Implement combining of items

src/ListHelp.hs

@@ -3,6 +3,7 @@ module ListHelp
     ,initOrNull
     , insertOver
     , swapIndices
+    , (!?)
     )where
 import Data.List
 
@@ -15,3 +16,18 @@ insertOver i x xs = take i xs ++ x : drop (i+1) xs
 
 swapIndices :: Int -> Int -> [a] -> [a]
 swapIndices i j xs = insertOver i (xs !! j) . insertOver j (xs !! i) $ xs
+
+--copied from Data.List.Extra:
+-- | A total variant of the list index function `(!!)`.
+--
+-- > [2,3,4] !? 1    == Just 3
+-- > [2,3,4] !? (-1) == Nothing
+-- > []      !? 0    == Nothing
+(!?) :: [a] -> Int -> Maybe a
+xs !? n
+  | n < 0     = Nothing
+             -- Definition adapted from GHC.List
+  | otherwise = foldr (\x r k -> case k of
+                                   0 -> Just x
+                                   _ -> r (k-1)) (const Nothing) xs n
+{-# INLINABLE (!?) #-}