Merge pull request #61 from xuwei-k/WriterT-Semigroup

relax WriterT instance constraints

docs/Control/Monad/Writer/Trans.md

@@ -19,12 +19,12 @@ The `MonadWriter` type class describes the operations supported by this monad.
 ##### Instances
 ``` purescript
 instance functorWriterT :: (Functor m) => Functor (WriterT w m)
-instance applyWriterT :: (Monoid w, Apply m) => Apply (WriterT w m)
+instance applyWriterT :: (Semigroup w, Apply m) => Apply (WriterT w m)
 instance applicativeWriterT :: (Monoid w, Applicative m) => Applicative (WriterT w m)
-instance altWriterT :: (Monoid w, Alt m) => Alt (WriterT w m)
-instance plusWriterT :: (Monoid w, Plus m) => Plus (WriterT w m)
+instance altWriterT :: (Alt m) => Alt (WriterT w m)
+instance plusWriterT :: (Plus m) => Plus (WriterT w m)
 instance alternativeWriterT :: (Monoid w, Alternative m) => Alternative (WriterT w m)
-instance bindWriterT :: (Monoid w, Monad m) => Bind (WriterT w m)
+instance bindWriterT :: (Semigroup w, Monad m) => Bind (WriterT w m)
 instance monadWriterT :: (Monoid w, Monad m) => Monad (WriterT w m)
 instance monadRecWriterT :: (Monoid w, MonadRec m) => MonadRec (WriterT w m)
 instance monadPlusWriterT :: (Monoid w, MonadPlus m) => MonadPlus (WriterT w m)

src/Control/Monad/Writer/Trans.purs

@@ -48,23 +48,23 @@ mapWriterT f m = WriterT $ f (runWriterT m)
 instance functorWriterT :: (Functor m) => Functor (WriterT w m) where
   map f = mapWriterT $ (<$>) \(Tuple a w) -> Tuple (f a) w
 
-instance applyWriterT :: (Monoid w, Apply m) => Apply (WriterT w m) where
+instance applyWriterT :: (Semigroup w, Apply m) => Apply (WriterT w m) where
   apply f v = WriterT $
     let k (Tuple a w) (Tuple b w') = Tuple (a b) (w <> w')
     in k <$> (runWriterT f) <*> (runWriterT v)
 
 instance applicativeWriterT :: (Monoid w, Applicative m) => Applicative (WriterT w m) where
   pure a = WriterT $ pure $ Tuple a mempty
 
-instance altWriterT :: (Monoid w, Alt m) => Alt (WriterT w m) where
+instance altWriterT :: (Alt m) => Alt (WriterT w m) where
   alt m n = WriterT $ runWriterT m <|> runWriterT n
 
-instance plusWriterT :: (Monoid w, Plus m) => Plus (WriterT w m) where
+instance plusWriterT :: (Plus m) => Plus (WriterT w m) where
   empty = WriterT empty
 
 instance alternativeWriterT :: (Monoid w, Alternative m) => Alternative (WriterT w m)
 
-instance bindWriterT :: (Monoid w, Monad m) => Bind (WriterT w m) where
+instance bindWriterT :: (Semigroup w, Monad m) => Bind (WriterT w m) where
   bind m k  = WriterT $ do
     Tuple a w <- runWriterT m
     Tuple b w' <- runWriterT (k a)