Monoid instances

Control/Applicative/Backwards.hs

@@ -143,3 +143,17 @@ instance (Contravariant f) => Contravariant (Backwards f) where
     contramap f = Backwards . contramap f . forwards
     {-# INLINE contramap #-}
 #endif
+
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Applicative f) => Semigroup (Backwards f a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Applicative f) => Monoid (Backwards f a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif

Control/Applicative/Lift.hs

@@ -128,6 +128,20 @@ instance (Foldable1 f) => Foldable1 (Lift f) where
     {-# INLINE foldMap1 #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Applicative f) => Semigroup (Lift f a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Applicative f) => Monoid (Lift f a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Projection to the other functor.
 unLift :: (Applicative f) => Lift f a -> f a
 unLift (Pure x) = pure x

Control/Monad/Trans/Accum.hs

@@ -247,6 +247,20 @@ instance (Monoid w, Functor m, MonadIO m) => MonadIO (AccumT w m) where
     liftIO = lift . liftIO
     {-# INLINE liftIO #-}
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monoid w, Monad m) => Semigroup (AccumT w m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monoid w, Monad m) => Monoid (AccumT w m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | @'look'@ is an action that fetches all the previously accumulated output.
 look :: (Monoid w, Monad m) => AccumT w m w
 look = AccumT $ \ w -> return (w, mempty)

Control/Monad/Trans/Cont.hs

@@ -206,6 +206,20 @@ instance (MonadIO m) => MonadIO (ContT r m) where
     liftIO = lift . liftIO
     {-# INLINE liftIO #-}
 
+#if MIN_VERSION_base(4,9,0)
+instance Semigroup a => Semigroup (ContT r m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance Monoid a => Monoid (ContT r m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | @callCC@ (call-with-current-continuation) calls its argument
 -- function, passing it the current continuation.  It provides
 -- an escape continuation mechanism for use with continuation

Control/Monad/Trans/Except.hs

@@ -281,6 +281,20 @@ instance Contravariant m => Contravariant (ExceptT e m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (ExceptT e m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (ExceptT e m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Signal an exception value @e@.
 --
 -- * @'runExceptT' ('throwE' e) = 'return' ('Left' e)@

Control/Monad/Trans/Identity.hs

@@ -185,6 +185,20 @@ instance (Contravariant f) => Contravariant (IdentityT f) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Applicative m) => Semigroup (IdentityT m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Applicative m) => Monoid (IdentityT m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Lift a unary operation to the new monad.
 mapIdentityT :: (m a -> n b) -> IdentityT m a -> IdentityT n b
 mapIdentityT f = IdentityT . f . runIdentityT

Control/Monad/Trans/Maybe.hs

@@ -229,6 +229,20 @@ instance Contravariant m => Contravariant (MaybeT m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (MaybeT m a) where
+  ma <> mb = (<>) <$> ma <*> mb
+  {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (MaybeT m a) where
+  mempty = pure mempty
+  {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Lift a @callCC@ operation to the new monad.
 liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
 liftCallCC callCC f =

Control/Monad/Trans/RWS/CPS.hs

@@ -272,6 +272,21 @@ instance MonadTrans (RWST r w s) where
 instance (MonadIO m) => MonadIO (RWST r w s m) where
     liftIO = lift . liftIO
     {-# INLINE liftIO #-}
+
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (RWST r w s m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (RWST r w s m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- ---------------------------------------------------------------------------
 -- Reader operations
 

Control/Monad/Trans/RWS/Lazy.hs

@@ -253,6 +253,20 @@ instance Contravariant m => Contravariant (RWST r w s m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monoid w, Monad m) => Semigroup (RWST r w s m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monoid w, Monad m) => Monoid (RWST r w s m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- ---------------------------------------------------------------------------
 -- Reader operations
 

Control/Monad/Trans/RWS/Strict.hs

@@ -257,6 +257,16 @@ instance Contravariant m => Contravariant (RWST r w s m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monoid w, Monad m) => Semigroup (RWST r w s m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+
+instance (Monoid a, Monoid w, Monad m) => Monoid (RWST r w s m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#endif
+
 -- ---------------------------------------------------------------------------
 -- Reader operations
 

Control/Monad/Trans/Reader.hs

@@ -236,6 +236,20 @@ instance Contravariant m => Contravariant (ReaderT r m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Applicative m) => Semigroup (ReaderT r m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Applicative m) => Monoid (ReaderT r m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 liftReaderT :: m a -> ReaderT r m a
 liftReaderT m = ReaderT (const m)
 {-# INLINE liftReaderT #-}

Control/Monad/Trans/Select.hs

@@ -154,6 +154,20 @@ instance (MonadIO m) => MonadIO (SelectT r m) where
     liftIO = lift . liftIO
     {-# INLINE liftIO #-}
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (SelectT r m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (SelectT r m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Convert a selection computation to a continuation-passing computation.
 selectToContT :: (Monad m) => SelectT r m a -> ContT r m a
 selectToContT (SelectT g) = ContT $ \ k -> g k >>= k

Control/Monad/Trans/State/Lazy.hs

@@ -277,6 +277,20 @@ instance Contravariant m => Contravariant (StateT s m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (StateT s m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (StateT s m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | Fetch the current value of the state within the monad.
 get :: (Monad m) => StateT s m s
 get = state $ \ s -> (s, s)

Control/Monad/Trans/State/Strict.hs

@@ -270,6 +270,16 @@ instance Contravariant m => Contravariant (StateT s m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (StateT s m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+
+instance (Monoid a, Monad m) => Monoid (StateT s m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#endif
+
 -- | Fetch the current value of the state within the monad.
 get :: (Monad m) => StateT s m s
 get = state $ \ s -> (s, s)

Control/Monad/Trans/Writer/CPS.hs

@@ -228,6 +228,20 @@ instance (MonadIO m) => MonadIO (WriterT w m) where
     liftIO = lift . liftIO
     {-# INLINE liftIO #-}
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monad m) => Semigroup (WriterT w m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monad m) => Monoid (WriterT w m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | @'tell' w@ is an action that produces the output @w@.
 tell :: (Monoid w, Monad m) => w -> WriterT w m ()
 tell w = writer ((), w)

Control/Monad/Trans/Writer/Lazy.hs

@@ -265,6 +265,20 @@ instance Contravariant m => Contravariant (WriterT w m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monoid w, Applicative m) => Semigroup (WriterT w m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monoid w, Applicative m) => Monoid (WriterT w m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | @'tell' w@ is an action that produces the output @w@.
 tell :: (Monad m) => w -> WriterT w m ()
 tell w = writer ((), w)

Control/Monad/Trans/Writer/Strict.hs

@@ -268,6 +268,20 @@ instance Contravariant m => Contravariant (WriterT w m) where
     {-# INLINE contramap #-}
 #endif
 
+#if MIN_VERSION_base(4,9,0)
+instance (Semigroup a, Monoid w, Applicative m) => Semigroup (WriterT w m a) where
+    ma <> mb = (<>) <$> ma <*> mb
+    {-# INLINE (<>) #-}
+#endif
+
+instance (Monoid a, Monoid w, Applicative m) => Monoid (WriterT w m a) where
+    mempty = pure mempty
+    {-# INLINE mempty #-}
+#if !MIN_VERSION_base(4,11,0)
+    ma `mappend` mb = (<>) <$> ma <*> mb
+    {-# INLINE mappend #-}
+#endif
+
 -- | @'tell' w@ is an action that produces the output @w@.
 tell :: (Monad m) => w -> WriterT w m ()
 tell w = writer ((), w)