Сумма элементов побочной диагонали

matr11 :: [[Int]] -> Int -> [Int] -> Int
matr11 []_ = []
matr11(x:xs)n = seln x n : matr11 xs n 
 
seln :: [Int] -> Int ->Int
seln []_ = 0
seln (y:ys)h = if h==1 then y else seln ys(h-1)

diag2 ys = 
    nx (reverse ys)
    where
        nx ((x : _) : xs) = x : nx (map tail xs)
        nx _ = []

sumd2 matr =  sum $ map (\ n -> (matr !! n) !! (k-n-1)) [0..k-1] 
              where k=length matr

matr11 :: [[Int]] -> Int -> Int
matr11 [] _ = 0
matr11 (x:xs) n = seln x n + matr11 xs (n+1) 
 
seln :: [Int] -> Int ->Int
seln [] _ = 0
seln (y:ys) h = if h==1 then y else seln ys (h-1)
 
testdata = [[1,2,3,4,5],[10,20,30,40,50],[100,200,300,400,500],
            [1000,2000,3000,4000,5000],[10000,20000,30000,40000,50000]]
 
main = do
    -- Верхняя побочная диагональ
    print $  matr11 testdata 2
    -- Нижняя побочная диагональ
    print $  matr11 testdata 0

sumn2 matr =  sum $ map (\ n -> (matr !! n) !! (n+1)) [0..k-2] 
              where k=length matr

diag2 = fst . foldr (\ a (s,i) -> (s + a !! i,i+1)) (0,0)

sum (filter (>10) (map (*2) [2..10]))

sum $ filter (>10) $ map (*2) [2..10]

(++) "qwe" $ take 5 "asdghk"

("qwe" ++) $ take 5 "asdghk"