Explain what extension functions are and their main purpose in programming.

Extension functions are a mechanism in some programming languages (e.g., Kotlin) that allows adding a new function to an existing class without the need for inheritance or modifying the original class's source code.

The main purpose of extension functions is to improve code readability and convenience by allowing new methods to be called as if they were part of the class. This is especially useful when working with library classes that the developer cannot modify.

Example in Kotlin:

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

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

Here, we added the isEmailValid function to the String class without changing the class itself.