cli using builtin package flag.go

package main

import (
	"flag"
	"fmt"
	"os"
	"strings"
)

type Cli struct {
	cmds map[string]*flag.FlagSet
	cb   *func()
}

func NewCli(callback *func()) *Cli {
	return &Cli{map[string]*flag.FlagSet{}, callback}
}

func (r *Cli) AddSub(name string) *flag.FlagSet {
	name = strings.TrimSpace(name)
	cmd := flag.NewFlagSet(name, flag.ExitOnError)
	r.cmds[name] = cmd
	return cmd
}

func (r *Cli) Parse() (string, *flag.FlagSet) {
	names := []string{}
	for name, _ := range r.cmds {
		names = append(names, name)
	}
	if len(os.Args) < 2 {
		fmt.Printf("expected subcommands: %s\n", strings.Join(names, ", "))
		if r.cb != nil {
			(*r.cb)()
		}
		os.Exit(1)
	}

	if cmd, ok := r.cmds[os.Args[1]]; ok {
		cmd.Parse(os.Args[2:])
		return os.Args[1], cmd
	} else {
		fmt.Printf("expected subcommands: %s\n", strings.Join(names, ", "))
		if r.cb != nil {
			(*r.cb)()
		}
		os.Exit(1)
	}
	return "", nil
}