{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE UndecidableInstances #-}

import Control.Monad.Error.Class
import Control.Monad.IO.Class
import Control.Monad.IO.Unlift
import Control.Exception (Exception, catch, throwIO)
import Control.Monad (liftM, ap)
import Data.Typeable (Typeable)

-- | A newtype wrapper that implements MonadError using IO exceptions
newtype ErrorT e m a = ErrorT { runErrorT :: m a }
  deriving (Functor, Applicative, Monad, MonadIO, MonadUnliftIO)

-- | Custom exception wrapper to hold our error type
data ErrorException e = ErrorException e
  deriving (Show, Eq, Typeable)

instance (Show e, Typeable e) => Exception (ErrorException e)

-- | MonadError instance that uses IO exceptions under the hood
instance (MonadUnliftIO m, Show e, Typeable e) => MonadError e (ErrorT e m) where
  throwError e = ErrorT $ liftIO $ throwIO (ErrorException e)
  
  catchError (ErrorT action) handler = ErrorT $ do
    withRunInIO $ \runInIO -> 
      runInIO action `catch` (\(ErrorException e) -> runInIO $ runErrorT $ handler e)

-- | Helper function to run an ErrorT computation
runErrorTSafe :: (MonadUnliftIO m, Show e, Typeable e) => ErrorT e m a -> m (Either e a)
runErrorTSafe (ErrorT action) = do
  withRunInIO $ \runInIO -> 
    (Right <$> runInIO action) `catch` (\(ErrorException e) -> return $ Left e)

-- | Example usage
example :: ErrorT String IO Int
example = do
  x <- return 10
  if x > 5
    then throwError "Number too large!"
    else return x

exampleWithCatch :: ErrorT String IO Int
exampleWithCatch = do
  result <- catchError example $ \err -> do
    liftIO $ putStrLn $ "Caught error: " ++ err
    return 0
  return result

-- | Test the implementation
main :: IO ()
main = do
  putStrLn "Testing MonadError implementation:"
  
  -- Test throwing an error
  result1 <- runErrorTSafe example
  putStrLn $ "Example result: " ++ show result1
  
  -- Test catching an error
  result2 <- runErrorTSafe exampleWithCatch
  putStrLn $ "Example with catch result: " ++ show result2