What is the main purpose of using extension functions in programming languages?

The main purpose of extension functions is to add new methods to existing classes without the need for inheritance or modifying the original class code. This improves code readability and usability by extending the functionality of classes, especially when they come from external libraries.

For example, in Kotlin, you can add a function to the String class:

fun String.isEmailValid(): Boolean {
    return this.contains("@") && this.contains(".")
}

val email = "test@example.com"
println(email.isEmailValid()) // true

Thus, extension functions make the code more expressive and allow for writing cleaner and more maintainable code.