refactor: common ifaces

interfaces/09-little-refactor/list.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"
+
+type list []*product
+
+func (l list) print() {
+	if len(l) == 0 {
+		fmt.Println("Sorry. We're waiting for delivery 🚚.")
+		return
+	}
+
+	for _, p := range l {
+		p.print()
+	}
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}

interfaces/09-little-refactor/main.go

@@ -0,0 +1,19 @@
+// 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
+
+func main() {
+	l := list{
+		{title: "moby dick", price: 10, released: toTimestamp(118281600)},
+		{title: "odyssey", price: 15, released: toTimestamp("733622400")},
+		{title: "hobbit", price: 25},
+	}
+
+	l.discount(.5)
+	l.print()
+}

interfaces/09-little-refactor/money.go

@@ -7,6 +7,10 @@
 
 package main
 
-type game struct {
-	product
+import "fmt"
+
+type money float64
+
+func (m money) string() string {
+	return fmt.Sprintf("$%.2f", m)
 }

interfaces/09-little-refactor/product.go

@@ -0,0 +1,26 @@
+// 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
+	released timestamp
+}
+
+func (p *product) print() {
+	fmt.Printf("%s: %s (%s)\n", p.title, p.price.string(), p.released.string())
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
+}

interfaces/09-little-refactor/timestamp.go

@@ -0,0 +1,45 @@
+// 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 (
+	"strconv"
+	"time"
+)
+
+// timestamp stores, formats and automatically prints a timestamp.
+type timestamp struct {
+	// timestamp anonymously embeds a time.
+	// no need to convert a time value to a timestamp value to use the methods of the time type.
+	time.Time
+}
+
+func (ts timestamp) string() string {
+	if ts.IsZero() { // same as: ts.Time.IsZero()
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout) // same as: ts.Time.Format(layout)
+}
+
+// toTimestamp returns a timestamp value depending on the type of `v`.
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/10-stringer/list.go

@@ -0,0 +1,33 @@
+// 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 []*product
+
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚.\n"
+	}
+
+	var str strings.Builder
+	for _, p := range l {
+		fmt.Fprintf(&str, "* %s\n", p)
+	}
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}

interfaces/10-stringer/main.go

@@ -0,0 +1,32 @@
+// 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() {
+	// The money type is a stringer.
+	// You don't need to call the String method when printing a value of it.
+	// var pocket money = 10
+	// fmt.Println(pocket)
+
+	l := list{
+		{title: "moby dick", price: 10, released: toTimestamp(118281600)},
+		{title: "odyssey", price: 15, released: toTimestamp("733622400")},
+		{title: "hobbit", price: 25},
+	}
+
+	// The list is a stringer.
+	// The `fmt.Print` function can print the `l`
+	// by calling `l`'s `String()` method.
+	//
+	// Underneath, `fmt.Print` uses a type switch to
+	// detect whether a type is a Stringer:
+	// https://golang.org/src/fmt/print.go#L627
+	fmt.Print(l)
+}

interfaces/10-stringer/product.go

@@ -7,21 +7,20 @@
 
 package main
 
-import "fmt"
+import (
+	"fmt"
+)
 
 type product struct {
-	Title string
-	Price money
-}
-
-func (p *product) discount(ratio float64) {
-	p.Price *= money(1 - ratio)
-}
-
-func (p *product) sum() money {
-	return p.Price
+	title    string
+	price    money
+	released timestamp
 }
 
 func (p *product) String() string {
-	return fmt.Sprintf("%-15s: %s", p.Title, p.Price)
+	return fmt.Sprintf("%s: %s (%s)", p.title, p.price, p.released)
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
 }

interfaces/10-stringer/timestamp.go

@@ -0,0 +1,43 @@
+// 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 (
+	"strconv"
+	"time"
+)
+
+// timestamp stores, formats and automatically prints a timestamp: it's a stringer.
+type timestamp struct {
+	time.Time
+}
+
+// String method makes the timestamp an fmt.stringer.
+func (ts timestamp) String() string {
+	if ts.IsZero() {
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
+}
+
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/11-sort/list.go

@@ -0,0 +1,54 @@
+// 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"
+	"sort"
+	"strings"
+)
+
+// product is now *product because sorting will unnecessarily copy the product values
+
+type list []*product
+
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚.\n"
+	}
+
+	sort.Sort(l)
+	var str strings.Builder
+	for _, p := range l {
+		fmt.Fprintf(&str, "* %s\n", p)
+	}
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}
+
+// by default `list` sorts by `title`.
+func (l list) Len() int           { return len(l) }
+func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+func (l list) Less(i, j int) bool { return l[i].title < l[j].title }
+
+// byRelease sorts by product release dates.
+type byRelease struct {
+	// byRelease embeds `list` and reuses list's Len and Swap methods.
+	list
+}
+
+func (bp byRelease) Less(i, j int) bool {
+	// Before() accepts a time.Time but `released` is not time.Time.
+	// `released.Time` is necessary.
+	return bp.list[i].released.Before(bp.list[j].released.Time)
+}

interfaces/11-sort/main.go

@@ -0,0 +1,28 @@
+// 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"
+	"sort"
+)
+
+func main() {
+	l := list{
+		{title: "moby dick", price: 10, released: toTimestamp(118281600)},
+		{title: "odyssey", price: 15, released: toTimestamp("733622400")},
+		{title: "hobbit", price: 25},
+	}
+
+	// sort.Sort(l)
+	// sort.Sort(sort.Reverse(l))
+	// sort.Sort(byRelease{l})
+	sort.Sort(sort.Reverse(byRelease{l}))
+
+	fmt.Print(l)
+}

interfaces/11-sort/product.go

@@ -0,0 +1,26 @@
+// 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
+	released timestamp
+}
+
+func (p *product) String() string {
+	return fmt.Sprintf("%s: %s (%s)", p.title, p.price, p.released)
+}
+
+func (p *product) discount(ratio float64) {
+	p.price *= money(1 - ratio)
+}

interfaces/11-sort/timestamp.go

@@ -0,0 +1,41 @@
+// 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 (
+	"strconv"
+	"time"
+)
+
+type timestamp struct {
+	time.Time
+}
+
+func (ts timestamp) String() string {
+	if ts.IsZero() {
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
+}
+
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/12-marshaler/database.json

@@ -0,0 +1,17 @@
+[
+	{
+		"title": "moby dick",
+		"price": 10,
+		"released": 118281600
+	},
+	{
+		"title": "odyssey",
+		"price": 15,
+		"released": 733622400
+	},
+	{
+		"title": "hobbit",
+		"price": 25,
+		"released": -62135596800
+	}
+]

interfaces/12-marshaler/json.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 (
+	"encoding/json"
+)
+
+func encode() ([]byte, error) {
+	l := list{
+		{Title: "moby dick", Price: 10, Released: toTimestamp(118281600)},
+		{Title: "odyssey", Price: 15, Released: toTimestamp("733622400")},
+		{Title: "hobbit", Price: 25},
+	}
+
+	return json.MarshalIndent(l, "", "\t")
+}
+
+func decode(data []byte) (l list, err error) {
+	if err := json.Unmarshal(data, &l); err != nil {
+		return nil, err
+	}
+	return l, nil
+}

interfaces/12-marshaler/list.go

@@ -0,0 +1,49 @@
+// 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"
+	"sort"
+	"strings"
+)
+
+type list []*product
+
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚.\n"
+	}
+
+	sort.Sort(l)
+	var str strings.Builder
+	for _, p := range l {
+		fmt.Fprintf(&str, "* %s\n", p)
+	}
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}
+
+// by default `list` sorts by `Title`.
+func (l list) Len() int           { return len(l) }
+func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+func (l list) Less(i, j int) bool { return l[i].Title < l[j].Title }
+
+// byRelease sorts by product release dates.
+type byRelease struct {
+	list
+}
+
+func (bp byRelease) Less(i, j int) bool {
+	return bp.list[i].Released.Before(bp.list[j].Released.Time)
+}

interfaces/12-marshaler/main.go

@@ -0,0 +1,21 @@
+// 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() {
+	// removed error handling for brevity (normally you shouldn't do this).
+	out, _ := encode()
+	fmt.Println(string(out))
+
+	l, _ := decode(out)
+	fmt.Print(l)
+}

interfaces/12-marshaler/product.go

@@ -0,0 +1,26 @@
+// 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    `json:"title"`
+	Price    money     `json:"price"`
+	Released timestamp `json:"released,omitempty"`
+}
+
+func (p *product) String() string {
+	return fmt.Sprintf("%s: %s (%s)", p.Title, p.Price, p.Released)
+}
+
+func (p *product) discount(ratio float64) {
+	p.Price *= money(1 - ratio)
+}

interfaces/12-marshaler/timestamp.go

@@ -0,0 +1,57 @@
+// 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 (
+	"strconv"
+	"time"
+)
+
+type timestamp struct {
+	time.Time
+}
+
+func (ts timestamp) String() string {
+	if ts.IsZero() {
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
+}
+
+// timestamp knows how to decode itself from json.
+// UnmarshalJSON is an implementation of the json.Unmarshaler interface.
+// json.Unmarshal and json.Decode call this method.
+func (ts *timestamp) UnmarshalJSON(data []byte) error {
+	*ts = toTimestamp(string(data))
+	return nil
+}
+
+// timestamp knows how to encode itself to json.
+// MarshalJSON is an implementation of the json.Marshaler interface.
+// json.Marshal and json.Encode call this method.
+func (ts timestamp) MarshalJSON() (out []byte, err error) {
+	// return ts.Time.MarshalJSON()
+	return strconv.AppendInt(out, ts.Unix(), 10), nil
+}
+
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/13-io/database.json

@@ -0,0 +1,17 @@
+[
+	{
+		"title": "moby dick",
+		"price": 10,
+		"released": 118281600
+	},
+	{
+		"title": "odyssey",
+		"price": 15,
+		"released": 733622400
+	},
+	{
+		"title": "hobbit",
+		"price": 25,
+		"released": -62135596800
+	}
+]

interfaces/13-io/json.go

@@ -0,0 +1,59 @@
+// 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 (
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+func encode() ([]byte, error) {
+	l := list{
+		{Title: "moby dick", Price: 10, Released: toTimestamp(118281600)},
+		{Title: "odyssey", Price: 15, Released: toTimestamp("733622400")},
+		{Title: "hobbit", Price: 25},
+	}
+
+	return json.MarshalIndent(l, "", "\t")
+}
+
+func decode(data []byte) (l list, err error) {
+	if err := json.Unmarshal(data, &l); err != nil {
+		return nil, err
+	}
+	return l, nil
+}
+
+func decodeFile(path string) (list, error) {
+	// ReadAll reads entire bytes from a file to memory.
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return decode(data)
+}
+
+func decodeFileObject(f *os.File) (list, error) {
+	data, err := ioutil.ReadAll(f)
+	if err != nil {
+		return nil, err
+	}
+	return decode(data)
+}
+
+func decodeReader(r io.Reader) (l list, err error) {
+	// data, err := ioutil.ReadAll(r)
+	// if err != nil {
+	// 	return nil, err
+	// }
+	// return decode(data)
+
+	return l, json.NewDecoder(r).Decode(&l)
+}

interfaces/13-io/list.go

@@ -0,0 +1,49 @@
+// 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"
+	"sort"
+	"strings"
+)
+
+type list []*product
+
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚.\n"
+	}
+
+	sort.Sort(l)
+	var str strings.Builder
+	for _, p := range l {
+		fmt.Fprintf(&str, "* %s\n", p)
+	}
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}
+
+// by default `list` sorts by `Title`.
+func (l list) Len() int           { return len(l) }
+func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+func (l list) Less(i, j int) bool { return l[i].Title < l[j].Title }
+
+// byRelease sorts by product release dates.
+type byRelease struct {
+	list
+}
+
+func (bp byRelease) Less(i, j int) bool {
+	return bp.list[i].Released.Before(bp.list[j].Released.Time)
+}

interfaces/13-io/main.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
+
+func main() {
+	// removed error handling for brevity (normally you shouldn't do this).
+
+	// encode to memory (to a byte slice)
+	// out, _ := encode()
+	// fmt.Println(string(out))
+
+	// read from memory (from a byte slice):
+	// l, _ := decode(out)
+
+	// read from a file by name:
+	// l, _ := decodeFile("database.json")
+
+	// read from a file #1 (not good):
+	// f, _ := os.Open("database.json")
+	// l, _ := decodeFileObject(f)
+	// f.Close()
+
+	// read from a file #2 (better):
+	// f, _ := os.Open("database.json")
+	// l, _ := decodeReader(f)
+	// f.Close()
+
+	// read from memory (from a string):
+	// const data = `[
+	// { "title": "moby dick", "price": 10, "released": 118281600 },
+	// { "title": "odyssey", "price": 15, "released": 733622400 },
+	// { "title": "hobbit", "price": 25, "released": -62135596800 }
+	// ]`
+	// l, _ := decodeReader(strings.NewReader(data))
+
+	// read from inanc's web server:
+	// res, _ := http.Get("https://inancgumus.github.io/x/database.json")
+	// l, _ := decodeReader(res.Body)
+	// res.Body.Close()
+
+	// fmt.Print(l)
+}

interfaces/13-io/product.go

@@ -0,0 +1,26 @@
+// 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    `json:"title"`
+	Price    money     `json:"price"`
+	Released timestamp `json:"released,omitempty"`
+}
+
+func (p *product) String() string {
+	return fmt.Sprintf("%s: %s (%s)", p.Title, p.Price, p.Released)
+}
+
+func (p *product) discount(ratio float64) {
+	p.Price *= money(1 - ratio)
+}

interfaces/13-io/timestamp.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 (
+	"strconv"
+	"time"
+)
+
+type timestamp struct {
+	time.Time
+}
+
+func (ts timestamp) String() string {
+	if ts.IsZero() {
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
+}
+
+func (ts *timestamp) UnmarshalJSON(data []byte) error {
+	*ts = toTimestamp(string(data))
+	return nil
+}
+
+func (ts timestamp) MarshalJSON() (out []byte, err error) {
+	return strconv.AppendInt(out, ts.Unix(), 10), nil
+}
+
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/14-composition/database.json

@@ -0,0 +1,17 @@
+[
+	{
+		"title": "moby dick",
+		"price": 10,
+		"released": 118281600
+	},
+	{
+		"title": "odyssey",
+		"price": 15,
+		"released": 733622400
+	},
+	{
+		"title": "hobbit",
+		"price": 25,
+		"released": -62135596800
+	}
+]

interfaces/14-composition/json.go

@@ -0,0 +1,59 @@
+// 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 (
+	"encoding/json"
+	"io"
+	"io/ioutil"
+	"os"
+)
+
+func encode() ([]byte, error) {
+	l := list{
+		{Title: "moby dick", Price: 10, Released: toTimestamp(118281600)},
+		{Title: "odyssey", Price: 15, Released: toTimestamp("733622400")},
+		{Title: "hobbit", Price: 25},
+	}
+
+	return json.MarshalIndent(l, "", "\t")
+}
+
+func decode(data []byte) (l list, err error) {
+	if err := json.Unmarshal(data, &l); err != nil {
+		return nil, err
+	}
+	return l, nil
+}
+
+func decodeFile(path string) (list, error) {
+	// ReadAll reads entire bytes from a file to memory.
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return decode(data)
+}
+
+func decodeFileObject(f *os.File) (list, error) {
+	data, err := ioutil.ReadAll(f)
+	if err != nil {
+		return nil, err
+	}
+	return decode(data)
+}
+
+func decodeReader(r io.Reader) (l list, err error) {
+	// data, err := ioutil.ReadAll(r)
+	// if err != nil {
+	// 	return nil, err
+	// }
+	// return decode(data)
+
+	return l, json.NewDecoder(r).Decode(&l)
+}

interfaces/14-composition/list.go

@@ -0,0 +1,49 @@
+// 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"
+	"sort"
+	"strings"
+)
+
+type list []*product
+
+func (l list) String() string {
+	if len(l) == 0 {
+		return "Sorry. We're waiting for delivery 🚚.\n"
+	}
+
+	sort.Sort(l)
+	var str strings.Builder
+	for _, p := range l {
+		fmt.Fprintf(&str, "* %s\n", p)
+	}
+	return str.String()
+}
+
+func (l list) discount(ratio float64) {
+	for _, p := range l {
+		p.discount(ratio)
+	}
+}
+
+// by default `list` sorts by `Title`.
+func (l list) Len() int           { return len(l) }
+func (l list) Swap(i, j int)      { l[i], l[j] = l[j], l[i] }
+func (l list) Less(i, j int) bool { return l[i].Title < l[j].Title }
+
+// byRelease sorts by product release dates.
+type byRelease struct {
+	list
+}
+
+func (bp byRelease) Less(i, j int) bool {
+	return bp.list[i].Released.Before(bp.list[j].Released.Time)
+}

interfaces/14-composition/main.go

@@ -0,0 +1,82 @@
+// 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
+
+// compress in bash:
+// gzip -k database.json
+
+func main() {
+	// removed error handling and x.Close() calls for brevity (normally you shouldn't do this).
+	// normally: fr.Close(), gr.Close() should also be called to release the resources.
+
+	// data, _ := ioutil.ReadFile("database.json")
+	// l, _ := decode(data)
+
+	// fmt.Print(string(data))
+	// fmt.Print(l)
+
+	// ---
+
+	// fr, _ := os.Open("database.json")
+	// tr := TeeReader(fr, os.Stdout)
+	// l, _ := decodeReader(tr)
+	// fmt.Print(l)
+
+	// ---
+
+	// fr, _ := os.Open("database.json.gz")
+	// gr, _ := gzip.NewReader(fr)
+	// tr := TeeReader(gr, os.Stdout)
+	// l, _ := decodeReader(tr)
+	// fmt.Print(l)
+
+	// ---
+
+	// TODO: FIX
+	// l, err := decodeGzip("database.json.gz", os.Stdout)
+	// l, err := decodeGzip("database.json.gz", ioutil.Discard)
+	// if err != nil {
+	// 	log.Fatalln(err)
+	// }
+	// fmt.Print("Products:\n", l)
+}
+
+// func decodeGzipFile(path string) (list, error) {
+// 	fr, err := os.Open("database.json.gz")
+// 	if err != nil {
+// 		return nil, err
+// 	}
+// 	defer fr.Close()
+
+// 	return decodeGzip(fr)
+// }
+
+// func decodeGzip(r io.Reader, w io.Writer) (list, error) {
+// 	fr, err := os.Open("database.json.gz")
+// 	if err != nil {
+// 		return nil, err
+// 	}
+
+// 	gr, err := gzip.NewReader(fr)
+// 	if err != nil {
+// 		fr.Close()
+// 		return nil, err
+// 	}
+
+// 	tr := TeeReader(gr, w)
+// 	l, err := decodeReader(tr)
+// 	if err != nil {
+// 		fr.Close()
+// 		gr.Close()
+// 		return nil, err
+// 	}
+
+// 	fr.Close()
+// 	gr.Close()
+// 	return l, nil
+// }

interfaces/14-composition/product.go

@@ -0,0 +1,26 @@
+// 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    `json:"title"`
+	Price    money     `json:"price"`
+	Released timestamp `json:"released,omitempty"`
+}
+
+func (p *product) String() string {
+	return fmt.Sprintf("%s: %s (%s)", p.Title, p.Price, p.Released)
+}
+
+func (p *product) discount(ratio float64) {
+	p.Price *= money(1 - ratio)
+}

interfaces/14-composition/tee.go

@@ -0,0 +1,34 @@
+// 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 "io"
+
+// TeeReader returns a Reader that writes to w what it reads from r.
+// All reads from r performed through it are matched with
+// corresponding writes to w. There is no internal buffering -
+// the write must complete before the read completes.
+// Any error encountered while writing is reported as a read error.
+func TeeReader(r io.Reader, w io.Writer) io.Reader {
+	return &teeReader{r, w}
+}
+
+type teeReader struct {
+	r io.Reader
+	w io.Writer
+}
+
+func (t *teeReader) Read(p []byte) (n int, err error) {
+	n, err = t.r.Read(p)
+	if n > 0 {
+		if n, err := t.w.Write(p[:n]); err != nil {
+			return n, err
+		}
+	}
+	return
+}

interfaces/14-composition/timestamp.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 (
+	"strconv"
+	"time"
+)
+
+type timestamp struct {
+	time.Time
+}
+
+func (ts timestamp) String() string {
+	if ts.IsZero() {
+		return "unknown"
+	}
+
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
+}
+
+func (ts *timestamp) UnmarshalJSON(data []byte) error {
+	*ts = toTimestamp(string(data))
+	return nil
+}
+
+func (ts timestamp) MarshalJSON() (out []byte, err error) {
+	return strconv.AppendInt(out, ts.Unix(), 10), nil
+}
+
+func toTimestamp(v interface{}) (ts timestamp) {
+	var t int
+
+	switch v := v.(type) {
+	case int:
+		t = v
+	case string:
+		t, _ = strconv.Atoi(v)
+	}
+
+	ts.Time = time.Unix(int64(t), 0)
+	return ts
+}

interfaces/_old/09-stringer/_handlemethods.go

@@ -0,0 +1,46 @@
+// 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/
+//
+
+// In the depths of the Go standard library's fmt package...
+// Printing functions use the handleMethods method.
+
+// Example:
+// var pocket money = 10
+// fmt.Println(pocket)
+
+//            the argument can be any type of value
+//      stores the pocket variable in the argument variable
+//                            ^
+//                            |
+func (p *pp) handleMethods(argument interface{}) (handled bool) {
+	// ...
+
+	// Checks whether a given argument is an error or an fmt.Stringer
+	switch v := argument.(type) {
+	// ...
+	// If the argument is a Stringer, calls its String() method
+	case Stringer:
+		// ...
+		//        pocket.String()
+		//              ^
+		//              |
+		p.fmtString(v.String(), verb)
+		return
+	}
+
+	// ...
+}
+
+/*
+The original `handleMethods` code is more involved:
+
+https://github.com/golang/go/blob/6f51082da77a1d4cafd5b7af0db69293943f4066/src/fmt/print.go#L574
+
+	-> 574#handleMethods(..)
+	-> 627#Stringer type check: If `v` is a Stringer, run:
+	-> 630#v.String()
+*/

interfaces/_old/12-reflection/money.go

@@ -7,6 +7,10 @@
 
 package main
 
-type puzzle struct {
-	product
+import "fmt"
+
+type money float64
+
+func (m money) String() string {
+	return fmt.Sprintf("$%.2f", m)
 }

interfaces/_old/13-reflection-2/money.go

@@ -7,6 +7,10 @@
 
 package main
 
-type toy struct {
-	product
+import "fmt"
+
+type money float64
+
+func (m money) String() string {
+	return fmt.Sprintf("$%.2f", m)
 }

interfaces/_old/14-io-reader/money.go

@@ -0,0 +1,16 @@
+// 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
+
+func (m money) String() string {
+	return fmt.Sprintf("$%.2f", m)
+}