{-# LANGUAGE FlexibleInstances #-}
{-# OPTIONS_GHC -fno-warn-tabs #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}
module Hcompta.Lib.Data.Monoid where

import Control.Monad (Monad(..), liftM2)
import Control.Monad.Trans.State.Strict as ST
import Data.Monoid (Monoid(..))
import qualified Hcompta.Lib.Control.Monad.Classes as MC
import System.IO (IO)

-- * Class 'Monoid1' (contain orphan instances)

-- | 'Monoid' lifted to unary type constructor.
class Monoid1 expr where
	mempty1  :: Monoid a => expr a
	mappend1 :: Monoid a => expr a -> expr a -> expr a
instance Monoid1 IO where
	mempty1  = mempty
	mappend1 = mappend
instance (Monoid1 m, Monad m) => Monoid1 (ST.StateT s m) where
	mempty1  = mempty
	mappend1 = mappend
instance (Monad m, Monoid a) => Monoid (ST.StateT s m a) where
	mempty  = return mempty
	mappend = liftM2 mappend
instance Monoid a => Monoid (IO a) where
	mempty  = return mempty
	mappend = liftM2 mappend
instance (Monoid1 m, Monad m) => Monoid1 (MC.WriterT w m) where
	mempty1  = mempty
	mappend1 = mappend
instance (Monad m, Monoid a) => Monoid (MC.WriterT w m a) where
	mempty  = return mempty
	mappend = liftM2 mappend