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{-# OPTIONS_GHC -Wall #-} ------------------------------------------------------------------------------- {- | Module : Control.MonadOr Description : MonadOr definition. Copyright : (c) Paulo Silva License : LGPL Maintainer : paufil@di.uminho.pt Stability : experimental Portability : portable Monad definition satisfying the Monoid and Left Catch laws as refered in <http://www.haskell.org/haskellwiki/MonadPlus_reform_proposal> -} ------------------------------------------------------------------------------- module Control.MonadOr ( MonadOr, mzero, morelse ) where import Control.Monad.Error import Control.Monad.State ------------------------------------------------------------------------------- class MonadPlus m => MonadOr m where morelse :: m a -> m a -> m a ------------------------------------------------------------------------------- instance MonadOr [] where morelse [] b = b morelse a _ = a ------------------------------------------------------------------------------- -- This instance is equal to mplus for maybe instance MonadOr Maybe where morelse = mplus ------------------------------------------------------------------------------- instance (Monad m, Error e) => MonadOr (ErrorT e m) where morelse = mplus ------------------------------------------------------------------------------- instance MonadOr m => MonadOr (StateT s m) where m `morelse` n = StateT $ \s -> runStateT m s `morelse` runStateT n s ------------------------------------------------------------------------------- |