Finding Maciej numbers

Maciej.hs

module Main where

fastlog :: Int -> Int -> Int
fastlog b n
  | n < 1     = -1
  | otherwise = 1 + fastlog b (n `div` b)

roundt :: Int -> Int
roundt n = m * (n `div` m)
    where m = 10 ^ (fastlog 10 n)

sumt :: Int -> Int
sumt n = (n * n + n) `div` 2

maciejt :: Int -> Bool
maciejt x = y == roundt y
    where y = sumt x

maciej :: [Int]
maciej = filter maciejt [1..]

main :: IO ()
main = mapM_ print maciej