Source File
	flag.go

Belonging Package
	flag

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Package flag implements command-line flag parsing.

# Usage

Define flags using [flag.String], [Bool], [Int], etc.

This declares an integer flag, -n, stored in the pointer nFlag, with type *int:

	import "flag"
	var nFlag = flag.Int("n", 1234, "help message for flag n")

If you like, you can bind the flag to a variable using the Var() functions.

	var flagvar int
	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.

After all flags are defined, call

	flag.Parse()

to parse the command line into the defined flags.

Flags may then be used directly. If you're using the flags themselves,
they are all pointers; if you bind to variables, they're values.

	fmt.Println("ip has value ", *ip)
	fmt.Println("flagvar has value ", flagvar)

After parsing, the arguments following the flags are available as the
slice [flag.Args] or individually as [flag.Arg](i).
The arguments are indexed from 0 through [flag.NArg]-1.

# Command line flag syntax

The following forms are permitted:

	-flag
	--flag   // double dashes are also permitted
	-flag=x
	-flag x  // non-boolean flags only

One or two dashes may be used; they are equivalent.
The last form is not permitted for boolean flags because the
meaning of the command

	cmd -x *

where * is a Unix shell wildcard, will change if there is a file
called 0, false, etc. You must use the -flag=false form to turn
off a boolean flag.

Flag parsing stops just before the first non-flag argument
("-" is a non-flag argument) or after the terminator "--".

Integer flags accept 1234, 0664, 0x1234 and may be negative.
Boolean flags may be:

	1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False

Duration flags accept any input valid for time.ParseDuration.

The default set of command-line flags is controlled by
top-level functions.  The [FlagSet] type allows one to define
independent sets of flags, such as to implement subcommands
in a command-line interface. The methods of [FlagSet] are
analogous to the top-level functions for the command-line
flag set.
*/
package flag

import (
	"encoding"
	"errors"
	"fmt"
	"io"
	"os"
	"reflect"
	"runtime"
	"slices"
	"strconv"
	"strings"
	"time"
)

// ErrHelp is the error returned if the -help or -h flag is invoked
// but no such flag is defined.
var ErrHelp = errors.New("flag: help requested")

// errParse is returned by Set if a flag's value fails to parse, such as with an invalid integer for Int.
// It then gets wrapped through failf to provide more information.
var errParse = errors.New("parse error")

// errRange is returned by Set if a flag's value is out of range.
// It then gets wrapped through failf to provide more information.
var errRange = errors.New("value out of range")

func numError(err error) error {
	ne, ok := err.(*strconv.NumError)
	if !ok {
		return err
	}
	if ne.Err == strconv.ErrSyntax {
		return errParse
	}
	if ne.Err == strconv.ErrRange {
		return errRange
	}
	return err
}

// -- bool Value
type boolValue bool

func newBoolValue(val bool, p *bool) *boolValue {
	*p = val
	return (*boolValue)(p)
}

func (b *boolValue) Set(s string) error {
	v, err := strconv.ParseBool(s)
	if err != nil {
		err = errParse
	}
	*b = boolValue(v)
	return err
}

func (b *boolValue) Get() any { return bool(*b) }

func (b *boolValue) String() string { return strconv.FormatBool(bool(*b)) }

func (b *boolValue) IsBoolFlag() bool { return true }

// optional interface to indicate boolean flags that can be
// supplied without "=value" text
type boolFlag interface {
	Value
	IsBoolFlag() bool
}

// -- int Value
type intValue int

func newIntValue(val int, p *int) *intValue {
	*p = val
	return (*intValue)(p)
}

func (i *intValue) Set(s string) error {
	v, err := strconv.ParseInt(s, 0, strconv.IntSize)
	if err != nil {
		err = numError(err)
	}
	*i = intValue(v)
	return err
}

func (i *intValue) Get() any { return int(*i) }

func (i *intValue) String() string { return strconv.Itoa(int(*i)) }

// -- int64 Value
type int64Value int64

func newInt64Value(val int64, p *int64) *int64Value {
	*p = val
	return (*int64Value)(p)
}

func (i *int64Value) Set(s string) error {
	v, err := strconv.ParseInt(s, 0, 64)
	if err != nil {
		err = numError(err)
	}
	*i = int64Value(v)
	return err
}

func (i *int64Value) Get() any { return int64(*i) }

func (i *int64Value) String() string { return strconv.FormatInt(int64(*i), 10) }

// -- uint Value
type uintValue uint

func newUintValue(val uint, p *uint) *uintValue {
	*p = val
	return (*uintValue)(p)
}

func (i *uintValue) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, strconv.IntSize)
	if err != nil {
		err = numError(err)
	}
	*i = uintValue(v)
	return err
}

func (i *uintValue) Get() any { return uint(*i) }

func (i *uintValue) String() string { return strconv.FormatUint(uint64(*i), 10) }

// -- uint64 Value
type uint64Value uint64

func newUint64Value(val uint64, p *uint64) *uint64Value {
	*p = val
	return (*uint64Value)(p)
}

func (i *uint64Value) Set(s string) error {
	v, err := strconv.ParseUint(s, 0, 64)
	if err != nil {
		err = numError(err)
	}
	*i = uint64Value(v)
	return err
}

func (i *uint64Value) Get() any { return uint64(*i) }

func (i *uint64Value) String() string { return strconv.FormatUint(uint64(*i), 10) }

// -- string Value
type stringValue string

func newStringValue(val string, p *string) *stringValue {
	*p = val
	return (*stringValue)(p)
}

func (s *stringValue) Set(val string) error {
	*s = stringValue(val)
	return nil
}

func (s *stringValue) Get() any { return string(*s) }

func (s *stringValue) String() string { return string(*s) }

// -- float64 Value
type float64Value float64

func newFloat64Value(val float64, p *float64) *float64Value {
	*p = val
	return (*float64Value)(p)
}

func (f *float64Value) Set(s string) error {
	v, err := strconv.ParseFloat(s, 64)
	if err != nil {
		err = numError(err)
	}
	*f = float64Value(v)
	return err
}

func (f *float64Value) Get() any { return float64(*f) }

func (f *float64Value) String() string { return strconv.FormatFloat(float64(*f), 'g', -1, 64) }

// -- time.Duration Value
type durationValue time.Duration

func newDurationValue(val time.Duration, p *time.Duration) *durationValue {
	*p = val
	return (*durationValue)(p)
}

func (d *durationValue) Set(s string) error {
	v, err := time.ParseDuration(s)
	if err != nil {
		err = errParse
	}
	*d = durationValue(v)
	return err
}

func (d *durationValue) Get() any { return time.Duration(*d) }

func (d *durationValue) String() string { return (*time.Duration)(d).String() }

// -- encoding.TextUnmarshaler Value
type textValue struct{ p encoding.TextUnmarshaler }

func newTextValue(val encoding.TextMarshaler, p encoding.TextUnmarshaler) textValue {
	ptrVal := reflect.ValueOf(p)
	if ptrVal.Kind() != reflect.Ptr {
		panic("variable value type must be a pointer")
	}
	defVal := reflect.ValueOf(val)
	if defVal.Kind() == reflect.Ptr {
		defVal = defVal.Elem()
	}
	if defVal.Type() != ptrVal.Type().Elem() {
		panic(fmt.Sprintf("default type does not match variable type: %v != %v", defVal.Type(), ptrVal.Type().Elem()))
	}
	ptrVal.Elem().Set(defVal)
	return textValue{p}
}

func (v textValue) Set(s string) error {
	return v.p.UnmarshalText([]byte(s))
}

func (v textValue) Get() interface{} {
	return v.p
}

func (v textValue) String() string {
	if m, ok := v.p.(encoding.TextMarshaler); ok {
		if b, err := m.MarshalText(); err == nil {
			return string(b)
		}
	}
	return ""
}

// -- func Value
type funcValue func(string) error

func (f funcValue) Set(s string) error { return f(s) }

func (f funcValue) String() string { return "" }

// -- boolFunc Value
type boolFuncValue func(string) error

func (f boolFuncValue) Set(s string) error { return f(s) }

func (f boolFuncValue) String() string { return "" }

func (f boolFuncValue) IsBoolFlag() bool { return true }

// Value is the interface to the dynamic value stored in a flag.
// (The default value is represented as a string.)
//
// If a Value has an IsBoolFlag() bool method returning true,
// the command-line parser makes -name equivalent to -name=true
// rather than using the next command-line argument.
//
// Set is called once, in command line order, for each flag present.
// The flag package may call the [String] method with a zero-valued receiver,
// such as a nil pointer.
type Value interface {
	String() string
	Set(string) error
}

// Getter is an interface that allows the contents of a [Value] to be retrieved.
// It wraps the [Value] interface, rather than being part of it, because it
// appeared after Go 1 and its compatibility rules. All [Value] types provided
// by this package satisfy the [Getter] interface, except the type used by [Func].
type Getter interface {
	Value
	Get() any
}

// ErrorHandling defines how [FlagSet.Parse] behaves if the parse fails.
type ErrorHandling int

// These constants cause [FlagSet.Parse] to behave as described if the parse fails.
const (
	ContinueOnError ErrorHandling = iota // Return a descriptive error.
	ExitOnError                          // Call os.Exit(2) or for -h/-help Exit(0).
	PanicOnError                         // Call panic with a descriptive error.
)

// A FlagSet represents a set of defined flags. The zero value of a FlagSet
// has no name and has [ContinueOnError] error handling.
//
// [Flag] names must be unique within a FlagSet. An attempt to define a flag whose
// name is already in use will cause a panic.
type FlagSet struct {
	// Usage is the function called when an error occurs while parsing flags.
	// The field is a function (not a method) that may be changed to point to
	// a custom error handler. What happens after Usage is called depends
	// on the ErrorHandling setting; for the command line, this defaults
	// to ExitOnError, which exits the program after calling Usage.
	Usage func()

	name          string
	parsed        bool
	actual        map[string]*Flag
	formal        map[string]*Flag
	args          []string // arguments after flags
	errorHandling ErrorHandling
	output        io.Writer         // nil means stderr; use Output() accessor
	undef         map[string]string // flags which didn't exist at the time of Set
}

// 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    Value  // value as set
	DefValue string // default value (as text); for usage message
}

// sortFlags returns the flags as a slice in lexicographical sorted order.
func sortFlags(flags map[string]*Flag) []*Flag {
	result := make([]*Flag, len(flags))
	i := 0
	for _, f := range flags {
		result[i] = f
		i++
	}
	slices.SortFunc(result, func(a, b *Flag) int {
		return strings.Compare(a.Name, b.Name)
	})
	return result
}

// Output returns the destination for usage and error messages. [os.Stderr] is returned if
// output was not set or was set to nil.
func (f *FlagSet) Output() io.Writer {
	if f.output == nil {
		return os.Stderr
	}
	return f.output
}

// Name returns the name of the flag set.
func (f *FlagSet) Name() string {
	return f.name
}

// ErrorHandling returns the error handling behavior of the flag set.
func (f *FlagSet) ErrorHandling() ErrorHandling {
	return f.errorHandling
}

// SetOutput sets the destination for usage and error messages.
// If output is nil, [os.Stderr] is used.
func (f *FlagSet) SetOutput(output io.Writer) {
	f.output = output
}

// VisitAll visits the flags in lexicographical order, calling fn for each.
// It visits all flags, even those not set.
func (f *FlagSet) VisitAll(fn func(*Flag)) {
	for _, flag := range sortFlags(f.formal) {
		fn(flag)
	}
}

// VisitAll visits the command-line flags in lexicographical order, calling
// fn for each. It visits all flags, even those not set.
func VisitAll(fn func(*Flag)) {
	CommandLine.VisitAll(fn)
}

// Visit visits the flags in lexicographical order, calling fn for each.
// It visits only those flags that have been set.
func (f *FlagSet) Visit(fn func(*Flag)) {
	for _, flag := range sortFlags(f.actual) {
		fn(flag)
	}
}

// Visit visits the command-line flags in lexicographical order, calling fn
// for each. It visits only those flags that have been set.
func Visit(fn func(*Flag)) {
	CommandLine.Visit(fn)
}

// Lookup returns the [Flag] structure of the named flag, returning nil if none exists.
func (f *FlagSet) Lookup(name string) *Flag {
	return f.formal[name]
}

// Lookup returns the [Flag] structure of the named command-line flag,
// returning nil if none exists.
func Lookup(name string) *Flag {
	return CommandLine.formal[name]
}

// Set sets the value of the named flag.
func (f *FlagSet) Set(name, value string) error {
	return f.set(name, value)
}
func (f *FlagSet) set(name, value string) error {
	flag, ok := f.formal[name]
	if !ok {
		// Remember that a flag that isn't defined is being set.
		// We return an error in this case, but in addition if
		// subsequently that flag is defined, we want to panic
		// at the definition point.
		// This is a problem which occurs if both the definition
		// and the Set call are in init code and for whatever
		// reason the init code changes evaluation order.
		// See issue 57411.
		_, file, line, ok := runtime.Caller(2)
		if !ok {
			file = "?"
			line = 0
		}
		if f.undef == nil {
			f.undef = map[string]string{}
		}
		f.undef[name] = fmt.Sprintf("%s:%d", file, line)

		return fmt.Errorf("no such flag -%v", name)
	}
	err := flag.Value.Set(value)
	if err != nil {
		return err
	}
	if f.actual == nil {
		f.actual = make(map[string]*Flag)
	}
	f.actual[name] = flag
	return nil
}

// Set sets the value of the named command-line flag.
func Set(name, value string) error {
	return CommandLine.set(name, value)
}

// isZeroValue determines whether the string represents the zero
// value for a flag.
func isZeroValue(flag *Flag, value string) (ok bool, err error) {
	// Build a zero value of the flag's Value type, and see if the
	// result of calling its String method equals the value passed in.
	// This works unless the Value type is itself an interface type.
	typ := reflect.TypeOf(flag.Value)
	var z reflect.Value
	if typ.Kind() == reflect.Pointer {
		z = reflect.New(typ.Elem())
	} else {
		z = reflect.Zero(typ)
	}
	// Catch panics calling the String method, which shouldn't prevent the
	// usage message from being printed, but that we should report to the
	// user so that they know to fix their code.
	defer func() {
		if e := recover(); e != nil {
			if typ.Kind() == reflect.Pointer {
				typ = typ.Elem()
			}
			err = fmt.Errorf("panic calling String method on zero %v for flag %s: %v", typ, flag.Name, e)
		}
	}()
	return value == z.Interface().(Value).String(), nil
}

// UnquoteUsage extracts a back-quoted name from the usage
// string for a flag and returns it and the un-quoted usage.
// Given "a `name` to show" it returns ("name", "a name to show").
// If there are no back quotes, the name is an educated guess of the
// type of the flag's value, or the empty string if the flag is boolean.
func UnquoteUsage(flag *Flag) (name string, usage string) {
	// Look for a back-quoted name, but avoid the strings package.
	usage = flag.Usage
	for i := 0; i < len(usage); i++ {
		if usage[i] == '`' {
			for j := i + 1; j < len(usage); j++ {
				if usage[j] == '`' {
					name = usage[i+1 : j]
					usage = usage[:i] + name + usage[j+1:]
					return name, usage
				}
			}
			break // Only one back quote; use type name.
		}
	}
	// No explicit name, so use type if we can find one.
	name = "value"
	switch fv := flag.Value.(type) {
	case boolFlag:
		if fv.IsBoolFlag() {
			name = ""
		}
	case *durationValue:
		name = "duration"
	case *float64Value:
		name = "float"
	case *intValue, *int64Value:
		name = "int"
	case *stringValue:
		name = "string"
	case *uintValue, *uint64Value:
		name = "uint"
	}
	return
}

// PrintDefaults prints, to standard error unless configured otherwise, the
// default values of all defined command-line flags in the set. See the
// documentation for the global function PrintDefaults for more information.
func (f *FlagSet) PrintDefaults() {
	var isZeroValueErrs []error
	f.VisitAll(func(flag *Flag) {
		var b strings.Builder
		fmt.Fprintf(&b, "  -%s", flag.Name) // Two spaces before -; see next two comments.
		name, usage := UnquoteUsage(flag)
		if len(name) > 0 {
			b.WriteString(" ")
			b.WriteString(name)
		}
		// Boolean flags of one ASCII letter are so common we
		// treat them specially, putting their usage on the same line.
		if b.Len() <= 4 { // space, space, '-', 'x'.
			b.WriteString("\t")
		} else {
			// Four spaces before the tab triggers good alignment
			// for both 4- and 8-space tab stops.
			b.WriteString("\n    \t")
		}
		b.WriteString(strings.ReplaceAll(usage, "\n", "\n    \t"))

		// Print the default value only if it differs to the zero value
		// for this flag type.
		if isZero, err := isZeroValue(flag, flag.DefValue); err != nil {
			isZeroValueErrs = append(isZeroValueErrs, err)
		} else if !isZero {
			if _, ok := flag.Value.(*stringValue); ok {
				// put quotes on the value
				fmt.Fprintf(&b, " (default %q)", flag.DefValue)
			} else {
				fmt.Fprintf(&b, " (default %v)", flag.DefValue)
			}
		}
		fmt.Fprint(f.Output(), b.String(), "\n")
	})
	// If calling String on any zero flag.Values triggered a panic, print
	// the messages after the full set of defaults so that the programmer
	// knows to fix the panic.
	if errs := isZeroValueErrs; len(errs) > 0 {
		fmt.Fprintln(f.Output())
		for _, err := range errs {
			fmt.Fprintln(f.Output(), err)
		}
	}
}

// PrintDefaults prints, to standard error unless configured otherwise,
// a usage message showing the default settings of all defined
// command-line flags.
// For an integer valued flag x, the default output has the form
//
//	-x int
//		usage-message-for-x (default 7)
//
// The usage message will appear on a separate line for anything but
// a bool flag with a one-byte name. For bool flags, the type is
// omitted and if the flag name is one byte the usage message appears
// on the same line. The parenthetical default is omitted if the
// default is the zero value for the type. The listed type, here int,
// can be changed by placing a back-quoted name in the flag's usage
// string; the first such item in the message is taken to be a parameter
// name to show in the message and the back quotes are stripped from
// the message when displayed. For instance, given
//
//	flag.String("I", "", "search `directory` for include files")
//
// the output will be
//
//	-I directory
//		search directory for include files.
//
// To change the destination for flag messages, call [CommandLine].SetOutput.
func PrintDefaults() {
	CommandLine.PrintDefaults()
}

// defaultUsage is the default function to print a usage message.
func (f *FlagSet) defaultUsage() {
	if f.name == "" {
		fmt.Fprintf(f.Output(), "Usage:\n")
	} else {
		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
	}
	f.PrintDefaults()
}

// NOTE: Usage is not just defaultUsage(CommandLine)
// because it serves (via godoc flag Usage) as the example
// for how to write your own usage function.

// Usage prints a usage message documenting all defined command-line flags
// to [CommandLine]'s output, which by default is [os.Stderr].
// It is called when an error occurs while parsing flags.
// The function is a variable that may be changed to point to a custom function.
// By default it prints a simple header and calls [PrintDefaults]; for details about the
// format of the output and how to control it, see the documentation for [PrintDefaults].
// Custom usage functions may choose to exit the program; by default exiting
// happens anyway as the command line's error handling strategy is set to
// [ExitOnError].
var Usage = func() {
	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
	PrintDefaults()
}

// NFlag returns the number of flags that have been set.
func (f *FlagSet) NFlag() int { return len(f.actual) }

// NFlag returns the number of command-line flags that have been set.
func NFlag() int { return len(CommandLine.actual) }

// Arg returns the i'th argument. Arg(0) is the first remaining argument
// after flags have been processed. Arg returns an empty string if the
// requested element does not exist.
func (f *FlagSet) Arg(i int) string {
	if i < 0 || i >= len(f.args) {
		return ""
	}
	return f.args[i]
}

// Arg returns the i'th command-line argument. Arg(0) is the first remaining argument
// after flags have been processed. Arg returns an empty string if the
// requested element does not exist.
func Arg(i int) string {
	return CommandLine.Arg(i)
}

// NArg is the number of arguments remaining after flags have been processed.
func (f *FlagSet) NArg() int { return len(f.args) }

// NArg is the number of arguments remaining after flags have been processed.
func NArg() int { return len(CommandLine.args) }

// Args returns the non-flag arguments.
func (f *FlagSet) Args() []string { return f.args }

// Args returns the non-flag command-line arguments.
func Args() []string { return CommandLine.args }

// 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 (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string) {
	f.Var(newBoolValue(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newBoolValue(value, p), name, usage)
}

// Bool defines a bool flag with specified name, default value, and usage string.
// The return value is the address of a bool variable that stores the value of the flag.
func (f *FlagSet) Bool(name string, value bool, usage string) *bool {
	p := new(bool)
	f.BoolVar(p, name, value, usage)
	return p
}

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

// 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 (f *FlagSet) IntVar(p *int, name string, value int, usage string) {
	f.Var(newIntValue(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newIntValue(value, p), name, usage)
}

// Int defines an int flag with specified name, default value, and usage string.
// The return value is the address of an int variable that stores the value of the flag.
func (f *FlagSet) Int(name string, value int, usage string) *int {
	p := new(int)
	f.IntVar(p, name, value, usage)
	return p
}

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

// 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 (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string) {
	f.Var(newInt64Value(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newInt64Value(value, p), name, usage)
}

// Int64 defines an int64 flag with specified name, default value, and usage string.
// The return value is the address of an int64 variable that stores the value of the flag.
func (f *FlagSet) Int64(name string, value int64, usage string) *int64 {
	p := new(int64)
	f.Int64Var(p, name, value, usage)
	return p
}

// Int64 defines an int64 flag with specified name, default value, and usage string.
// The return value is the address of an int64 variable that stores the value of the flag.
func Int64(name string, value int64, usage string) *int64 {
	return CommandLine.Int64(name, value, usage)
}

// 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 (f *FlagSet) UintVar(p *uint, name string, value uint, usage string) {
	f.Var(newUintValue(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newUintValue(value, p), name, usage)
}

// Uint defines a uint flag with specified name, default value, and usage string.
// The return value is the address of a uint variable that stores the value of the flag.
func (f *FlagSet) Uint(name string, value uint, usage string) *uint {
	p := new(uint)
	f.UintVar(p, name, value, usage)
	return p
}

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

// 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 (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string) {
	f.Var(newUint64Value(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newUint64Value(value, p), name, usage)
}

// Uint64 defines a uint64 flag with specified name, default value, and usage string.
// The return value is the address of a uint64 variable that stores the value of the flag.
func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64 {
	p := new(uint64)
	f.Uint64Var(p, name, value, usage)
	return p
}

// Uint64 defines a uint64 flag with specified name, default value, and usage string.
// The return value is the address of a uint64 variable that stores the value of the flag.
func Uint64(name string, value uint64, usage string) *uint64 {
	return CommandLine.Uint64(name, value, usage)
}

// 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 (f *FlagSet) StringVar(p *string, name string, value string, usage string) {
	f.Var(newStringValue(value, p), name, usage)
}

// 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 string, value string, usage string) {
	CommandLine.Var(newStringValue(value, p), name, usage)
}

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

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

// 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 (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string) {
	f.Var(newFloat64Value(value, p), name, usage)
}

// 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) {
	CommandLine.Var(newFloat64Value(value, p), name, usage)
}

// Float64 defines a float64 flag with specified name, default value, and usage string.
// The return value is the address of a float64 variable that stores the value of the flag.
func (f *FlagSet) Float64(name string, value float64, usage string) *float64 {
	p := new(float64)
	f.Float64Var(p, name, value, usage)
	return p
}

// Float64 defines a float64 flag with specified name, default value, and usage string.
// The return value is the address of a float64 variable that stores the value of the flag.
func Float64(name string, value float64, usage string) *float64 {
	return CommandLine.Float64(name, value, usage)
}

// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
// The argument p points to a time.Duration variable in which to store the value of the flag.
// The flag accepts a value acceptable to time.ParseDuration.
func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	f.Var(newDurationValue(value, p), name, usage)
}

// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
// The argument p points to a time.Duration variable in which to store the value of the flag.
// The flag accepts a value acceptable to time.ParseDuration.
func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
	CommandLine.Var(newDurationValue(value, p), name, usage)
}

// Duration defines a time.Duration flag with specified name, default value, and usage string.
// The return value is the address of a time.Duration variable that stores the value of the flag.
// The flag accepts a value acceptable to time.ParseDuration.
func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
	p := new(time.Duration)
	f.DurationVar(p, name, value, usage)
	return p
}

// Duration defines a time.Duration flag with specified name, default value, and usage string.
// The return value is the address of a time.Duration variable that stores the value of the flag.
// The flag accepts a value acceptable to time.ParseDuration.
func Duration(name string, value time.Duration, usage string) *time.Duration {
	return CommandLine.Duration(name, value, usage)
}

// TextVar defines a flag with a specified name, default value, and usage string.
// The argument p must be a pointer to a variable that will hold the value
// of the flag, and p must implement encoding.TextUnmarshaler.
// If the flag is used, the flag value will be passed to p's UnmarshalText method.
// The type of the default value must be the same as the type of p.
func (f *FlagSet) TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
	f.Var(newTextValue(value, p), name, usage)
}

// TextVar defines a flag with a specified name, default value, and usage string.
// The argument p must be a pointer to a variable that will hold the value
// of the flag, and p must implement encoding.TextUnmarshaler.
// If the flag is used, the flag value will be passed to p's UnmarshalText method.
// The type of the default value must be the same as the type of p.
func TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
	CommandLine.Var(newTextValue(value, p), name, usage)
}

// Func defines a flag with the specified name and usage string.
// Each time the flag is seen, fn is called with the value of the flag.
// If fn returns a non-nil error, it will be treated as a flag value parsing error.
func (f *FlagSet) Func(name, usage string, fn func(string) error) {
	f.Var(funcValue(fn), name, usage)
}

// Func defines a flag with the specified name and usage string.
// Each time the flag is seen, fn is called with the value of the flag.
// If fn returns a non-nil error, it will be treated as a flag value parsing error.
func Func(name, usage string, fn func(string) error) {
	CommandLine.Func(name, usage, fn)
}

// BoolFunc defines a flag with the specified name and usage string without requiring values.
// Each time the flag is seen, fn is called with the value of the flag.
// If fn returns a non-nil error, it will be treated as a flag value parsing error.
func (f *FlagSet) BoolFunc(name, usage string, fn func(string) error) {
	f.Var(boolFuncValue(fn), name, usage)
}

// BoolFunc defines a flag with the specified name and usage string without requiring values.
// Each time the flag is seen, fn is called with the value of the flag.
// If fn returns a non-nil error, it will be treated as a flag value parsing error.
func BoolFunc(name, usage string, fn func(string) error) {
	CommandLine.BoolFunc(name, usage, fn)
}

// Var defines a flag with the specified name and usage string. The type and
// value of the flag are represented by the first argument, of type [Value], which
// typically holds a user-defined implementation of [Value]. For instance, the
// caller could create a flag that turns a comma-separated string into a slice
// of strings by giving the slice the methods of [Value]; in particular, [Set] would
// decompose the comma-separated string into the slice.
func (f *FlagSet) Var(value Value, name string, usage string) {
	// Flag must not begin "-" or contain "=".
	if strings.HasPrefix(name, "-") {
		panic(f.sprintf("flag %q begins with -", name))
	} else if strings.Contains(name, "=") {
		panic(f.sprintf("flag %q contains =", name))
	}

	// Remember the default value as a string; it won't change.
	flag := &Flag{name, usage, value, value.String()}
	_, alreadythere := f.formal[name]
	if alreadythere {
		var msg string
		if f.name == "" {
			msg = f.sprintf("flag redefined: %s", name)
		} else {
			msg = f.sprintf("%s flag redefined: %s", f.name, name)
		}
		panic(msg) // Happens only if flags are declared with identical names
	}
	if pos := f.undef[name]; pos != "" {
		panic(fmt.Sprintf("flag %s set at %s before being defined", name, pos))
	}
	if f.formal == nil {
		f.formal = make(map[string]*Flag)
	}
	f.formal[name] = flag
}

// Var defines a flag with the specified name and usage string. The type and
// value of the flag are represented by the first argument, of type [Value], which
// typically holds a user-defined implementation of [Value]. For instance, the
// caller could create a flag that turns a comma-separated string into a slice
// of strings by giving the slice the methods of [Value]; in particular, [Set] would
// decompose the comma-separated string into the slice.
func Var(value Value, name string, usage string) {
	CommandLine.Var(value, name, usage)
}

// sprintf formats the message, prints it to output, and returns it.
func (f *FlagSet) sprintf(format string, a ...any) string {
	msg := fmt.Sprintf(format, a...)
	fmt.Fprintln(f.Output(), msg)
	return msg
}

// failf prints to standard error a formatted error and usage message and
// returns the error.
func (f *FlagSet) failf(format string, a ...any) error {
	msg := f.sprintf(format, a...)
	f.usage()
	return errors.New(msg)
}

// usage calls the Usage method for the flag set if one is specified,
// or the appropriate default usage function otherwise.
func (f *FlagSet) usage() {
	if f.Usage == nil {
		f.defaultUsage()
	} else {
		f.Usage()
	}
}

// parseOne parses one flag. It reports whether a flag was seen.
func (f *FlagSet) parseOne() (bool, error) {
	if len(f.args) == 0 {
		return false, nil
	}
	s := f.args[0]
	if len(s) < 2 || s[0] != '-' {
		return false, nil
	}
	numMinuses := 1
	if s[1] == '-' {
		numMinuses++
		if len(s) == 2 { // "--" terminates the flags
			f.args = f.args[1:]
			return false, nil
		}
	}
	name := s[numMinuses:]
	if len(name) == 0 || name[0] == '-' || name[0] == '=' {
		return false, f.failf("bad flag syntax: %s", s)
	}

	// it's a flag. does it have an argument?
	f.args = f.args[1:]
	hasValue := false
	value := ""
	for i := 1; i < len(name); i++ { // equals cannot be first
		if name[i] == '=' {
			value = name[i+1:]
			hasValue = true
			name = name[0:i]
			break
		}
	}

	flag, ok := f.formal[name]
	if !ok {
		if name == "help" || name == "h" { // special case for nice help message.
			f.usage()
			return false, ErrHelp
		}
		return false, f.failf("flag provided but not defined: -%s", name)
	}

	if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
		if hasValue {
			if err := fv.Set(value); err != nil {
				return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
			}
		} else {
			if err := fv.Set("true"); err != nil {
				return false, f.failf("invalid boolean flag %s: %v", name, err)
			}
		}
	} else {
		// It must have a value, which might be the next argument.
		if !hasValue && len(f.args) > 0 {
			// value is the next arg
			hasValue = true
			value, f.args = f.args[0], f.args[1:]
		}
		if !hasValue {
			return false, f.failf("flag needs an argument: -%s", name)
		}
		if err := flag.Value.Set(value); err != nil {
			return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)
		}
	}
	if f.actual == nil {
		f.actual = make(map[string]*Flag)
	}
	f.actual[name] = flag
	return true, nil
}

// Parse parses flag definitions from the argument list, which should not
// include the command name. Must be called after all flags in the [FlagSet]
// are defined and before flags are accessed by the program.
// The return value will be [ErrHelp] if -help or -h were set but not defined.
func (f *FlagSet) Parse(arguments []string) error {
	f.parsed = true
	f.args = arguments
	for {
		seen, err := f.parseOne()
		if seen {
			continue
		}
		if err == nil {
			break
		}
		switch f.errorHandling {
		case ContinueOnError:
			return err
		case ExitOnError:
			if err == ErrHelp {
				os.Exit(0)
			}
			os.Exit(2)
		case PanicOnError:
			panic(err)
		}
	}
	return nil
}

// Parsed reports whether f.Parse has been called.
func (f *FlagSet) Parsed() bool {
	return f.parsed
}

// Parse parses the command-line flags from [os.Args][1:]. Must be called
// after all flags are defined and before flags are accessed by the program.
func Parse() {
	// Ignore errors; CommandLine is set for ExitOnError.
	CommandLine.Parse(os.Args[1:])
}

// Parsed reports whether the command-line flags have been parsed.
func Parsed() bool {
	return CommandLine.Parsed()
}

// CommandLine is the default set of command-line flags, parsed from [os.Args].
// The top-level functions such as [BoolVar], [Arg], and so on are wrappers for the
// methods of CommandLine.
var CommandLine = NewFlagSet(os.Args[0], ExitOnError)

func init() {
	// Override generic FlagSet default Usage with call to global Usage.
	// Note: This is not CommandLine.Usage = Usage,
	// because we want any eventual call to use any updated value of Usage,
	// not the value it has when this line is run.
	CommandLine.Usage = commandLineUsage
}

func commandLineUsage() {
	Usage()
}

// NewFlagSet returns a new, empty flag set with the specified name and
// error handling property. If the name is not empty, it will be printed
// in the default usage message and in error messages.
func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
	f := &FlagSet{
		name:          name,
		errorHandling: errorHandling,
	}
	f.Usage = f.defaultUsage
	return f
}

// Init sets the name and error handling property for a flag set.
// By default, the zero [FlagSet] uses an empty name and the
// [ContinueOnError] error handling policy.
func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
	f.name = name
	f.errorHandling = errorHandling
}

The pages are generated with Golds v0.7.0-preview. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @zigo_101 (reachable from the left QR code) to get the latest news of Golds.