Project Euler 78 (14s)

pr78.hs

import Data.Array

pentagonals :: [Int]
pentagonals = [ n*(3*n-1) `div` 2 | n <- pentagonalIndices ]
    where
        pentagonalIndices :: [Int]
        pentagonalIndices = concatMap (\x -> [x,-x]) [1..]

partition :: Array Int Integer
partition = listArray (0, 100000) $ 1 : 1 : map partition' [2..]
    where
        partition' n = sum (zipWith ($) (cycle [(partition !), (partition !), negate . (!) partition, negate . (!) partition]) $ indices n)
        indices :: Int -> [Int]
        indices n = takeWhile (>= 0) [n - x | x <- pentagonals]
    
main =
    print . fst . head . filter (\x -> snd x `mod` 1000000 == 0) . assocs $ partition