How to create nested and hierarchical flags for complex CLI structures in Go?

To create nested and hierarchical flags for complex CLI structures in Go, you can utilize the flag package from the standard library along with some custom code. Here's a step-by-step guide:

Import the required packages:

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

Define a custom FlagSet struct that embeds the standard flag.FlagSet struct:

type FlagSet struct {
	*flag.FlagSet
	parent *FlagSet // reference to the parent FlagSet
}

Create a constructor function to initialize a new FlagSet:

func NewFlagSet(name string, parent *FlagSet) *FlagSet {
	fs := flag.NewFlagSet(name, flag.ExitOnError)
	return &FlagSet{
		FlagSet: fs,
		parent:  parent,
	}
}

Implement a new Var method on FlagSet to add nested flags:

func (fs *FlagSet) Var(p flag.Value, name string, usage string) {
	fs.FlagSet.Var(p, name, usage)
}

Create a new Lookup method on FlagSet to support nested flag lookup:

func (fs *FlagSet) Lookup(name string) *flag.Flag {
	f := fs.FlagSet.Lookup(name)
	if f != nil {
		return f
	}

	if fs.parent != nil {
		return fs.parent.Lookup(name)
	}

	return nil
}

Implement a Parse method on FlagSet to parse the defined flags:

func (fs *FlagSet) Parse(args []string) {
	fs.FlagSet.Parse(args)
	// Handle additional logic specific to the nested flag structure if needed
}

Finally, define the CLI structure using the FlagSet and set up nested flags:

func main() {
	root := NewFlagSet("cli", nil)
	sub := NewFlagSet("sub", root)

	// Add flags to each FlagSet
	root.StringVar(&globalFlag, "global", "", "Global flag")
	sub.StringVar(&subFlag, "subflag", "", "Sub command flag")

	// Parse the command-line arguments
	root.Parse(os.Args[1:])

	// Access flags
	fmt.Printf("Global flag: %s\n", globalFlag)
	fmt.Printf("Sub command flag: %s\n", subFlag)
}

By following these steps, you can create a nested and hierarchical flag structure for your complex CLI application in Go. You can extend this approach further by adding more methods to the FlagSet struct as per your requirements.