Void type is an odd duck that is only used to indicate that an expression does not have a value. Examples of expressions of Void type are the while-expression and calls to functions which have no return value. The only time Void can be used to explicitly declare a type is to declare the return type of a function which does not return a value:

function gab(val : Number) : Void {
   var remains = val;
   while (remains > 1.0) {
      println( remains );
      remains = remains / 2;
   }
}