modify now returns the new state too

Author: garyb

src/Control/Monad/State/Class.purs

@@ -36,6 +36,7 @@ gets f = state \s -> Tuple (f s) s
 put :: forall m s. MonadState s m => s -> m Unit
 put s = state \_ -> Tuple unit s
 
--- | Modify the state by applying a function to the current state.
-modify :: forall s m. MonadState s m => (s -> s) -> m Unit
-modify f = state \s -> Tuple unit (f s)
+-- | Modify the state by applying a function to the current state. The returned
+-- | value is the new state value.
+modify :: forall s m. MonadState s m => (s -> s) -> m s
+modify f = state \s -> let s' = f s in Tuple s' s'

test/Example/State.purs

@@ -8,7 +8,7 @@ import Effect (Effect)
 import Effect.Console (log)
 
 incState :: State Int Unit
-incState = modify (_ + 1)
+incState = void $ modify (_ + 1)
 
 testState :: State Int String
 testState = do

test/Example/StateEff.purs

@@ -23,7 +23,7 @@ pop = unsafePartial do
 push :: Int -> Stack Unit
 push x = do
   lift $ log $ "Pushing " <> show x
-  modify $ (:) x
+  _ <- modify (x : _)
   pure unit
 
 testState :: Stack Int