Преобразователи Монад

В нескольких прошлых частях серии, мы изучили множество новых монад. В 3 части мы увидели как часто вещи как Maybe и IO могут быть монадами. Затем в 4 и 5 частях мы изучили Reader, Writer и State монады. С этими монадами на поясе, вы возмоно думаете как можно их объединять. Ответ, как мы обнаружи в этой части, это преобразователи монад.

С пониманием монад, вы открываете больше Haskell возможностей. Но вам всё ещё нужны идеи библиотек Haskell, который позволят вам их испытать.

Пример Мотивации

Ранее, мы уже видели как монада maybe помогает избежать треугольника судьбы шаблонов кода. Без них, нам нужно проверять каждую функцию на успех. Однако, примеры на которые мы смотрим, где всё является чистым кодом предполагает следующее:

main1 :: IO ()
main1 = do
  maybeUserName <- readUserName
  case maybeUserName of
    Nothing -> print "Invalid user name!"
    Just (uName) -> do
      maybeEmail <- readEmail
      case maybeEmail of
        Nothing -> print "Invalid email!"
        Just (email) -> do
          maybePassword <- readPassword
          Case maybePassword of
            Nothing -> print "Invalid Password"
            Just password -> login uName email password

readUserName :: IO (Maybe String)
readUserName = do
  putStrLn "Please enter your username!"
  str <- getLine
  if length str > 5
    then return $ Just str
    else return Nothing

readEmail :: IO (Maybe String)
readEmail = do
  putStrLn "Please enter your email!"
  str <- getLine
  if '@' `elem` str && '.' `elem` str
    then return $ Just str
    else return Nothing

readPassword :: IO (Maybe String)
readPassword = do
  putStrLn "Please enter your Password!"
  str <- getLine
  if length str < 8 || null (filter isUpper str) || null (filter isLower str)
    then return Nothing
    else return $ Just str

login :: String -> String -> String -> IO ()
...

В этом примере, все наши потенциальные проблемы кода идут из IO монады. Как мы може использовать Maybe монаду когда мы уже внутри другой монады?

MONAD TRANSFORMERS

Luckily, we can get the desired behavior by using monad transformers to combine monads. In this example, we'll wrap the IO actions within a transformer called MaybeT.

A monad transformer is fundamentally a wrapper type. It is generally parameterized by another monadic type. You can then run actions from the inner monad, while adding your own customized behavior for combining actions in this new monad. The common transformers add T to the end of an existing monad. Here's the definition of MaybeT:

newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) }

instance (Monad m) => Monad (MaybeT m) where
  return = lift . return
  x >>= f = MaybeT $ do
    v <- runMaybeT x
    case v of
      Nothing -> return Nothing
      Just y  -> runMaybeT (f y)

So MaybeT itself is simply a newtype. It in turn contains a wrapper around a Maybe value. If the type m is a monad, we can also make a monad out of MaybeT.

Let's consider our example. We want to use MaybeT to wrap the IO monad, so we can run IO actions. This means our new monad is MaybeT IO. Our three helper functions all return strings, so they each get the type MaybeT IO String. To convert the old IO code into the MaybeT monad, all we need to do is wrap the IO action in the MaybeT constructor.

readUserName' :: MaybeT IO String
readUserName' = MaybeT $ do
  putStrLn "Please enter your Username!"
  str <- getLine
  if length str > 5
    then return $ Just str
    else return Nothing

readEmail' :: MaybeT IO String
readEmail' = MaybeT $ do
  putStrLn "Please enter your Email!"
  str <- getLine
  if length str > 5
    then return $ Just str
    else return Nothing

readPassword' :: MaybeT IO String
readPassword' = MaybeT $ do
  putStrLn "Please enter your Password!"
  str <- getLine
  if length str < 8 || null (filter isUpper str) || null (filter isLower str)
    then return Nothing
    else return $ Just str

Now we can wrap all three of these calls into a single monadic action, and do a single pattern match to get the results. We'll use the runMaybeT function to unwrap the Maybe value from the MaybeT:

main2 :: IO ()
main2 = do
  maybeCreds <- runMaybeT $ do
    usr <- readUserName
    email <- readEmail
    pass <- readPassword
    return (usr, email, pass)
  case maybeCreds of
    Nothing -> print "Couldn't login!"
    Just (u, e, p) -> login u e p

And this new code will have the proper short-circuiting behavior of the Maybe monad! If any of the read functions fail, our code will immediately return Nothing.

ADDING MORE LAYERS

Here's the best part about monad transformers. Since our newly created type is a monad itself, we can wrap it inside another transformer! Pretty much all common monads have transformer types in the same way the MaybeT is a transformer for the ordinary Maybe monad.

For a quick example, suppose we had an Env type containing some user information. We could wrap this environment in a Reader. However, we want to still have access to IO functionality, so we'll use the ReaderT transformer. Then we can wrap the result in MaybeT transformer.

type Env = (Maybe String, Maybe String, Maybe String)

readUserName'' :: MaybeT (ReaderT Env IO) String
readUserName'' = MaybeT $ do
  (maybeOldUser, _, _) <- ask
  case maybeOldUser of
    Just str -> return $ Just str
    Nothing -> do
      -- lift allows normal IO functions from inside ReaderT Env IO!
      lift $ putStrLn "Please enter your Username!"
      input <- lift getLine
      if length input > 5
        then return (Just input)
        else return Nothing

Notice we had to use lift to run the IO function getLine. In a monad transformer, the lift function allows you to run actions in the underlying monad. This behavior is encompassed by the MonadTrans class:

class MonadTrans t where
  lift :: (Monad m) => m a -> t m a

So using lift in the ReaderT Env IO action allows IO functions. Using the type template from the class, we can substitute Reader Env for t, and IO for m.

Within a MaybeT (ReaderT Env IO) function, calling lift would allow you to run a Reader function. We don't need this above since the bulk of the code lies in Reader actions wrapped by the MaybeT constructor.

To understand the concept of lifting, think of your monad layer as a stack. When you have a ReaderT Env IO action, imagine a Reader Env monad on top of the IO monad. An IO action exists on the bottom layer. So to run it from the upper layer, you need to lift it up. If your stack is more than two layers, you can lift multiple times. Calling lift twice from the MaybeT (ReaderT Env IO) monad will allow you to call IO functions.

It's inconvenient to have to know how many times to call lift to get to a particular level of the chain. Thus helper functions are frequently used for this. Additionally, since monad transformers can run several layers deep, the types can get complicated. So it is typical to use type synonyms liberally.

type TripleMonad a = MaybeT (ReaderT Env IO) a

performReader :: ReaderT Env IO a -> TripleMonad a
performReader = lift

performIO :: IO a -> TripleMonad a
performIO = lift . lift

TYPECLASSES

As a similar idea, there are some typeclasses which allow you to make certain assumptions about the monad stack below. For instance, you often don't care what the exact stack is, but you just need IO to exist somewhere on the stack. This is the purpose of the MonadIO typeclass:

class (Monad m) => MonadIO m where
  liftIO :: IO a -> m a

We can use this behavior to get a function to print even when we don't know its exact monad:

debugFunc :: (MonadIO m) => String -> m ()
debugFunc input = liftIO $ putStrLn ("Successfully produced input: " ++ input)

So even though this function doesn't explicitly live in MaybeT IO, we can write a version of our main function to use it.

main3 :: IO ()
main3 = do
  maybeCreds <- runMaybeT $ do
    usr <- readUserName'
    debugFunc usr
    email <- readEmail'
    debugFunc email
    pass <- readPassword'
    debugFunc pass
    return (usr, email, pass)
  case maybeCreds of
    Nothing -> print "Couldn't login!"
    Just (u, e, p) -> login u e p

One final note: You cannot, in general, wrap another monad with the IO monad using a transformer. You can, however, make the other monadic value the return type of an IO action.

func :: IO (Maybe String)
-- This type makes sense

func2 :: IO_T (ReaderT Env (Maybe)) string
-- This does not exist

SUMMARY

Now that you know how to combine your monads together, you're almost done with understanding the key concepts of monads! You could probably go out now and start writing some pretty complex code! But to truly master monads, you should know how to make your own, and there's one final concept that you should understand for that. This is the idea of type "laws". Each of the structures we've gone over in this series has a series of laws associated with it. And for your instances of these classes to make sense, they should follow the laws! Check out part 7 to make sure you know what's going on!

Now that you can write some pretty complex code, you need to know some of the libraries that will help you use it! Download our Production Checklist for a summary of some awesome libraries to help you apply your skills! Haskell has many tools for tasks like building web APIs and a