Can't find error in code (Project Euler #11 (Haskell))

Question

I can't seem to find out why my code isn't working. I think I'm correct and all the examples I've seen do the same thing I do. But when I execute it, I get a different answer. Could someone please help me in finding out what's wrong? (adj4 is my final answer)

type Grid = [[Integer]]

twentyGrid :: [Integer]
twentyGrid = [8,2,22,97,38,15,0,40,0,75,4,5,7,78,52,12,50,77,91,8,49,49,99,40,17,81,18,57,60,87,17,40,98,43,69,48,4,56,62,0,81,49,31,73,55,79,14,29,93,71,40,67,53,88,30,3,49,13,36,65,52,70,95,23,4,60,11,42,69,24,68,56,1,32,56,71,37,2,36,91,22,31,16,71,51,67,63,89,41,92,36,54,22,40,40,28,66,33,13,80,24,47,32,60,99,3,45,2,44,75,33,53,78,36,84,20,35,17,12,50,32,98,81,28,64,23,67,10,26,38,40,67,59,54,70,66,18,38,64,70,67,26,20,68,2,62,12,20,95,63,94,39,63,8,40,91,66,49,94,21,24,55,58,5,66,73,99,26,97,17,78,78,96,83,14,88,34,89,63,72,21,36,23,9,75,0,76,44,20,45,35,14,0,61,33,97,34,31,33,95,78,17,53,28,22,75,31,67,15,94,3,80,4,62,16,14,9,53,56,92,16,39,5,42,96,35,31,47,55,58,88,24,0,17,54,24,36,29,85,57,86,56,0,48,35,71,89,7,5,44,44,37,44,60,21,58,51,54,17,58,19,80,81,68,5,94,47,69,28,73,92,13,86,52,17,77,4,89,55,40,4,52,8,83,97,35,99,16,7,97,57,32,16,26,26,79,33,27,98,66,88,36,68,87,57,62,20,72,3,46,33,67,46,55,12,32,63,93,53,69,4,42,16,73,38,25,39,11,24,94,72,18,8,46,29,32,40,62,76,36,20,69,36,41,72,30,23,88,34,62,99,69,82,67,59,85,74,4,36,16,20,73,35,29,78,31,90,1,74,31,49,71,48,86,81,16,23,57,5,54,1,70,54,71,83,51,54,69,16,92,33,48,61,43,52,1,89,19,67,48]



rows :: Int -> [Integer] -> Grid --split grid into rows of length n
rows n [] = []
rows n xs = (take n xs):(rows n (drop n xs))

cols :: Int -> [Integer] -> Grid
cols n  = (transpose.rows n)

rowGrid :: Grid
rowGrid = rows 20 twentyGrid

colGrid :: Grid
colGrid = cols 20 twentyGrid

getDiag ::  Grid-> [Integer] --get a diagonal
getDiag [] = []
getDiag xss
    | head xss == [] = []
    | otherwise  = ((head.head) xss) : (getDiag ((map(drop 1).(drop 1)) xss))

adjac :: Int -> [Integer] -> [[Integer]] -- get all possible combinations of n numbers
adjac 0 xs = []
adjac n [] = []
adjac n (x:xs)
    | length (x:xs) > n = (take n (x:xs)) : (adjac n xs)
    | otherwise          = [x:xs]

diags :: Grid -> Grid
diags [] = []
diags (xs:xss)
    | (xs == []) = []
    | otherwise   =(getDiag (xs:xss)): (diags (map (drop 1) (xs:xss)))

upDiag :: Grid --get upper diaonals
upDiag = diags rowGrid

lowDiag :: Grid -- get lower diagonals
lowDiag = diags colGrid

allCells :: Grid --every diagonal column and row merged together
allCells = rowGrid ++ colGrid ++ upDiag ++ lowDiag

adj4 :: Integer --find largest product of 4 adjacent numbers
adj4 = (maximum.map maximum.map (map product).map (adjac 4)) allCells

testAdj :: [[Integer]]
testAdj = (map (map product).map (adjac 4)) allCells

Answer 1

It looks like you are handling the diagonals that go from upper left to lower right, both above the main diagonal ( upDiag ) and below the main diagnoal ( lowDiag ). However, you don't seem to be handling diagonals in the other direction. For example, if you look in the top-left corner of the grid:

08 02 22 97
49 49 99 40 ...
81 49 31 73
52 70 95 23
...

you have no code checking diagonals like the bolded one.