module ListHelp 
    ( module Data.List
    , initOrNull
    , insertOver
    , swapIndices
    , (!?)
    )where
import Data.List

initOrNull :: [a] -> [a]
initOrNull [] = []
initOrNull xs = init xs

insertOver :: Int -> a -> [a] -> [a]
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 (!?) #-}