https://youtu.be/Cxkqrg8FCt8

solution.hs

elem :: (Eq a) => a -> [a] -> Bool
elem _ [] = False
elem e (x:xs) = (e == x) || (elem e xs)

nub :: (Eq a) => [a] -> [a]
nub [] = []
nub (x:xs)
 | x `elem` xs = nub xs
 | otherwise   = x : nub xs
 
isAsc :: [Int] -> Bool
isAsc [] = True
isAsc [x] = True
isAsc (x:y:xs) = (x <= y) && isAsc (y:xs)

hasPath :: [(Int,Int)] -> Int -> Int -> Bool
hasPath [] x y = x == y
hasPath xs x y
 | x == y    = True
 | otherwise =
  let xs' = [ (n,m) | (n,m) <- xs, n /= x ] in 
  or [ hasPath xs' m y | (n,m) <- xs, n == x ]