add: interfaces assertion and composition (wip)

interfaces/04-assertion/list.go

@@ -0,0 +1,84 @@
+// For more tutorials: https://blog.learngoprogramming.com
+//
+// Copyright © 2018 Inanc Gumus
+// Learn Go Programming Course
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+//
+
+package main
+
+import "fmt"
+
+type printer interface {
+	print()
+}
+
+type list []printer
+
+func (l list) print() {
+	if len(l) == 0 {
+		fmt.Println("Sorry. Our store is closed. We're waiting for the delivery 🚚.")
+		return
+	}
+
+	for _, it := range l {
+		it.print()
+	}
+}
+
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
+	}
+
+	for _, it := range l {
+		if it, ok := it.(discounter); ok {
+			it.discount(ratio)
+		}
+	}
+}
+
+//
+// type assertion can pull out the real value behind an interface value.
+//
+// it can also check whether the value convertable to a given type.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		// remember: interface is a type
+// 		if it, ok := it.(interface{ discount(float64) }); ok {
+// 			it.discount(ratio)
+// 		}
+// 	}
+// }
+
+//
+// type switch can pull out the real value behind an interface value.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		switch it := it.(type) {
+// 		case *game:
+// 			it.discount(ratio)
+// 		case *puzzle:
+// 			it.discount(ratio)
+// 		}
+// 	}
+// }
+
+//
+// type switch can find the type behind an interface value.
+//
+// func (l list) discount(ratio float64) {
+// 	for _, it := range l {
+// 		switch it.(type) {
+// 		case *game:
+// 			fmt.Print("it's a *game!   --> ")
+// 		case *puzzle:
+// 			fmt.Print("it's a *puzzle! --> ")
+// 		default:
+// 			fmt.Print("neither         --> ")
+// 		}
+// 		it.print()
+// 	}
+// }