Examples of Fibonacci number function

fibonacci.hs

fib 0 = 0
fib 1 = 1
fib 2 = 1
fib n = fib (n - 1) + fib (n - 2)
-- Get the 100th number 
fib 100
-- But it takes too long.
-- You can do it with zipWith like the following line and it works much faster
fibs = 0 : 1 : 1 : zipWith (+) (drop 1 fibs) (drop 2 fibs)

-- Get the 100th number. Why 99? because the fist number is 0th element.
fibs !! 99 
-- 218922995834555169026

-- This can fix that issue.
fibAt n = fibs !! (n - 1) where fibs = 0 : 1 : 1 : zipWith (+) (drop 1 fibs) (drop 2 fibs)

-- The 100th fibonacci number
fibAt 100
-- 218922995834555169026