import Control.Exception
import Control.Monad.Trans.State
import Control.Monad.IO.Class
tryIO :: IO a -> IO (Either IOException a)
tryIO = try
foo :: StateT String IO Int
foo = do
s <- get
liftIO $ readIO s
tryState :: StateT String IO a -> StateT String IO (Either IOException a)
tryState (StateT f) = StateT $ \s0 -> do
eres <- tryIO $ f s0
return $ case eres of
Left e -> (Left e, s0)
Right (x, s1) -> (Right x, s1)
main :: IO ()
main = flip evalStateT "123" $ do
let go = tryState foo >>= liftIO . print
go
put "456"
go
put "hello"
go
put "789"
go
