Flags: add user-defined flag types. The change is really no code; it's just publishing

the set() method and add() functions.  But we rename add() to Var() for consistency.
Also rename FlagValue to Value for simplicity.

Also, delete the check for multiple settings for a flag.  This makes it possible to
define a flag that collects values, such as into a slice of strings.

type flagVar []string

func (f *flagVar) String() string {
	return fmt.Sprint(v)
}

func (f *flagVar) Set(value string) bool {
	if v == nil {
		v = make(flagVar, 1)
	} else {
		nv := make(flagVar, len(v)+1)
		copy(nv, v)
		v = nv
	}
	v[len(v)-1] = value
	return true
}

var v flagVar

func main() {
	flag.Var(&v, "testV", "multiple values build []string")
	flag.Parse()
	fmt.Printf("v = %v\n", v)
}

R=rsc
CC=golang-dev
https://golang.org/cl/842041

src/pkg/flag/flag.go

@@ -15,6 +15,10 @@
 		func init() {
 			flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
 		}
+	Or you can create custom flags that satisfy the Value interface (with
+	pointer receivers) and couple them to flag parsing by
+		flag.Var(&flagVal, "name", "help message for flagname")
+	For such flags, the default value is just the initial value of the variable.
 
 	2) After all flags are defined, call
 		flag.Parse()
@@ -63,7 +67,7 @@ func newBoolValue(val bool, p *bool) *boolValue {
 	return &boolValue{p}
 }
 
-func (b *boolValue) set(s string) bool {
+func (b *boolValue) Set(s string) bool {
 	v, err := strconv.Atob(s)
 	*b.p = v
 	return err == nil
@@ -81,7 +85,7 @@ func newIntValue(val int, p *int) *intValue {
 	return &intValue{p}
 }
 
-func (i *intValue) set(s string) bool {
+func (i *intValue) Set(s string) bool {
 	v, err := strconv.Atoi(s)
 	*i.p = int(v)
 	return err == nil
@@ -99,7 +103,7 @@ func newInt64Value(val int64, p *int64) *int64Value {
 	return &int64Value{p}
 }
 
-func (i *int64Value) set(s string) bool {
+func (i *int64Value) Set(s string) bool {
 	v, err := strconv.Atoi64(s)
 	*i.p = v
 	return err == nil
@@ -117,7 +121,7 @@ func newUintValue(val uint, p *uint) *uintValue {
 	return &uintValue{p}
 }
 
-func (i *uintValue) set(s string) bool {
+func (i *uintValue) Set(s string) bool {
 	v, err := strconv.Atoui(s)
 	*i.p = uint(v)
 	return err == nil
@@ -135,7 +139,7 @@ func newUint64Value(val uint64, p *uint64) *uint64Value {
 	return &uint64Value{p}
 }
 
-func (i *uint64Value) set(s string) bool {
+func (i *uint64Value) Set(s string) bool {
 	v, err := strconv.Atoui64(s)
 	*i.p = uint64(v)
 	return err == nil
@@ -153,7 +157,7 @@ func newStringValue(val string, p *string) *stringValue {
 	return &stringValue{p}
 }
 
-func (s *stringValue) set(val string) bool {
+func (s *stringValue) Set(val string) bool {
 	*s.p = val
 	return true
 }
@@ -170,7 +174,7 @@ func newFloatValue(val float, p *float) *floatValue {
 	return &floatValue{p}
 }
 
-func (f *floatValue) set(s string) bool {
+func (f *floatValue) Set(s string) bool {
 	v, err := strconv.Atof(s)
 	*f.p = v
 	return err == nil
@@ -188,7 +192,7 @@ func newFloat64Value(val float64, p *float64) *float64Value {
 	return &float64Value{p}
 }
 
-func (f *float64Value) set(s string) bool {
+func (f *float64Value) Set(s string) bool {
 	v, err := strconv.Atof64(s)
 	*f.p = v
 	return err == nil
@@ -196,19 +200,19 @@ func (f *float64Value) set(s string) bool {
 
 func (f *float64Value) String() string { return fmt.Sprintf("%v", *f.p) }
 
-// FlagValue is the interface to the dynamic value stored in a flag.
+// Value is the interface to the dynamic value stored in a flag.
 // (The default value is represented as a string.)
-type FlagValue interface {
+type Value interface {
 	String() string
-	set(string) bool
+	Set(string) bool
 }
 
 // A Flag represents the state of a flag.
 type Flag struct {
-	Name     string    // name as it appears on command line
-	Usage    string    // help message
-	Value    FlagValue // value as set
-	DefValue string    // default value (as text); for usage message
+	Name     string // name as it appears on command line
+	Usage    string // help message
+	Value    Value  // value as set
+	DefValue string // default value (as text); for usage message
 }
 
 type allFlags struct {
@@ -249,7 +253,7 @@ func Set(name, value string) bool {
 	if !ok {
 		return false
 	}
-	ok = f.Value.set(value)
+	ok = f.Value.Set(value)
 	if !ok {
 		return false
 	}
@@ -294,21 +298,10 @@ func NArg() int { return len(os.Args) - flags.first_arg }
 // Args returns the non-flag command-line arguments.
 func Args() []string { return os.Args[flags.first_arg:] }
 
-func add(name string, value FlagValue, usage string) {
-	// Remember the default value as a string; it won't change.
-	f := &Flag{name, usage, value, value.String()}
-	_, alreadythere := flags.formal[name]
-	if alreadythere {
-		fmt.Fprintln(os.Stderr, "flag redefined:", name)
-		panic("flag redefinition") // Happens only if flags are declared with identical names
-	}
-	flags.formal[name] = f
-}
-
 // BoolVar defines a bool flag with specified name, default value, and usage string.
 // The argument p points to a bool variable in which to store the value of the flag.
 func BoolVar(p *bool, name string, value bool, usage string) {
-	add(name, newBoolValue(value, p), usage)
+	Var(newBoolValue(value, p), name, usage)
 }
 
 // Bool defines a bool flag with specified name, default value, and usage string.
@@ -322,7 +315,7 @@ func Bool(name string, value bool, usage string) *bool {
 // IntVar defines an int flag with specified name, default value, and usage string.
 // The argument p points to an int variable in which to store the value of the flag.
 func IntVar(p *int, name string, value int, usage string) {
-	add(name, newIntValue(value, p), usage)
+	Var(newIntValue(value, p), name, usage)
 }
 
 // Int defines an int flag with specified name, default value, and usage string.
@@ -336,7 +329,7 @@ func Int(name string, value int, usage string) *int {
 // Int64Var defines an int64 flag with specified name, default value, and usage string.
 // The argument p points to an int64 variable in which to store the value of the flag.
 func Int64Var(p *int64, name string, value int64, usage string) {
-	add(name, newInt64Value(value, p), usage)
+	Var(newInt64Value(value, p), name, usage)
 }
 
 // Int64 defines an int64 flag with specified name, default value, and usage string.
@@ -350,7 +343,7 @@ func Int64(name string, value int64, usage string) *int64 {
 // UintVar defines a uint flag with specified name, default value, and usage string.
 // The argument p points to a uint variable in which to store the value of the flag.
 func UintVar(p *uint, name string, value uint, usage string) {
-	add(name, newUintValue(value, p), usage)
+	Var(newUintValue(value, p), name, usage)
 }
 
 // Uint defines a uint flag with specified name, default value, and usage string.
@@ -364,7 +357,7 @@ func Uint(name string, value uint, usage string) *uint {
 // Uint64Var defines a uint64 flag with specified name, default value, and usage string.
 // The argument p points to a uint64 variable in which to store the value of the flag.
 func Uint64Var(p *uint64, name string, value uint64, usage string) {
-	add(name, newUint64Value(value, p), usage)
+	Var(newUint64Value(value, p), name, usage)
 }
 
 // Uint64 defines a uint64 flag with specified name, default value, and usage string.
@@ -378,7 +371,7 @@ func Uint64(name string, value uint64, usage string) *uint64 {
 // StringVar defines a string flag with specified name, default value, and usage string.
 // The argument p points to a string variable in which to store the value of the flag.
 func StringVar(p *string, name, value string, usage string) {
-	add(name, newStringValue(value, p), usage)
+	Var(newStringValue(value, p), name, usage)
 }
 
 // String defines a string flag with specified name, default value, and usage string.
@@ -392,7 +385,7 @@ func String(name, value string, usage string) *string {
 // FloatVar defines a float flag with specified name, default value, and usage string.
 // The argument p points to a float variable in which to store the value of the flag.
 func FloatVar(p *float, name string, value float, usage string) {
-	add(name, newFloatValue(value, p), usage)
+	Var(newFloatValue(value, p), name, usage)
 }
 
 // Float defines a float flag with specified name, default value, and usage string.
@@ -406,7 +399,7 @@ func Float(name string, value float, usage string) *float {
 // Float64Var defines a float64 flag with specified name, default value, and usage string.
 // The argument p points to a float64 variable in which to store the value of the flag.
 func Float64Var(p *float64, name string, value float64, usage string) {
-	add(name, newFloat64Value(value, p), usage)
+	Var(newFloat64Value(value, p), name, usage)
 }
 
 // Float64 defines a float64 flag with specified name, default value, and usage string.
@@ -417,6 +410,19 @@ func Float64(name string, value float64, usage string) *float64 {
 	return p
 }
 
+// Var defines a user-typed flag with specified name, default value, and usage string.
+// The argument p points to a Value variable in which to store the value of the flag.
+func Var(value Value, name string, usage string) {
+	// Remember the default value as a string; it won't change.
+	f := &Flag{name, usage, value, value.String()}
+	_, alreadythere := flags.formal[name]
+	if alreadythere {
+		fmt.Fprintln(os.Stderr, "flag redefined:", name)
+		panic("flag redefinition") // Happens only if flags are declared with identical names
+	}
+	flags.formal[name] = f
+}
+
 
 func (f *allFlags) parseOne(index int) (ok bool, next int) {
 	s := os.Args[index]
@@ -455,14 +461,8 @@ func (f *allFlags) parseOne(index int) (ok bool, next int) {
 			break
 		}
 	}
-	flag, alreadythere := flags.actual[name]
-	if alreadythere {
-		fmt.Fprintf(os.Stderr, "flag specified twice: -%s\n", name)
-		Usage()
-		os.Exit(2)
-	}
 	m := flags.formal
-	flag, alreadythere = m[name] // BUG
+	flag, alreadythere := m[name] // BUG
 	if !alreadythere {
 		fmt.Fprintf(os.Stderr, "flag provided but not defined: -%s\n", name)
 		Usage()
@@ -470,13 +470,13 @@ func (f *allFlags) parseOne(index int) (ok bool, next int) {
 	}
 	if f, ok := flag.Value.(*boolValue); ok { // special case: doesn't need an arg
 		if has_value {
-			if !f.set(value) {
+			if !f.Set(value) {
 				fmt.Fprintf(os.Stderr, "invalid boolean value %t for flag: -%s\n", value, name)
 				Usage()
 				os.Exit(2)
 			}
 		} else {
-			f.set("true")
+			f.Set("true")
 		}
 	} else {
 		// It must have a value, which might be the next argument.
@@ -491,7 +491,7 @@ func (f *allFlags) parseOne(index int) (ok bool, next int) {
 			Usage()
 			os.Exit(2)
 		}
-		ok = flag.Value.set(value)
+		ok = flag.Value.Set(value)
 		if !ok {
 			fmt.Fprintf(os.Stderr, "invalid value %s for flag: -%s\n", value, name)
 			Usage()

src/pkg/flag/flag_test.go

@@ -78,3 +78,22 @@ func TestEverything(t *testing.T) {
 		}
 	}
 }
+
+// Declare a user-defined flag.
+// TODO: do the work to make this test better by resetting flag state
+// and manipulating os.Args.
+type flagVar []string
+
+func (f *flagVar) String() string {
+	return "foo"
+}
+
+func (f *flagVar) Set(value string) bool {
+	return true
+}
+
+var v flagVar
+
+func init() {
+	Var(&v, "testV", "usage")
+}