monad-stm: added Semigroup and Monoid instances

Author: coot

io-classes/src/Control/Monad/Class/MonadSTM/Internal.hs

@@ -1319,6 +1319,12 @@ deriving instance MonadSTM m => Monad       (WrappedSTM t r m)
 deriving instance MonadSTM m => Alternative (WrappedSTM t r m)
 deriving instance MonadSTM m => MonadPlus   (WrappedSTM t r m)
 
+instance ( Semigroup a, MonadSTM m ) => Semigroup (WrappedSTM t r m a) where
+    a <> b = (<>) <$> a <*> b
+instance ( Monoid a , MonadSTM m ) => Monoid (WrappedSTM t r m a) where
+    mempty = pure mempty
+
+
 -- note: this (and the following) instance requires 'UndecidableInstances'
 -- extension because it violates 3rd Paterson condition, however `STM m` will
 -- resolve to a concrete type of kind (Type -> Type), and thus no larger than

io-sim/src/Control/Monad/IOSim/Types.hs

@@ -178,6 +178,12 @@ data SimA s a where
 
 newtype STM s a = STM { unSTM :: forall r. (a -> StmA s r) -> StmA s r }
 
+instance Semigroup a => Semigroup (STM s a) where
+    a <> b = (<>) <$> a <*> b
+
+instance Monoid a => Monoid (STM s a) where
+    mempty = pure mempty
+
 runSTM :: STM s a -> StmA s a
 runSTM (STM k) = k ReturnStm