diff --git a/go.mod b/go.mod
index 5527ed9..0dfc42b 100644
--- a/go.mod
+++ b/go.mod
@@ -6,4 +6,5 @@ require (
 	github.com/arl/statsviz v0.5.1 // indirect
 	github.com/gorilla/websocket v1.4.2 // indirect
 	golang.org/x/net v0.2.0 // indirect
+	gopl.io v0.0.0-20211004154805-1ae3ec64947b // indirect
 )
diff --git a/go.sum b/go.sum
index edf24fe..2f26eb6 100644
--- a/go.sum
+++ b/go.sum
@@ -2,5 +2,13 @@ github.com/arl/statsviz v0.5.1 h1:3HY0ZEB738JtguWsD1Tf1pFJZiCcWUmYRq/3OTYKaSI=
 github.com/arl/statsviz v0.5.1/go.mod h1:zDnjgRblGm1Dyd7J5YlbH7gM1/+HRC+SfkhZhQb5AnM=
 github.com/gorilla/websocket v1.4.2 h1:+/TMaTYc4QFitKJxsQ7Yye35DkWvkdLcvGKqM+x0Ufc=
 github.com/gorilla/websocket v1.4.2/go.mod h1:YR8l580nyteQvAITg2hZ9XVh4b55+EU/adAjf1fMHhE=
+golang.org/x/net v0.0.0-20210929193557-e81a3d93ecf6/go.mod h1:9nx3DQGgdP8bBQD5qxJ1jj9UTztislL4KSBs9R2vV5Y=
 golang.org/x/net v0.2.0 h1:sZfSu1wtKLGlWI4ZZayP0ck9Y73K1ynO6gqzTdBVdPU=
 golang.org/x/net v0.2.0/go.mod h1:KqCZLdyyvdV855qA2rE3GC2aiw5xGR5TEjj8smXukLY=
+golang.org/x/sys v0.0.0-20201119102817-f84b799fce68/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/sys v0.0.0-20210423082822-04245dca01da/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
+golang.org/x/term v0.0.0-20201126162022-7de9c90e9dd1/go.mod h1:bj7SfCRtBDWHUb9snDiAeCFNEtKQo2Wmx5Cou7ajbmo=
+golang.org/x/text v0.3.6/go.mod h1:5Zoc/QRtKVWzQhOtBMvqHzDpF6irO9z98xDceosuGiQ=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
+gopl.io v0.0.0-20211004154805-1ae3ec64947b h1:7X7imR4tIDID9aTy9Algx201qZnnRi1ANBum/C9fFWY=
+gopl.io v0.0.0-20211004154805-1ae3ec64947b/go.mod h1:Ma727rQcq6c2t9FT4cyk3IutUQU2j0mtNkG5po6QChQ=
diff --git a/src/study/day9Interface/FlagValue.go b/src/study/day9Interface/FlagValue.go
new file mode 100644
index 0000000..663ef21
--- /dev/null
+++ b/src/study/day9Interface/FlagValue.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+)
+
+type Celsius float64
+type Fahrenheit float64
+type Kelvin 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 KToC(k Kelvin) Celsius     { return Celsius(k - 273.15) }
+
+// 练习7.7 为啥默认值是带有°C的字符串
+// 因为Celsius类型实现了String()方法，
+// 在flag.CommandLine.Var(&f, name, usage)时，
+// 会调用f.String()方法，给Flag结构体的DefValue属性就是带有°C这个符号的
+// 而default的这句话是在flag.Parse()调用其他函数中执行输出的
+// 见Flag.FlagSet.defaultUsage() 和 PrintDefaults()方法
+
+func (c Celsius) String() string { return fmt.Sprintf("%gC", c) }
+
+// *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
+	case "K", "°K":
+		f.Celsius = KToC(Kelvin(value))
+		return nil
+	}
+	return fmt.Errorf("invalid temperature %q", s)
+}
+
+// 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
+}
+
+var temp = CelsiusFlag("temp", 20.0, "the temperature")
+
+func main() {
+	flag.Parse()
+	fmt.Println(*temp)
+}