go-solid-open-closed-5dfc2e81a33fd7a8f2eebc5a

main.go

package main

import "fmt"

type square struct {
	width, height int
}

func main() {
	s := square{width: 10, height: 10}
	printArea(s)
}

// printArea prints an area number of the shape
func printArea(s square) {
	fmt.Printf("the shape area is: %d\n", s.width*s.height)
}

open_closed_applied_main.go

// a shape interface which provides its area calculation method
type shape interface {
	area() float64
}

// printArea prints an area number of the shape
func printArea(s shape) {
	fmt.Printf("the shape area is: %.2f\n", s.area())
}

print_area_with_type_assertion.go

// printArea prints an area number of the shape
func printArea(variousShape interface{}) {
	switch s := variousShape.(type) {
	case square:
		fmt.Printf("the shape area is: %d\n", s.width*s.height)
	case rectangle:
		fmt.Printf("the shape area is: %d\n", s.width*s.height)
	}
}