MyState homework solution

MyState.hs

data MyState s a = MyState (s -> (a,s))

get :: MyState s s
get = MyState (\s -> (s, s))

put :: s -> MyState s ()
put s = MyState (\_ -> ((), s))

modify :: (s -> s) -> MyState s ()
modify f = MyState (\s -> ((), f s))

instance Functor (MyState s) where
  -- Why did they put (MyState s) a instead of MyState s a?
  -- fmap :: (a -> b) -> (MyState s) a -> (MyState s) b
  fmap f (MyState g) = MyState $ \s ->
    let (x, s') = g s
        y       = f x
    in  (y, s')

instance Applicative (MyState s) where
  pure x = MyState $ \s -> (x, s)

  -- (<*>) :: (MyState s) (a->b) -> (MyState s) a -> (MyState s) b
  (<*>) (MyState f) (MyState g) = MyState $ \s ->
    let (f', s' ) = f s
        (a , s'') = g s'
    in  (f' a, s'')


instance Monad (MyState s) where
  return = pure

  -- (>>=) :: (MyState s) a -> (a-> (MyState s) b) -> (MyState s) b
  (>>=) (MyState f) f' = MyState $ \s ->
    let (a, s')    = f s
        MyState g' = f' a
        (b, s'')   = g' s'
    in  (b, s'')


main = print "Hello World!"

MyStateT.hs

data MyStateT s m a = MyStateT (s -> m (a, s))
 
get :: Monad m => MyStateT s m s
get = undefined
 
put :: Monad m => s -> MyStateT s m ()
put = undefined
 
modify :: Monad m => (s -> s) -> MyStateT s m ()
modify = undefined
 
instance Monad m => Functor (MyStateT s m) where
    -- fmap :: (a -> b) -> (MyStateT s m) a -> (MyStateT s m) b
    fmap = undefined
    
instance Monad m => Applicative (MyStateT s m) where
    -- pure :: a -> (MyStateT s m) a
    pure = undefined
    
    -- (<*>) :: (MyStateT s m) (a -> b) -> (MyStateT s m) a -> (MyStateT s m) b
    (<*>) = undefined
 
instance Monad m => Monad (MyStateT s m) where
    -- return :: a -> (MyStateT s m) a
    return = pure
    
    -- (>>=) :: (MyStateT s m) a -> (a -> (MyStateT s m) b) -> (MyStateT s m) b
    (>>=) = undefined
    
instance MonadTrans (MyStateT s) where
    -- lift :: Monad m => m a -> MyStateT s m a
    lift = undefined