while print flag , the placeholder if need but not set.

print flag type first.

Signed-off-by: jokemanfire <hu.dingyang@zte.com.cn>

examples_test.go

@@ -143,7 +143,7 @@ func ExampleCommand_Run_appHelp() {
 	//    help, h        Shows a list of commands or help for one command
 	//
 	// GLOBAL OPTIONS:
-	//    --name value   a name to say (default: "bob")
+	//    --name string  a name to say (default: "bob")
 	//    --help, -h     show help
 	//    --version, -v  print the version
 }

flag.go

@@ -180,6 +180,11 @@ type LocalFlag interface {
 	IsLocal() bool
 }
 
+// FlagType is an interface to detect if a flag is a string, bool, etc.
+type FlagType interface {
+	GetFlagType() string
+}
+
 func newFlagSet(name string, flags []Flag) (*flag.FlagSet, error) {
 	set := flag.NewFlagSet(name, flag.ContinueOnError)
 
@@ -304,9 +309,14 @@ func stringifyFlag(f Flag) string {
 	}
 	placeholder, usage := unquoteUsage(df.GetUsage())
 	needsPlaceholder := df.TakesValue()
-
+	// if needsPlaceholder is true, placeholder is empty
 	if needsPlaceholder && placeholder == "" {
-		placeholder = defaultPlaceholder
+		// try to get type from flag
+		if v1, ok := f.(FlagType); ok && v1.GetFlagType() != "" {
+			placeholder = v1.GetFlagType()
+		} else {
+			placeholder = defaultPlaceholder
+		}
 	}
 
 	defaultValueString := ""

flag_impl.go

@@ -98,6 +98,21 @@ func (f *FlagBase[T, C, V]) GetValue() string {
 	return fmt.Sprintf("%v", f.Value)
 }
 
+// GetFlagType returns the type of the flag.
+func (f *FlagBase[T, C, V]) GetFlagType() string {
+	ty := reflect.TypeOf(f.Value)
+	if ty == nil {
+		return ""
+	}
+	// if it is a Slice, then return the slice type. Will nested slices be used in the future?
+	if ty.Kind() == reflect.Slice {
+		elemType := ty.Elem()
+		return "[]" + elemType.Name()
+	}
+	//TODO. Hashmap is a bit difficult to judge, it will be fixed in the future
+	return ty.Name()
+}
+
 // Apply populates the flag given the flag set and environment
 func (f *FlagBase[T, C, V]) Apply(set *flag.FlagSet) error {
 	tracef("apply (flag=%[1]q)", f.Name)