module Main where import Text.Read import System.Environment import System.Exit (exitWith, ExitCode(ExitFailure)) import Data.List (intercalate) mergeSort :: Ord a => [a] -> [a] mergeSort xs = head $ mergeSort' $ fmap (:) xs where mergeSort' :: Ord a => [[a]] -> [[a]] mergeSort'  =  mergeSort' [x] = [x] mergeSort' (x1:x2:xs) = mergeSort' $ merge x1 x2:mergeSort' xs merge :: Ord a => [a] -> [a] -> [a] merge x  = x merge  y = y merge (x:xs) (y:ys) | x < y = x:merge xs (y:ys) | otherwise = y:merge (x:xs) ys headMaybe :: [a] -> Maybe a headMaybe  = Nothing headMaybe (x:xs) = Just x -- Converts string in format "1, 2, 3" to a Maybe list of int stringToListMaybe :: String -> Maybe [Int] stringToListMaybe str = readMaybe $ "[" ++ str ++ "]" :: Maybe [Int] -- Ensure that a list contains at least two elements verifyListLength :: Ord a => [a] -> Maybe [a] verifyListLength  = Nothing verifyListLength [x] = Nothing verifyListLength (x:xs) = Just (x:xs) listToString :: [Int] -> String listToString = intercalate ", " . map show main :: IO () main = do args <- getArgs let xs = headMaybe args >>= stringToListMaybe >>= verifyListLength case xs of Nothing -> do putStrLn "Usage: please provide a list of at least two integers to sort in the format \"1, 2, 3, 4, 5\"" exitWith $ ExitFailure 1 Just xs -> putStrLn $ listToString $ mergeSort xs
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