diagonals.hs

import Control.Monad ((<=<))
import Data.Maybe (catMaybes, listToMaybe)
import Safe 

xss = [
    [1,2,3],
    [4,5,6],
    [7,8,9]
    ]

xss' = [
    [1,2,3,3],
    [4,5,6,6],
    [7,8,9,9]
    ]

xss2 = [
    [1,2,3],
    [4,5,6],
    [7,8,9],
    [70,80,90]
    ]


getDiag xs = catMaybes $ zipWith ($) getters xs
    where
        getters = iterate (<=< tailMay) listToMaybe
        
getAllSomeDir lifter xss@(xs:_) = [lifter f xss | f <- getters]
    where
        getters = zipWith const (iterate (. tail) id) xss


getAllOneWay :: [[a]] -> [[[a]]]
getAllOneWay xss = do
    rightCombs <- getAllSomeDir fmap xss
    allCombs <- getAllSomeDir id rightCombs
    return allCombs

getAllBothWays xss = getAllOneWay xss ++ getAllOneWay reved
    where
        reved = fmap reverse xss

getAllDiagonals = fmap getDiag . getAllBothWays

main = do

    putStrLn "\n"
    mapM_ print $ getAllDiagonals xss
    
    putStrLn "\n"
    mapM_ print $ getAllDiagonals xss2

    putStrLn "\n"
    mapM_ print $ getAllDiagonals xss'



    -- [ | _ <- head xss, ]