正确遵循 class 但代码不被接受

Question

I want it to show the desired n final numbers, but the GHCi is giving an error in the code

nLast :: int -> [a] -> [a]
nLast 0 _ = []
nLast _ [] = []
nLast n (x:xs) = nultimos (n-1) xs

Answer 1

You can work with two enumerators over the list. You give one of the enumeraters a head start of n items and each time you let both the enumerators make one hope. If the first runner reaches the empty list, then we know that the second runner has n items they still need to enumerate over.

We thus can implement this with:

nLast :: Int -> [a] -> [a]
nLast n ls = go (drop n ls) ls
    where go (_:xs) (_:ys) = …
          go [] ys = …

here you still need to fill in the … parts. I leave this as an exercise.

In the above example there is a pattern that we do not cover, a pattern where the first list is a non-empty list (_:_) and the second is empty [] . We know that this can never happen, it might still be better however to add a clause for this.