Euler problem 14

14.hs

import Data.List

collatz :: Int -> Int
collatz 1 = 0
collatz n
  | odd n  = force $ (collatz $! 3*n+1) + 1
  | even n = force $ (collatz $! div n 2) + 1

force x = x `seq` x

longest = foldl' check 0 [1..1000000]
  where
    check a b = if collatz b > a then collatz b else a

main = putStrLn . show $ longest