Haskell Cookbook

How to do it...

1. Open src/Main.hs. The file will define the Main module with the function main :: IO (). 
2. Here is the module definition. Add declaration of our function, fib. The function takes an integer to denote the index of the fibonacci number and returns the fibonacci number at the given index:

        fib :: Integer -> Integer

3. Handle the base cases using pattern matching and pattern guards. We have three cases:

• Case I--Index is negative. This is an error case:

        fib n | n < 0 = error "invalid index"

• Case II--Index is zero. We provide the 0^th value as a seed value:

        fib 0 = 0

• Case III-- Index is one. We provide the 1^st value as the seed value:

        fib 1 = 1

4. Handle the generic case, that is, calculate the n^th fibonacci number from the  ...