refactor: stringer

interfaces/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/09-stringer/book.go

@@ -16,13 +16,15 @@ type book struct {
 	published timestamp
 }
 
-// book satisfies the fmt.Stringer
+// String method makes the book an fmt.Stringer.
+// The list was calling the embedded product type's String(),
+// now it calls the book.String().
 func (b *book) String() string {
 	// product.String() has a pointer receiver.
-	// That's why you need to manually take the product's address here.
+	// Therefore, you need to manually take the product's address.
 	//
-	// If you pass: "b.product", Go would pass it as a copy to Sprintf.
+	// If you pass: "b.product", Go would pass it as a copy to `Sprintf`.
 	// In that case, Go can't deference b.product automatically.
-	// It's because: b.product would be different value—a copy.
+	// It's because: b.product would be a different value, a copy.
 	return fmt.Sprintf("%s - (%s)", &b.product, b.published)
 }

interfaces/09-stringer/game.go

@@ -8,7 +8,8 @@
 package main
 
 type game struct {
-	// game satisfies the fmt.Stringer
-	// because the product satisfies the fmt.Stringer
+	// game is an fmt.Stringer
+	// because the product is an fmt.Stringer
+	// and the game embeds the product
 	product
 }

interfaces/09-stringer/list.go

@@ -15,31 +15,38 @@ import (
 type item interface {
 	discount(ratio float64)
 
+	// ~~ interface embedding ~~
+	//
 	// item interface embeds the fmt.Stringer interface.
 	//
-	// Go adds all the methods of the fmt.Stringer
-	// to the item interface.
-	fmt.Stringer // same (see below): String() string
+	// Go adds the methods of the fmt.Stringer
+	// to this item interface.
+	//
+	// same as this:
 	// String() string
+	fmt.Stringer
 }
-
 type list []item
 
-// list satisfies the fmt.Stringer
+// String method makes the list an 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.
+	// use the strings.Builder when you're combining
+	// a long 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.
+		// the builder.WriteString doesn't know about the stringer interface.
+		// because it takes a string argument.
+		// so, you need to call the String method yourself.
 		str.WriteString(it.String())
 		str.WriteRune('\n')
+
+		// or slower way:
+		// fmt.Fprintln(&str, it)
 	}
 
 	return str.String()

interfaces/09-stringer/main.go

@@ -10,10 +10,15 @@ 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)
+
 	store := list{
 		&book{product{"moby dick", 10}, toTimestamp(118281600)},
 		&book{product{"odyssey", 15}, toTimestamp("733622400")},
-		&book{product{"hobbit", 25}, unknown},
+		&book{product{"hobbit", 25}, toTimestamp(nil)},
 		&puzzle{product{"rubik's cube", 5}},
 		&game{product{"minecraft", 20}},
 		&game{product{"tetris", 5}},
@@ -22,12 +27,12 @@ func main() {
 
 	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.
+	// The list is a stringer.
+	// The `fmt.Print` function can print the `store`
+	// by calling `store`'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(store)
-
-	// timestamp is useful even if it's zero.
-	var ts timestamp
-	fmt.Println(ts)
 }

interfaces/09-stringer/money.go

@@ -11,7 +11,7 @@ import "fmt"
 
 type money float64
 
-// money satisfies the fmt.Stringer
+// String makes the money an fmt.Stringer
 func (m money) String() string {
 	return fmt.Sprintf("$%.2f", m)
 }

interfaces/09-stringer/product.go

@@ -14,7 +14,7 @@ type product struct {
 	price money
 }
 
-// product satisfies the fmt.Stringer
+// String makes the product an fmt.Stringer
 func (p *product) String() string {
 	return fmt.Sprintf("%-15s: %s", p.title, p.price)
 }

interfaces/09-stringer/puzzle.go

@@ -8,7 +8,8 @@
 package main
 
 type puzzle struct {
-	// game satisfies the fmt.Stringer
-	// because the product satisfies the fmt.Stringer
+	// puzzle is an fmt.Stringer
+	// because the product is an fmt.Stringer
+	// and the puzzle embeds the product
 	product
 }

interfaces/09-stringer/timestamp.go

@@ -12,40 +12,42 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
+// timestamp stores, formats and automatically prints a timestamp: it's a stringer.
+type timestamp struct {
+	// timestamp embeds a time, therefore it can be used as a time value.
+	// there is no need to convert a time value to a timestamp value.
+	time.Time
+}
 
-// unknown is the zero value of a timestamp.
-var unknown = timestamp(time.Time{})
-
-// Timestamp prints timestamps, it's a stringer.
-// Timestamp is useful even if it's zero.
-type timestamp time.Time
-
-// String makes the timestamp a stringer.
+// String method makes the timestamp an fmt.stringer.
 func (ts timestamp) String() string {
-	t := time.Time(ts)
-
-	if t.IsZero() {
+	if ts.IsZero() {
 		return "unknown"
 	}
 
-	return t.Format(layout)
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+	return ts.Format(layout)
 }
 
-// toTimestamp was "book.format" before.
-// Now it returns a timestamp value depending on the type of `v`.
+// toTimestamp returns a timestamp value depending on the type of `v`.
+// toTimestamp was "book.format()" before.
 func toTimestamp(v interface{}) timestamp {
 	var t int
 
 	switch v := v.(type) {
 	case int:
+		// book{title: "moby dick", price: 10, published: 118281600},
 		t = v
 	case string:
+		// book{title: "odyssey", price: 15, published: "733622400"},
 		t, _ = strconv.Atoi(v)
 	default:
-		return unknown
+		// book{title: "hobbit", price: 25},
+		return timestamp{}
 	}
 
-	return timestamp(time.Unix(int64(t), 0))
+	return timestamp{
+		Time: time.Unix(int64(t), 0),
+	}
 }

interfaces/09-stringer/toy.go

@@ -8,7 +8,8 @@
 package main
 
 type toy struct {
-	// game satisfies the fmt.Stringer
-	// because the product satisfies the fmt.Stringer
+	// toy is an fmt.Stringer
+	// because the product is an fmt.Stringer
+	// and the toy embeds the product
 	product
 }

interfaces/10-marshaler/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -58,6 +55,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }
 

interfaces/11-decode-toiface/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -57,6 +54,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }
 

interfaces/12-reflection/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -57,6 +54,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }
 

interfaces/13-reflection-2/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -57,6 +54,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }
 

interfaces/14-io-reader/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -57,6 +54,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }
 

interfaces/15-io-reader-composition/timestamp.go

@@ -12,9 +12,6 @@ import (
 	"time"
 )
 
-// Mon Jan 2 15:04:05 -0700 MST 2006
-const layout = "2006/01"
-
 // unknown is the zero value of a timestamp.
 var unknown = timestamp(time.Time{})
 
@@ -57,6 +54,9 @@ func (ts timestamp) String() string {
 		return "unknown"
 	}
 
+	// Mon Jan 2 15:04:05 -0700 MST 2006
+	const layout = "2006/01"
+
 	return t.Format(layout)
 }