Working with recursive functions

In this recipe, we will calculate fibonacci numbers by writing a simple recursive function. Fibonacci numbers are the numbers appearing in the fibonacci sequence.

Fibonacci numbers are calculated as follows:

F_n = F_n-1 + F_n-2

We need to provide the first two seed fibonacci numbers in the sequence so that we can calculate the next set of fibonacci numbers. We will seed the sequence with 0 and 1. Our sequence will look like this:

0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144 ....

We will start indexing with 0. Hence, the 0^th fibonacci number is 0, 1^st fibonacci number is 1, .. 5^th fibonacci number is 5 ... and so on.

Getting ready

Create a new project, fibonacci, using stack with the simple template:

    stack new fibonacci simple

This will create a fibonacci directory and will add only Main.hs in the src folder. If you build the project, it will create the executable fibonacci. The default implementation will print hello world. 

How to do it...

1. Open src/Main.hs. The file...