good good study, day day up!

vendor/gopl.io/ch7/tempconv/tempconv.go

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+// See page 180.
+
+// Package tempconv performs Celsius and Fahrenheit temperature computations.
+package tempconv
+
+import (
+	"flag"
+	"fmt"
+)
+
+type Celsius float64
+type Fahrenheit float64
+
+func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9.0/5.0 + 32.0) }
+func FToC(f Fahrenheit) Celsius { return Celsius((f - 32.0) * 5.0 / 9.0) }
+
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
+
+/*
+//!+flagvalue
+package flag
+
+// Value is the interface to the value stored in a flag.
+type Value interface {
+	String() string
+	Set(string) error
+}
+//!-flagvalue
+*/
+
+//!+celsiusFlag
+// *celsiusFlag satisfies the flag.Value interface.
+type celsiusFlag struct{ Celsius }
+
+func (f *celsiusFlag) Set(s string) error {
+	var unit string
+	var value float64
+	fmt.Sscanf(s, "%f%s", &value, &unit) // no error check needed
+	switch unit {
+	case "C", "°C":
+		f.Celsius = Celsius(value)
+		return nil
+	case "F", "°F":
+		f.Celsius = FToC(Fahrenheit(value))
+		return nil
+	}
+	return fmt.Errorf("invalid temperature %q", s)
+}
+
+//!-celsiusFlag
+
+//!+CelsiusFlag
+
+// CelsiusFlag defines a Celsius flag with the specified name,
+// default value, and usage, and returns the address of the flag variable.
+// The flag argument must have a quantity and a unit, e.g., "100C".
+func CelsiusFlag(name string, value Celsius, usage string) *Celsius {
+	f := celsiusFlag{value}
+	flag.CommandLine.Var(&f, name, usage)
+	return &f.Celsius
+}
+
+//!-CelsiusFlag

vendor/gopl.io/ch7/tempconv/tempconv.go.~master~

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+// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
+// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
+
+
+// Package tempconv performs Celsius and Fahrenheit temperature computations.
+package tempconv
+
+import (
+	"flag"
+	"fmt"
+)
+
+type Celsius float64
+type Fahrenheit float64
+
+func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9.0/5.0 + 32.0) }
+func FToC(f Fahrenheit) Celsius { return Celsius((f - 32.0) * 5.0 / 9.0) }
+
+func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
+
+/*
+//!+flagvalue
+package flag
+
+// Value is the interface to the value stored in a flag.
+type Value interface {
+	String() string
+	Set(string) error
+}
+//!-flagvalue
+*/
+
+//!+celsiusflag
+// *celsiusFlag satisfies the flag.Value interface.
+type celsiusFlag struct{ Celsius }
+
+func (f *celsiusFlag) Set(s string) error {
+	var unit string
+	var value float64
+	fmt.Sscanf(s, "%f%s", &value, &unit) // no error check needed
+	switch unit {
+	case "C", "°C":
+		f.Celsius = Celsius(value)
+		return nil
+	case "F", "°F":
+		f.Celsius = FToC(Fahrenheit(value))
+		return nil
+	}
+	return fmt.Errorf("invalid temperature %q", s)
+}
+
+//!-celsiusflag
+
+//!+Celsiusflag
+// CelsiusFlag defines a Celsius flag with the specified name,
+// default value, and usage, and returns the address of the flag variable.
+// The flag argument must have a quantity and a unit, e.g., "100C".
+func CelsiusFlag(name string, value Celsius, usage string) *Celsius {
+	f := celsiusFlag{value}
+	flag.CommandLine.Var(&f, name, usage)
+	return &f.Celsius
+}
+
+//!-Celsiusflag