Kotlin allows you to nest functions, one within another. We can declare and use a function within another function.

When you declare a function within another function, the nested functions, visibility will stay exclusively within the parent function and cannot be accessed from outside.

So, let's have an example:

fun main(args: Array<String>) { 
    fun nested():String { 
        return "String from nested function" 
    } 
    println("Nested Output: ${nested()}") 
} 

In the preceding program, we declared and used a function—nested()—inside the main function.

The following is the output, if you're curious:

So, as we've got our basics brushed up in functions, let's move forward in functional programming. In the following section, we will learn about function types.