RosettaCodeData/Task/Hailstone-sequence/Haskell/hailstone-sequence-1.hs

import Data.List (maximumBy)
import Data.Ord (comparing)

collatz :: Int -> Int
collatz n
  | even n = n `div` 2
  | otherwise = 3 * n + 1

hailstone :: Int -> [Int]
hailstone = takeWhile (/= 1) . iterate collatz

longestChain :: Int
longestChain =
  fst $
  maximumBy (comparing snd) $ (,) <*> (length . hailstone) <$> [1 .. 100000]

--TEST -------------------------------------------------------------------------
main :: IO ()
main =
  mapM_
    putStrLn
    [ "Collatz sequence for 27: "
    , (show . hailstone) 27
    , "The number " ++ show longestChain
    , "has the longest hailstone sequence for any number less then 100000. "
    , "The sequence has length: " ++ (show . length . hailstone $ longestChain)
    ]