Haskell non-exhaustive patterns in function with `otherwise`

Question

I am using the following function:

combinations :: Int -> [a] -> [[a]]
combinations k xs = combinations\' (length xs) k xs
  where combinations\' n k\' l@(         


        

Answer 1

Because of l@(x:xs) in combinations' n k' l@(y:ys) you're missing the case combinations _ _ [].

The guard null l will always be False.