cli/Hcompta/Lib/Data/Monoid.hs

   1 {-# LANGUAGE FlexibleInstances #-}
   2 {-# OPTIONS_GHC -fno-warn-tabs #-}
   3 {-# OPTIONS_GHC -fno-warn-orphans #-}
   4 module Hcompta.Lib.Data.Monoid where
   5
   6 import Control.Monad (Monad(..), liftM2)
   7 import Control.Monad.Trans.State.Strict as ST
   8 import Data.Monoid (Monoid(..))
   9 import qualified Hcompta.Lib.Control.Monad.Classes as MC
  10 import System.IO (IO)
  11
  12 -- * Class 'Monoid1' (contain orphan instances)
  13
  14 -- | 'Monoid' lifted to unary type constructor.
  15 class Monoid1 expr where
  16         mempty1  :: Monoid a => expr a
  17         mappend1 :: Monoid a => expr a -> expr a -> expr a
  18 instance Monoid1 IO where
  19         mempty1  = mempty
  20         mappend1 = mappend
  21 instance (Monoid1 m, Monad m) => Monoid1 (ST.StateT s m) where
  22         mempty1  = mempty
  23         mappend1 = mappend
  24 instance (Monad m, Monoid a) => Monoid (ST.StateT s m a) where
  25         mempty  = return mempty
  26         mappend = liftM2 mappend
  27 instance Monoid a => Monoid (IO a) where
  28         mempty  = return mempty
  29         mappend = liftM2 mappend
  30 instance (Monoid1 m, Monad m) => Monoid1 (MC.WriterT w m) where
  31         mempty1  = mempty
  32         mappend1 = mappend
  33 instance (Monad m, Monoid a) => Monoid (MC.WriterT w m a) where
  34         mempty  = return mempty
  35         mappend = liftM2 mappend
  36