add: string interface

interfaces/09-stringer/list.go

@@ -0,0 +1,47 @@
+// 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"
+	"strings"
+)
+
+type list []fmt.Stringer
+
+// list satisfies the fmt.Stringer
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚."
+	}
+
+	// use strings.Builder when you're combining unknown
+	// list of strings together.
+	var str strings.Builder
+
+	for _, it := range l {
+		// the builder doesn't know about the stringer interface.
+		// that's why you need to call String method here.
+		str.WriteString(it.String())
+		str.WriteRune('\n')
+	}
+
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	type discounter interface {
+		discount(float64)
+	}
+
+	for _, it := range l {
+		if it, ok := it.(discounter); ok {
+			it.discount(ratio)
+		}
+	}
+}