refactor: common ifaces

interfaces/10-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/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/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)
+}

interfaces/10-stringer/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/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
+}