add Monoid instance (#115)

src/Control/Monad/Cont/Trans.purs

@@ -8,6 +8,7 @@ module Control.Monad.Cont.Trans
 
 import Prelude
 
+import Control.Apply (lift2)
 import Control.Monad.Cont.Class (class MonadCont, callCC)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader, ask, local)
 import Control.Monad.State.Class (class MonadState, state)
@@ -67,3 +68,10 @@ instance monadReaderContT :: MonadReader r1 m => MonadReader r1 (ContT r m) wher
 
 instance monadStateContT :: MonadState s m => MonadState s (ContT r m) where
   state = lift <<< state
+
+instance semigroupContT :: (Apply m, Semigroup a) => Semigroup (ContT r m a) where
+  append = lift2 (<>)
+
+instance monoidContT :: (Applicative m, Monoid a) => Monoid (ContT r m a) where
+  mempty = pure mempty
+

src/Control/Monad/Except/Trans.purs

@@ -10,6 +10,7 @@ import Prelude
 
 import Control.Alt (class Alt)
 import Control.Alternative (class Alternative)
+import Control.Apply (lift2)
 import Control.Monad.Cont.Class (class MonadCont, callCC)
 import Control.Monad.Error.Class (class MonadThrow, class MonadError, throwError, catchError)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader, ask, local)
@@ -136,3 +137,10 @@ instance monadWriterExceptT :: MonadWriter w m => MonadWriter w (ExceptT e m) wh
     pure case a of
       Left e -> Tuple (Left e) identity
       Right (Tuple r f) -> Tuple (Right r) f
+
+instance semigroupExceptT :: (Monad m, Semigroup a) => Semigroup (ExceptT e m a) where
+  append = lift2 (<>)
+
+instance monoidExceptT :: (Monad m, Monoid a) => Monoid (ExceptT e m a) where
+  mempty = pure mempty
+

src/Control/Monad/Maybe/Trans.purs

@@ -9,6 +9,7 @@ import Prelude
 
 import Control.Alt (class Alt)
 import Control.Alternative (class Alternative)
+import Control.Apply (lift2)
 import Control.Monad.Cont.Class (class MonadCont, callCC)
 import Control.Monad.Error.Class (class MonadThrow, class MonadError, catchError, throwError)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader, ask, local)
@@ -121,3 +122,10 @@ instance monadWriterMaybeT :: MonadWriter w m => MonadWriter w (MaybeT m) where
     pure case a of
       Nothing -> Tuple Nothing identity
       Just (Tuple v f) -> Tuple (Just v) f
+
+instance semigroupMaybeT :: (Monad m, Semigroup a) => Semigroup (MaybeT m a) where
+  append = lift2 (<>)
+
+instance monoidMaybeT :: (Monad m, Monoid a) => Monoid (MaybeT m a) where
+  mempty = pure mempty
+

src/Control/Monad/RWS/Trans.purs

@@ -10,6 +10,7 @@ import Prelude
 
 import Control.Alt (class Alt, (<|>))
 import Control.Alternative (class Alternative)
+import Control.Apply (lift2)
 import Control.Lazy (class Lazy)
 import Control.Monad.Error.Class (class MonadThrow, class MonadError, throwError, catchError)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader)
@@ -130,3 +131,10 @@ instance monadRecRWST :: (MonadRec m, Monoid w) => MonadRec (RWST r w s m) where
 
 instance plusRWST :: Plus m => Plus (RWST r w s m) where
   empty = RWST \ _ _ -> empty
+
+instance semigroupRWST :: (Bind m, Monoid w, Semigroup a) => Semigroup (RWST r w s m a) where
+  append = lift2 (<>)
+
+instance monoidRWST :: (Monad m, Monoid w, Monoid a) => Monoid (RWST r w s m a) where
+  mempty = pure mempty
+

src/Control/Monad/State/Trans.purs

@@ -10,6 +10,7 @@ import Prelude
 
 import Control.Alt (class Alt, (<|>))
 import Control.Alternative (class Alternative)
+import Control.Apply (lift2)
 import Control.Lazy (class Lazy)
 import Control.Monad.Cont.Class (class MonadCont, callCC)
 import Control.Monad.Error.Class (class MonadThrow, class MonadError, catchError, throwError)
@@ -137,3 +138,10 @@ instance monadWriterStateT :: MonadWriter w m => MonadWriter w (StateT s m) wher
       StateT m' -> do
         Tuple (Tuple a f) s' <- m' s
         pure $ Tuple (Tuple a s') f
+
+instance semigroupStateT :: (Monad m, Semigroup a) => Semigroup (StateT s m a) where
+  append = lift2 (<>)
+
+instance monoidStateT :: (Monad m, Monoid a) => Monoid (StateT s m a) where
+  mempty = pure mempty
+

src/Control/Monad/Writer/Trans.purs

@@ -10,6 +10,7 @@ import Prelude
 
 import Control.Alt (class Alt, (<|>))
 import Control.Alternative (class Alternative)
+import Control.Apply (lift2)
 import Control.Monad.Cont.Class (class MonadCont, callCC)
 import Control.Monad.Error.Class (class MonadThrow, class MonadError, catchError, throwError)
 import Control.Monad.Reader.Class (class MonadAsk, class MonadReader, ask, local)
@@ -125,3 +126,10 @@ instance monadWriterWriterT :: (Monoid w, Monad m) => MonadWriter w (WriterT w m
   pass (WriterT m) = WriterT do
     Tuple (Tuple a f) w <- m
     pure $ Tuple a (f w)
+
+instance semigroupWriterT :: (Apply m, Semigroup w, Semigroup a) => Semigroup (WriterT w m a) where
+  append = lift2 (<>)
+
+instance monoidWriterT :: (Applicative m, Monoid w, Monoid a) => Monoid (WriterT w m a) where
+  mempty = pure mempty
+