Functions are defined using the fun keyword with optional parameters and a return value. The parameter list must always be present, even if no parameters are defined. For example, this function has no parameters and it returns a String value:

    fun hello() : String = "hello world" 

Each parameter is in the form name: type. The following function accepts two parameters of the type String and also returns a String value:

    fun hello(name: String, location: String): String =
      "hello to you  $name at $location"

If a function does not return any meaningful value, then it is defined to return Unit. As discussed in Chapter 2, Kotlin Basics, Unit is analogous to the Java and C void types. By using a class that is part of a type hierarchy-rather than a special type, such as void-the type system in Kotlin can be made more regular. Every function must return a value, and this value could be Unit.

Functions returning Unit can omit the return type for procedure-style...