99 questions/Solutions/20

(*) Remove the K'th element from a list.

removeAt :: Int -> [a] -> (a, [a])
removeAt k xs = case back of
        [] -> error "removeAt: index too large"
        x:rest -> (x, front ++ rest)
  where (front, back) = splitAt (k - 1) xs

Simply use the splitAt to split after k - 1 elements. If the original list has fewer than k elements, the second list will be empty, and there will be no element to extract. Note that we treat 1 as the first element in the list.

or

removeAt n xs = (xs !! (n - 1), take (n - 1) xs ++ drop n xs)

or, a safe version of that

removeAt n xs | n > 0 && n <= length xs = (Just (xs !! index), take index xs ++ drop n xs)
              | otherwise = (Nothing, xs)
              where index = n - 1

or, a list comprehension based one:

removeAt :: (Enum a) => Int -> [a] -> (a, [a])
removeAt n xs = ((xs !! (n-1)), [ x | (i, x) <- zip [1..] xs, i /= n ])

Another solution that avoids throwing an error and using ++ operators. Treats 1 as the first element in the list.

removeAt :: Int -> [a] -> (Maybe a, [a])
removeAt _ [] = (Nothing, [])
removeAt 1 (x:xs) = (Just x, xs)
removeAt k (x:xs) = let (a, r) = removeAt (k - 1) xs in (a, x:r)

Another solution that also uses Maybe to indicate failure:

removeAt :: Int -> [a] -> (Maybe a, [a]) 
removeAt _ [] = (Nothing, [])
removeAt 0 xs = (Nothing, xs)
removeAt nr xs 	| nr > length xs = (Nothing, xs)
		| otherwise = (Just (xs !! nr), fst splitted ++ (tail . snd) splitted)
			where splitted = splitAt nr xs

And yet another solution (without error checking):

removeAt :: Int -> [a] -> (a, [a])
removeAt n xs = let (front, back) = splitAt n xs in (last front, init front ++ back)

Similar, point-free style:

removeAt n = (\(a, b) -> (head b, a ++ tail b)) . splitAt (n - 1)

A simple recursive solution:

removeAt 1 (x:xs) = (x, xs)
removeAt n (x:xs) = (l, x:r)
	where (l, r) = removeAt (n - 1) xs