P18.hs

triangle :: [[Int]]
triangle = [[75],
            [95,64],
            [17,47,82],
            [18,35,87,10],
            [20,04,82,47,65],
            [19,01,23,75,03,34],
            [88,02,77,73,07,63,67],
            [99,65,04,28,06,16,70,92],
            [41,41,26,56,83,40,80,70,33],
            [41,48,72,33,47,32,37,16,94,29],
            [53,71,44,65,25,43,91,52,97,51,14],
            [70,11,33,28,77,73,17,78,39,68,17,57],
            [91,71,52,38,17,14,91,43,58,50,27,29,48],
            [63,66,04,68,89,53,67,30,73,16,69,87,40,31],
            [04,62,98,27,23,09,70,98,73,93,38,53,60,04,23]]

calcRow :: [Int] -> [Int] -> [Int]
calcRow maxes = calcRow' (Nothing : map Just maxes)
  where
    calcRow' (Nothing:Just m:ms) (r:rs) = (m+r) : calcRow' (Just m:ms) rs
    calcRow' (Just m1:Just m2:ms) (r:rs) = max (m1+r) (m2+r) : calcRow' (Just m2:ms) rs
    calcRow' [Just m] [r] = [m+r]

totals :: [Int]
totals = foldl1 calcRow triangle

answer :: Int
answer = maximum totals

main :: IO ()
main = putStrLn . show $ answer