proposal: flag: add slice flag

[zkep]

Proposal Details

Proposal Details

In practice, array-type fields are often used, and they should be supported, perhaps not necessarily required, but useful

Such as string slice, using a single name to accept multiple values, such a requirement exists in command-line programs

// -- strings Value
type stringsValue []string

func newStringsValue(val []string, p *[]string) *stringsValue {
	*p = val
	return (*stringsValue)(p)
}

func (s *stringsValue) Set(val string) error {
	*s = append(*s, val)
	return nil
}

func (s *stringsValue) Get() any { return []string(*s) }

func (s *stringsValue) String() string { return fmt.Sprint([]string(*s)) }

// StringsVar defines many string flag with specified name, default value, and usage string.
// The argument p points to many string variable in which to store the value of the flag.
func (f *FlagSet) StringsVar(p *[]string, name string, value []string, usage string) {
	f.Var(newStringsValue(value, p), name, usage)
}


// StringsVar defines many string flag with specified name, default value, and usage string.
// The argument p points to many string variable in which to store the value of the flag.
func StringsVar(p *[]string, name string, value []string, usage string) {
	CommandLine.Var(newStringsValue(value, p), name, usage)
}

// Strings defines many string flag with specified name, default value, and usage string.
// The return value is the address of many string variable that stores the value of the flag.
func (f *FlagSet) Strings(name string, value []string, usage string) *[]string {
	p := new([]string)
	f.StringsVar(p, name, value, usage)
	return p
}

// Strings defines many string flag with specified name, default value, and usage string.
// The return value is the address of many string variable that stores the value of the flag.
func Strings(name string, value []string, usage string) *[]string {
	return CommandLine.Strings(name, value, usage)
}

[ianlancetaylor]

CC @robpike

[robpike]

Dup of #40155