hail.hs

-- Collatz sequence.  See rosettacode.org/wiki/Hailstone_sequence#Haskell

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

main :: IO () 
main = putStrLn $ "Collatz sequence for 27: "
          ++ (show.hailstone $ 27)
          ++ "\n"
          ++ "The number "
          ++ (show longestChain)
          ++" has the longest hailstone sequence"
          ++" for any number less then 100000. "
          ++"The sequence has length " 
          ++ (show.length.hailstone $ longestChain)
 
hailstone :: Integer -> [Integer]
hailstone = takeWhile (/=1) . (iterate collatz)
   where collatz n = if even n then n `div` 2 else 3*n+1
 
longestChain :: Integer
longestChain = fst $ maximumBy (comparing snd) $ map pairUp [1..100000]
   where pairUp i = (i, (length.hailstone) i)

--  ------------For GNU Emacs ------------
--  Local Variables:
--  compile-command: "ghc -Wall -O2 hail.hs"
--  End: