape (see the input) -- has no output
func Calculate(x Shape) {
	v, ok := x.(square)
	if ok {
		fmt.Println("Is a square:", v)
	}

	fmt.Println(x.Area())
	fmt.Println(x.Perimeter())
}

func main() {
	x := square{X: 10}
	fmt.Println("Perimeter:", x.Perimeter())
	Calculate(x)
}