tests for State and StateT:

src/Course/State.hs

@@ -67,8 +67,12 @@ instance Applicative (State s) where
     error "todo: Course.State (<*>)#instance (State s)"
 
 -- | Implement the `Bind` instance for `State s`.
+--
 -- >>> runState ((const $ put 2) =<< put 1) 0
 -- ((),2)
+--
+-- >>> let modify f = State (\s -> ((), f s)) in runState (modify (+1) >>= \() -> modify (*2)) 7
+-- ((),16)
 instance Monad (State s) where
   (=<<) ::
     (a -> State s b)

src/Course/StateT.hs

@@ -77,6 +77,9 @@ instance Monad f => Applicative (StateT s f) where
 --
 -- >>> runStateT ((const $ putT 2) =<< putT 1) 0
 -- ((),2)
+--
+-- >>> let modify f = StateT (\s -> pure ((), f s)) in runStateT (modify (+1) >>= \() -> modify (*2)) 7
+-- ((),16)
 instance Monad f => Monad (StateT s f) where
   (=<<) ::
     (a -> StateT s f b)