Project Euler Problem 078

import Control.Monad.ST
import Data.List
import qualified Data.Array.Unboxed as U
import Data.Array.ST


dpf :: Int -> U.UArray Int Int
dpf n = runSTUArray (
        do{ a <- (newArray (0,n) 0) :: ST s (STUArray s Int Int);
            writeArray a 0 1;
            sequence_ [
              do{sa <- sequence $ [fmap (*sign)$ readArray a (i-p) | (p,sign) <- takeWhile((<=i).fst) pentaa1];
                 sb <- sequence $ [fmap (*sign)$ readArray a (i-p) | (p,sign) <- takeWhile((<=i).fst) pentaa2];
                 writeArray a i ((sum sa+sum sb)`mod` (10^(6::Integer)))}
                 | i<-[1..n]];
            return a}) where
  pentaa1 = zip (scanl1(+) [1,4..]) (cycle [1,-1])
  pentaa2 = zip (scanl1(+) [2,5..]) (cycle [1,-1])
ans1 :: Maybe (Int, Int)
ans1 =   find ((==0).snd) $ U.assocs $ dpf 100000
main :: IO ()
main = print ans1