add: string interface

interfaces/09-stringer/book.go

@@ -0,0 +1,50 @@
+// 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"
+	"strconv"
+	"time"
+)
+
+type book struct {
+	product
+	published interface{}
+}
+
+// book satisfies the fmt.Stringer
+func (b *book) String() string {
+	p := format(b.published)
+
+	// product.String has a pointer receiver.
+	// that's why we need to take its address here.
+	//
+	// when you put a value in an interface, the interface
+	// cannot run that value's type's value-receiver methods.
+	return fmt.Sprintf("%s - (%v)", &b.product, p)
+}
+
+func format(v interface{}) string {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	default:
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
+	u := time.Unix(int64(t), 0)
+	return u.Format(layout)
+}

interfaces/09-stringer/game.go

@@ -0,0 +1,14 @@
+// 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
+
+type game struct {
+	// game satisfies the fmt.Stringer
+	// because the product satisfies the fmt.Stringer
+	product
+}

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)
+		}
+	}
+}

interfaces/09-stringer/main.go

@@ -0,0 +1,29 @@
+// 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"
+
+func main() {
+	store := list{
+		&book{product{"moby dick", 10}, 118281600},
+		&book{product{"odyssey", 15}, "733622400"},
+		&book{product{"hobbit", 25}, nil},
+		&puzzle{product{"rubik's cube", 5}},
+		&game{product{"minecraft", 20}},
+		&game{product{"tetris", 5}},
+		&toy{product{"yoda", 150}},
+	}
+
+	store.discount(.5)
+
+	// the store doesn't have a print method anymore.
+	// but the Print function can print it.
+	// it's because, the list satisfies the stringer.
+	fmt.Print(store)
+}

interfaces/09-stringer/money.go

@@ -0,0 +1,17 @@
+// 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 money float64
+
+// money satisfies the fmt.Stringer
+func (m money) String() string {
+	return fmt.Sprintf("$%.2f", m)
+}

interfaces/09-stringer/product.go

@@ -0,0 +1,24 @@
+// 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 product struct {
+	title string
+	price money
+}
+
+// product satisfies the fmt.Stringer
+func (p *product) String() string {
+	return fmt.Sprintf("%-15s: %s", p.title, p.price)
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
+}

interfaces/09-stringer/puzzle.go

@@ -0,0 +1,14 @@
+// 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
+
+type puzzle struct {
+	// game satisfies the fmt.Stringer
+	// because the product satisfies the fmt.Stringer
+	product
+}

interfaces/09-stringer/toy.go

@@ -0,0 +1,14 @@
+// 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
+
+type toy struct {
+	// game satisfies the fmt.Stringer
+	// because the product satisfies the fmt.Stringer
+	product
+}