MonadError.hs

#line 117 "src/util.anansi"

#line 30 "src/introduction.anansi"
-- Copyright (C) 2009-2010 John Millikin <jmillikin@gmail.com>
-- 
-- This program is free software: you can redistribute it and/or modify
-- it under the terms of the GNU General Public License as published by
-- the Free Software Foundation, either version 3 of the License, or
-- any later version.
-- 
-- This program is distributed in the hope that it will be useful,
-- but WITHOUT ANY WARRANTY; without even the implied warranty of
-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-- GNU General Public License for more details.
-- 
-- You should have received a copy of the GNU General Public License
-- along with this program.  If not, see <http://www.gnu.org/licenses/>.

#line 118 "src/util.anansi"
{-# LANGUAGE TypeFamilies #-}
module DBus.Util.MonadError
	( ErrorT (..)
	, throwError
	) where
import Control.Monad.Trans.Class
import Control.Monad.State
import Control.Monad.Error.Class

newtype ErrorT e m a = ErrorT { runErrorT :: m (Either e a) }

instance Functor m => Functor (ErrorT e m) where
	fmap f = ErrorT . fmap (fmap f) . runErrorT

instance Monad m => Monad (ErrorT e m) where
	return = ErrorT . return . Right
	(>>=) m k = ErrorT $ do
		x <- runErrorT m
		case x of
			Left l -> return $ Left l
			Right r -> runErrorT $ k r

instance MonadTrans (ErrorT e) where
	lift = ErrorT . liftM Right

instance Monad m => MonadError (ErrorT e m) where
	type ErrorType (ErrorT e m) = e
	throwError = ErrorT . return . Left
	catchError m h = ErrorT $ do
		x <- runErrorT m
		case x of
			Left l -> runErrorT $ h l
			Right r -> return $ Right r

instance MonadState m => MonadState (ErrorT e m) where
	type StateType (ErrorT e m) = StateType m
	get = lift get
	put = lift . put