1. import Data.List
2. import Data.Ord
3.  
4. nextCollatz n
5.   | even n = div n 2
6.   | otherwise = n * 3 + 1
7.  
8. collatzChain lst@(x:xs)
9.   | x == 1 = lst
10.   | otherwise = collatzChain ((nextCollatz x):lst)
11.  
12. collatz n = collatzChain [n]
13.  
14. longest = maximumBy (comparing (length . collatz)) [1..1000000]
15. main = putStrLn $ show longest