Alternative for MonadBaseControl IO

gistfile1.hs

{-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances #-}

import Control.Applicative
import Control.Exception.Lifted
import Control.Monad.State
import Control.Monad.Trans
import Control.Monad.Trans.Control

import Prelude hiding (catch)

instance (MonadBaseControl IO m) => Alternative m where
  -- empty :: m a
  empty = throwIO $ userError "empty IO"
  -- (<|>) :: m a -> m a -> m a
  a <|> b = a `catch` (\(SomeException _) -> b)

main :: IO ()
main = do
  undefined <|> putStrLn "hello"

  (`evalStateT` ()) $ do
    undefined <|> liftIO (putStrLn "world")