SafeListAccess.hs

data BoundsError = BoundsError { listLength :: Int, attemptedToGet :: Int } deriving Show

getItem :: Int -> Int -> [a] -> Either a BoundsError
getItem traversedLength tryingToGet [] = Right $ BoundsError traversedLength (tryingToGet + traversedLength)
getItem _ 0 (x:_) = Left x
getItem traversedLength tryingToGet (x:xs) = getItem (traversedLength + 1) (tryingToGet - 1) xs

main :: IO ()
main = do
  print $ getItem 0 50 [0..10]
  print [0..10]