How to implement flag validation and custom validation functions in Go?

To implement flag validation and custom validation functions in Go, you can follow these steps:

Import the flag package: First, import the built-in flag package to use its functionalities for command-line argument parsing.

import "flag"

Define custom flag variables: Declare the required flag variables with their respective types. For example, using flag.String() to declare a string flag:

var myFlag string
flag.StringVar(&myFlag, "myFlag", "", "Description of the flag")

Implement a flag validation function (optional): If you want to apply additional validation logic for the flag values, you can define a custom flag validation function. The function should have the correct signature according to flag.Value interface:

func validateFlag(value string) error {
    // Add your validation logic here
    if value == "invalid" {
        return fmt.Errorf("invalid flag value")
    }
    return nil
}

Implement the flag's custom validation function (optional): If you want to apply specific validation logic for a particular flag, you can define a custom validation function for that flag. The function should have the correct signature according to flag.Value interface:

func (f *flagValue) Set(value string) error {
    // Add your validation logic here
    if value == "invalid" {
        return fmt.Errorf("invalid flag value")
    }
    f.value = value
    return nil
}

Set custom flag validation functions (optional): Set the custom validation functions for the respective flags using flag.Var() method:

flag.Var(&myFlag, "myFlag", "Description of the flag")

Parse the command-line arguments: Finally, call flag.Parse() to parse the command-line arguments and validate the flags:

flag.Parse()

Here's an example where we apply custom flag validation and validation functions for a string flag:

import (
	"flag"
	"fmt"
)

type flagValue struct {
	value string
}

func (f *flagValue) String() string {
	return f.value
}

func (f *flagValue) Set(value string) error {
	// Add your validation logic here
	if len(value) < 3 {
		return fmt.Errorf("flag value must have at least 3 characters")
	}
	f.value = value
	return nil
}

func main() {
	var myFlag flagValue
	flag.Var(&myFlag, "myFlag", "Description of the flag")
	flag.Parse()

	fmt.Println(myFlag)
}

Now, when you run the program with the flag -myFlag, it will validate the input based on your custom validation logic.