How it works...

As a result, we are going to get the next 20 Fibonacci numbers printed to the console:

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181]

The additional seed parameter in the generateSequence() provides a starting value. The nextFunction() function is applied to the seed while computing the second value. Later on, it is generating each following element using its preceding value. However, in the case of the Fibonacci sequence, we have two initial values and we need a pair of preceding values in order to compute the next value. For this reason, we wrapped them in Pair type instances. Basically, we are defining a sequence of  Pair<Int, Int> type elements, and in each nextFunction() call, we are returning a new pair that holds the values updated accordingly. At the end, we just need to use the map() function to replace each Pair element with the value of its first property. As a result, we are getting an infinite sequence of integer types returning the subsequent Fibonacci numbers.