Finally, what I ended up doing:

f :: Integer -> Integer -> Integer

f acc x
    | x == 1 = acc
        | even x = f (acc + 1) (x `div` 2)
        | otherwise = f (acc + 1) (3 * x + 1)

g :: Integer -> (Integer, Integer)
g x = (f 1 x, x)

answer = (foldl' max (0,0)) $ map g [1 .. 999999]


main = putStrLn( show answer)