MonadBaseControl from the
monad-control package is a confusing typeclass, and its methods have complicated types. For many people, it’s nothing more than scary, impossible-to-understand magic that is, for some reason, needed when lifting certain kinds of operations. Few resources exist that adequately explain how, why, and when it works, which sadly seems to have resulted in some FUD about its use.
There’s no doubt that the machinery of
MonadBaseControl is complex, and the role it plays in practice is often subtle. However, its essence is actually much simpler than it appears, and I promise it can be understood by mere mortals. In this blog post, I hope to provide a complete survey of
MonadBaseControl—how it works, how it’s designed, and how it can go wrong—in a way that is accessible to anyone with a firm grasp of monads and monad transformers. To start, we’ll motivate
MonadBaseControl by reinventing it ourselves.
The higher-order action problem
Say we have a function with the following type:1
foo :: IO a -> IO a
If we have an action built from a transformer stack like
bar :: StateT X IO Y
then we might wish to apply
bar, but that is ill-typed, since
IO is not the same as
StateT X IO. In cases like these, we often use
lift, but it’s not good enough here:
lift adds a new monad transformer to an action, but here we need to remove a transformer. So we need a function with a type like this:
unliftState :: StateT X IO Y -> IO Y
However, if you think about that type just a little bit, it’s clear something’s wrong: it throws away information, namely the state. You may remember that a
StateT X IO Y action is equivalent to a function of type
X -> IO (Y, X), so our hypothetical
unliftState function has two problems:
We have no
Xto use as the initial state.
We’ll lose any modifications
barmade to the state, since the result type is just
Clearly, we’ll need something more sophisticated, but what?
A naïve solution
foo doesn’t know anything about the state, we can’t easily thread it through
foo itself. However, by using
runStateT explicitly, we could do some of the state management ourselves:
foo' :: StateT s IO a -> StateT s IO a foo' m = do s <- get (v, s') <- lift $ foo (runStateT m s) put s' pure v
Do you see what’s going on there? It’s not actually very complicated: we get the current state, then pass it as the initial state to
runStateT. This produces an action
IO (a, s) that has closed over the current state. We can pass that action to
foo without issue, since
foo is polymorphic in the action’s return type. Finally, all we have to do is
put the modified state back into the enclosing
StateT computation, and we can get on with our business.
That strategy works okay when we only have one monad transformer, but it gets hairy quickly as soon as we have two or more. For example, if we had
baz :: ExceptT X (StateT Y IO) Z, then we could do the same trick by getting the underlying
Y -> IO (Either X Z, Y)
function, closing over the state, restoring it, and doing the appropriate case analysis to re-raise any
ExceptT errors, but that’s a lot of work to do for every single function! What we’d like to do instead is somehow abstract over the pattern we used to write
foo' in a way that scales to arbitrary monad transformers.
The essence of
To build a more general solution for “unlifting” arbitrary monad transformers, we need to start thinking about monad transformer state. The technique we used to implement
foo' operated on the following process:
Capture the action’s input state and close over it.
Package up the action’s output state with its result and run it.
Restore the action’s output state into the enclosing transformer.
Return the action’s result.
StateT s, it turns out that the input state and output state are both
s, but other monad transformers have state, too. Consider the input and output state for the following common monad transformers:
|transformer||representation||input state||output state|
Notice how the input state is whatever is to the left of the
->, while the output state is whatever extra information gets produced alongside the result. Using the same reasoning, we can also deduce the input and output state for compositions of multiple monad transformers, such as the following:
|transformer||representation||input state||output state|
Notice that when monad transformers are composed, their states are composed, too. This is useful to keep in mind, since our goal is to capture the four steps above in a typeclass, polymorphic in the state of the monad transformers we need to lift through. At minimum, we need two new operations: one to capture the input state and close over it (step 1) and one to restore the output state (step 3). One class we might come up with could look like this:
class MonadBase b m => MonadBaseControl b m | m -> b where type InputState m type OutputState m captureInputState :: m (InputState m) closeOverInputState :: m a -> InputState m -> b (a, OutputState m) restoreOutputState :: OutputState m -> m ()
If we can write instances of that typeclass for various transformers, we can use the class’s operations to implement
foo' in a generic way that works with any combination of them:
foo' :: MonadBaseControl IO m => m a -> m a foo' m = do s <- captureInputState let m' = closeOverInputState m s (v, s') <- liftBase $ foo m' restoreOutputState s' pure v
So how do we implement those instances? Let’s start with
IO, since that’s the base case:
instance MonadBaseControl IO IO where type InputState IO = () type OutputState IO = () captureInputState = pure () closeOverInputState m () = m <&> (, ()) restoreOutputState () = pure ()
Not very exciting. The
StateT s instance, on the other hand, is significantly more interesting:
instance MonadBaseControl b m => MonadBaseControl b (StateT s m) where type InputState (StateT s m) = (s, InputState m) type OutputState (StateT s m) = (s, OutputState m) captureInputState = (,) <$> get <*> lift captureInputState closeOverInputState m (s, ss) = do ((v, s'), ss') <- closeOverInputState (runStateT m s) ss pure (v, (s', ss')) restoreOutputState (s, ss) = lift (restoreOutputState ss) *> put s
This instance alone includes most of the key ideas behind
MonadBaseControl. There’s a lot going on, so let’s break it down, step by step:
Start by examining the definitions of
OutputState. Are they what you expected? You’d be forgiven for expecting the following:
type InputState (StateT s m) = s type OutputState (StateT s m) = s
After all, that’s what we wrote in the table, isn’t it?
However, if you give it a try, you’ll find it doesn’t work.
OutputStatemust capture the state of the entire monad, not just a single transformer layer, so we have to combine the
StateT sstate with the state of the underlying monad. In the simplest case we get
InputState (StateT s IO) = (s, ())
which is boring, but in a more complex case, we need to get something like this:
InputState (StateT s (ReaderT IO)) = (s, (r, ()))
InputState (StateT s m)combines
InputState min a tuple, and
OutputStatedoes the same.
Moving on, take a look at
closeOverInputState. Just as
OutputStatecapture the state of the entire monad, these functions need to be inductive in the same way.
captureInputStateacquires the current state using
get, and it combines it with the remaining monadic state using
closeOverInputStateuses the captured state to peel off the outermost
StateTlayer, then calls
closeOverInputStaterecursively to peel off the rest of them.
restoreOutputStaterestores the state of the underlying monad stack, then restores the
StateTstate, ensuring everything ends up back the way it’s supposed to be.
Take the time to digest all that—work through it yourself if you need to—as it’s a dense piece of code. Once you feel comfortable with it, take a look at the instances for
WriterT as well:
instance MonadBaseControl b m => MonadBaseControl b (ReaderT r m) where type InputState (ReaderT r m) = (r, InputState m) type OutputState (ReaderT r m) = OutputState m captureInputState = (,) <$> ask <*> lift captureInputState closeOverInputState m (s, ss) = closeOverInputState (runReaderT m s) ss restoreOutputState ss = lift (restoreOutputState ss) instance (MonadBaseControl b m, Monoid w) => MonadBaseControl b (WriterT w m) where type InputState (WriterT w m) = InputState m type OutputState (WriterT w m) = (w, OutputState m) captureInputState = lift captureInputState closeOverInputState m ss = do ((v, s'), ss') <- closeOverInputState (runWriterT m) ss pure (v, (s', ss')) restoreOutputState (s, ss) = lift (restoreOutputState ss) *> tell s
Make sure you understand these instances, too. It should be easier this time, since they share most of their structure with the
StateT instance, but note the asymmetry that arises from the differing input and output states. (It may even help to try and write these instances yourself, focusing on the types whenever you get stuck.)
If you feel alright with them, then congratulations: you’re already well on your way to grokking
Hiding the input state
So far, our implementation of
MonadBaseControl works, but it’s actually slightly more complicated than it needs to be. As it happens, all valid uses of
MonadBaseControl will always end up performing the following pattern:
s <- captureInputState let m' = closeOverInputState m s
That is, we close over the input state as soon as we capture it. We can therefore combine
closeOverInputState into a single function:
captureAndCloseOverInputState :: m a -> m (b (a, OutputState m))
What’s more, we no longer need the
InputState associated type at all! This is an improvement, since it simplifies the API and removes the possibility for any misuse of the input state, since it’s never directly exposed. On the other hand, it has a more complicated type: it produces a monadic action that returns another monadic action. This can be a little more difficult to grok, which is why I presented the original version first, but it may help to consider how the above type arises naturally from the following definition:
captureAndCloseOverInputState m = closeOverInputState m <$> captureInputState
Let’s update the
MonadBaseControl class to incorporate this simplification:
class MonadBase b m => MonadBaseControl b m | m -> b where type OutputState m captureAndCloseOverInputState :: m a -> m (b (a, OutputState m)) restoreOutputState :: OutputState m -> m ()
We can then update all the instances to use the simpler API by simply fusing the definitions of
instance MonadBaseControl IO IO where type OutputState IO = () captureAndCloseOverInputState m = pure (m <&> (, ())) restoreOutputState () = pure () instance MonadBaseControl b m => MonadBaseControl b (StateT s m) where type OutputState (StateT s m) = (s, OutputState m) captureAndCloseOverInputState m = do s <- get m' <- lift $ captureAndCloseOverInputState (runStateT m s) pure $ do ((v, s'), ss') <- m' pure (v, (s', ss')) restoreOutputState (s, ss) = lift (restoreOutputState ss) *> put s instance MonadBaseControl b m => MonadBaseControl b (ReaderT r m) where type OutputState (ReaderT r m) = OutputState m captureAndCloseOverInputState m = do s <- ask lift $ captureAndCloseOverInputState (runReaderT m s) restoreOutputState ss = lift (restoreOutputState ss) instance (MonadBaseControl b m, Monoid w) => MonadBaseControl b (WriterT w m) where type OutputState (WriterT w m) = (w, OutputState m) captureAndCloseOverInputState m = do m' <- lift $ captureAndCloseOverInputState (runWriterT m) pure $ do ((v, s'), ss') <- m' pure (v, (s', ss')) restoreOutputState (s, ss) = lift (restoreOutputState ss) *> tell s
This is already very close to a full
MonadBaseControl implementation. The
captureAndCloseOverInputState implementations are getting a little out of hand, but bear with me—they’ll get simpler before this blog post is over.
Coping with partiality
MonadBaseControl class now works with
WriterT, but one transformer we haven’t considered is
ExceptT. Let’s try to extend our table from before with a row for
|transformer||representation||input state||output state|
Hmm… what is the output state for
The answer can’t be
e, since we might not end up with an
e—the computation might not fail.
Maybe e would be closer… could that work?
Well, let’s try it. Let’s write a
MonadBaseControl instance for
instance MonadBaseControl b m => MonadBaseControl b (ExceptT e m) where type OutputState (ExceptT e m) = (Maybe e, OutputState m) captureAndCloseOverInputState m = do m' <- lift $ captureAndCloseOverInputState (runExceptT m) pure $ do ((v, s'), ss') <- m' pure (v, (s', ss')) restoreOutputState (s, ss) = lift (restoreOutputState ss) *> case s of Just e -> throwError e Nothing -> pure ()
Sadly, the above implementation doesn’t typecheck; it is rejected with the following type error:
• Couldn't match type ‘Either e a’ with ‘(a, Maybe e)’ Expected type: m (b ((a, Maybe e), OutputState m)) Actual type: m (b (Either e a, OutputState m)) • In the second argument of ‘($)’, namely ‘captureAndCloseOverInputState (runExceptT m)’ In a stmt of a 'do' block: m' <- lift $ captureAndCloseOverInputState (runExceptT m) In the expression: do m' <- lift $ captureAndCloseOverInputState (runExceptT m) return do ((v, s'), ss') <- m' pure (v, (s', ss'))
We promised a
(a, Maybe e), but we have an
Either e a, and there’s certainly no way to get the former from the latter. Are we stuck? (If you’d like, take a moment to think about how you’d solve this type error before moving on, as it may be helpful for understanding the following solution.)
The fundamental problem here is partiality. The type of the
captureAndCloseOverInputState method always produces an action in the base monad that includes an
a in addition to some other output state. But
ExceptT is different: when it an error is raised, it doesn’t produce an
a at all—it only produces an
e. Therefore, as written, it’s impossible to give
Of course, we’d very much like to give
MonadBaseControl instance, so that isn’t very satisfying. Somehow, we need to change
captureAndCloseOverInputState so that it doesn’t always need to produce an
a. There are a few ways we could accomplish that, but an elegant way to do it is this:
class MonadBase b m => MonadBaseControl b m | m -> b where type WithOutputState m a captureAndCloseOverInputState :: m a -> m (b (WithOutputState m a)) restoreOutputState :: WithOutputState m a -> m a
We’ve replaced the old
OutputState associated type with a new
WithOutputState type, and the key difference between them is that
WithOutputState describes the type of a combination of the result (of type
a) and the output state, rather than describing the type of the output state alone. For total monad transformers like
WithOutputState m a will just be a tuple of the result value and the output state, the same as before. For example, here’s an updated
MonadBaseControl instance for
instance MonadBaseControl b m => MonadBaseControl b (StateT s m) where type WithOutputState (StateT s m) a = WithOutputState m (a, s) captureAndCloseOverInputState m = do s <- get lift $ captureAndCloseOverInputState (runStateT m s) restoreOutputState ss = do (a, s) <- lift $ restoreOutputState ss put s pure a
Before we consider how this helps us with
ExceptT, let’s pause for a moment and examine the revised
StateT instance in detail, as there are some new things going on here:
Take a close look at the definition of
WithOutputState (StateT s m) a. Note that we’ve defined it to be
WithOutputState m (a, s), not
(WithOutputState m a, s). Consider, for a moment, the difference between these types. Can you see why we used the former, not the latter?
If it’s unclear to you, that’s okay—let’s illustrate the difference with an example. Consider two similar monad transformer stacks:
m1 :: StateT s (ExceptT e IO) a m2 :: ExceptT e (StateT s IO) a
Both these stacks contain
ExceptT, but they are layered in a different order. What’s the difference? Well, consider what
m2return once fully unwrapped:
runExceptT (runStateT m1 s) :: m (Either e (a, s)) runStateT (runExceptT m2) s :: m (Either e a, s)
These results are meaningfully different: in
m1, the state is discarded if an error is raised, but in
m2, the final state is always returned, even if the computation is aborted. What does this mean for
Here’s the important detail: the state is discarded when
StateT, not the other way around. This can be counterintuitive, since the
sends up inside the
StateTconstructor is on the outside and vice versa. This is really just a property of how monad transformers compose, not anything specific to
MonadBaseControl, so an explanation of why this happens is outside the scope of this blog post, but the relevant insight is that the
StateT s m acontrols the eventual action’s output state.
If we had defined
WithOutputState (StateT s m) ato be
(WithOutputState m a, s), we’d be in a pickle, since
mwould be unable to influence the presence of
sin the output state. Therefore, we have no choice but to use
WithOutputState m (a, s). (If you are still confused by this, try it yourself; you’ll find that there’s no way to make the other definition typecheck.)
Now that we’ve developed an intuitive understanding of why
WithOutputStatemust be defined the way it is, let’s look at things from another perspective. Consider the type of
runStateT :: StateT s m a -> s -> m (a, s)
Note that the result type is
m (a, s), with the
mon the outside. As it happens, this correspondence simplifies the definition of
captureAndCloseOverInputState, since we no longer have to do any fiddling with its result—it’s already in the proper shape, so we can just return it directly.
Finally, this instance illustrates an interesting change to
restoreOutputState. Since the
ais now packed inside the
WithOutputState m avalue, the caller of
captureAndCloseOverInputStateneeds some way to get the
aback out! Conveniently,
restoreOutputStatecan play that role, both restoring the output state and unpacking the result.
Even ignoring partial transformers like
ExceptT, this is an improvement over the old API, as it conveniently prevents the programmer from forgetting to call
restoreOutputState. However, as we’ll see shortly, it is much more than a convenience: once
ExceptTcomes into play, it is essential!
With those details addressed, let’s return to
ExceptT. Using the new interface, writing an instance for
ExceptT is not only possible, it’s actually rather easy:
instance MonadBaseControl b m => MonadBaseControl b (ExceptT e m) where type WithOutputState (ExceptT e m) a = WithOutputState m (Either e a) captureAndCloseOverInputState m = lift $ captureAndCloseOverInputState (runExceptT m) restoreOutputState ss = either throwError pure =<< lift (restoreOutputState ss)
This instance illustrates why it’s so crucial that
restoreOutputState have the aforementioned dual role: it must handle the case where no
a exists at all! In the case of
ExceptT, it restores the state in the enclosing monad by re-raising an error.
Now all that’s left to do is update the other instances:
instance MonadBaseControl IO IO where type WithOutputState IO a = a captureAndCloseOverInputState = pure restoreOutputState = pure instance MonadBaseControl b m => MonadBaseControl b (ReaderT r m) where type WithOutputState (ReaderT r m) a = WithOutputState m a captureAndCloseOverInputState m = do s <- ask lift $ captureAndCloseOverInputState (runReaderT m s) restoreOutputState ss = lift $ restoreOutputState ss instance (MonadBaseControl b m, Monoid w) => MonadBaseControl b (WriterT w m) where type WithOutputState (WriterT w m) a = WithOutputState m (a, w) captureAndCloseOverInputState m = lift $ captureAndCloseOverInputState (runWriterT m) restoreOutputState ss = do (a, s) <- lift $ restoreOutputState ss tell s pure a
Finally, we can update our lifted variant of
foo to use the new interface so it will work with transformer stacks that include
foo' :: MonadBaseControl IO m => m a -> m a foo' m = do m' <- captureAndCloseOverInputState m restoreOutputState =<< liftBase (foo m')
At this point, it’s worth considering something: although getting the
MonadBaseControl class and instances right was a lot of work, the resulting
foo' implementation is actually incredibly simple. That’s a good sign, since we only have to write the
MonadBaseControl instances once (in a library), but we have to write functions like
foo' quite often.
Scaling to the real
MonadBaseControl class we implemented in the previous section is complete. It is a working, useful class that is equivalent in power to the “real”
MonadBaseControl class in the
monad-control library. However, if you compare the two, you’ll notice that the version in
monad-control looks a little bit different. What gives?
Let’s compare the two classes side by side:
-- ours class MonadBase b m => MonadBaseControl b m | m -> b where type WithOutputState m a captureAndCloseOverInputState :: m a -> m (b (WithOutputState m a)) restoreOutputState :: WithOutputState m a -> m a -- theirs class MonadBase b m => MonadBaseControl b m | m -> b where type StM m a liftBaseWith :: (RunInBase m b -> b a) -> m a restoreM :: StM m a -> m a
Let’s start with the similarities, since those are easy:
WithOutputStateassociated type is precisely equivalent to their
StMassociated type, they just use a (considerably) shorter name.
restoreOutputStatemethod is precisely equivalent to their
restoreMmethod, simply under a different name.
liftBaseWith. Those two methods both do similar things, but they aren’t identical, and that’s where all the differences lie. To understand
liftBaseWith, let’s start by inlining the definition of the
RunInBase type alias so we can see the fully-expanded type:
liftBaseWith :: MonadBaseControl b m => ((forall c. m c -> b (StM m c)) -> b a) -> m a
That type is complicated! However, if we break it down, hopefully you’ll find it’s not as scary as it first appears. Let’s reimplement the
foo' example from before using
liftBaseWith to show how this version of
foo' :: MonadBaseControl IO m => m a -> m a foo' m = do s <- liftBaseWith $ \runInBase -> foo (runInBase m) restoreM s
This is, in some ways, superficially similar to the version we wrote using our version of
MonadBaseControl. Just like in our version, we capture the input state, apply
foo in the
IO monad, then restore the state. But what exactly is doing the state capturing, and what is
Let’s start by adding a type annotation to
runInBase to help make it a little clearer what’s going on:
foo' :: forall m a. MonadBaseControl IO m => m a -> m a foo' m = do s <- liftBaseWith $ \(runInBase :: forall b. m b -> IO (StM m b)) -> foo (runInBase m) restoreM s
That type should look sort of recognizable. If we replace
WithOutputState, then we get a type that looks very similar to that of our original
closeOverInputState function, except it doesn’t need to take the input state as an argument. How does that work?
Here’s the trick:
liftBaseWith starts by capturing the input state, just as before. However, it then builds a function,
runInBase, which is like
closeOverInputState partially-applied to the input state it captured. It hands that function to us, and we’re free to apply it to
m, which produces the
IO (StM m a) action we need, and we can now pass that action to
foo. The result is returned in the outer monad, and we restore the state using
Sharing the input state
At first, this might seem needlessly complicated. When we first started, we separated capturing the input state and closing over it into two separate operations (
closeOverInputState), but we eventually combined them so that we could keep the input state hidden. Why does
monad-control split them back into two operations again?
As it turns out, when lifting
foo, there’s no advantage to the more complicated API of
monad-control. In fact, we could implement our
captureAndCloseOverInputState operation in terms of
liftBaseWith, and we could use that to implement
foo' the same way we did before:
captureAndCloseOverInputState :: MonadBaseControl b m => m a -> m (b (StM m a)) captureAndCloseOverInputState m = liftBaseWith $ \runInBase -> pure (runInBase m) foo' :: MonadBaseControl IO m => m a -> m a foo' m = do m' <- captureAndCloseOverInputState m restoreM =<< liftBase (foo m')
However, that approach has a downside once we need to lift more complicated functions.
foo is exceptionally simple, as it only accepts a single input argument, but what if we wanted to lift a more complicated function that took two monadic arguments, such as this one:
bar :: IO a -> IO a -> IO a
We could implement that by calling
captureAndCloseOverInputState twice, like this:
bar' :: MonadBaseControl IO m => m a -> m a -> m a bar' ma mb = do ma' <- captureAndCloseOverInputState ma mb' <- captureAndCloseOverInputState mb restoreM =<< liftBase (bar ma' mb')
However, that would capture the monadic state twice, which is rather inefficient. By using
liftBaseWith, the state capturing is done just once, and it’s shared between all calls to
bar' :: MonadBaseControl IO m => m a -> m a -> m a bar' ma mb = do s <- liftBaseWith $ \runInBase -> bar (runInBase ma) (runInBase mb) restoreM s
By providing a “running” function (
runInBase) instead of direct access to the input state,
liftBaseWith allows sharing the captured input state between multiple actions without exposing it directly.
Sidebar: continuation-passing and impredicativity
One last point before we move on: although the above explains why
captureAndCloseOverInputState is insufficient, you may be left wondering why
liftBaseWith can’t just return
runInBase. Why does it need to be given a continuation? After all, it would be nicer if we could just write this:
bar' :: MonadBaseControl IO m => m a -> m a -> m a bar' ma mb = do runInBase <- askRunInBase restoreM =<< liftBase (bar (runInBase ma) (runInBase mb))
To understand the problem with a hypothetical
askRunInBase function, remember that the type of
runInBase is polymorphic:
runInBase :: forall a. m a -> b (StM m a)
This is important, since if you need to lift a function with a type like
baz :: IO b -> IO c -> IO (Either b c)
then you’ll want to instantiate that
a variable with two different types. We’d need to retain that power in
askRunInBase, so it would need to have the following type:
askRunInBase :: MonadBaseControl b m => m (forall a. m a -> b (StM m a))
Sadly, that type is illegal in Haskell. Type constructors must be applied to monomorphic types, but in the above type signature,
m is applied to a polymorphic type.2 The
RankNTypes GHC extension introduces a single exception: the
(->) type constructor is special and may be applied to polymorphic types. That’s why
liftBaseWith is legal, but
askRunInBase is not: since
liftBaseWith is passed a higher-order function that receives
runInBase as an argument, the polymorphic type appears immediately under an application of
(->), which is allowed.
The aforementioned restriction means we’re basically out of luck, but if you really want
askRunInBase, there is a workaround. GHC is perfectly alright with a field of a datatype being polymorphic, so we can define a newtype that wraps a suitably-polymorphic function:
newtype RunInBase b m = RunInBase (forall a. m a -> b (StM m a))
We can now alter
askRunInBase to return our newtype, and we can implement it in terms of
askRunInBase :: MonadBaseControl b m => m (RunInBase b m) askRunInBase = liftBaseWith $ \runInBase -> pure $ RunInBase runInBase
askRunInBase, we have to pattern match on the
RunInBase constructor, but it isn’t very noisy, since we can do it directly in a
do binding. For example, we could implement a lifted version of
baz this way:
baz' :: MonadBaseControl IO m => m a -> m b -> m (Either a b) baz' ma mb = do RunInBase runInBase <- askRunInBase s <- liftBase (baz (runInBase ma) (runInBase mb)) bitraverse restoreM restoreM s
As of version 220.127.116.11,
monad-control does not provide a newtype like
RunInBase, so it also doesn’t provide a function like
askRunInBase. For now, you’ll have to use
liftBaseWith, but it might be a useful future addition to the library.
At this point in the blog post, we’ve covered the essentials of
MonadBaseControl: how it works, how it’s designed, and how you might go about using it. However, so far, we’ve only considered situations where
MonadBaseControl works well, and I’ve intentionally avoided examples where the technique breaks down. In this section, we’re going to take a look at the pitfalls and drawbacks of
MonadBaseControl, plus some ways they can be mitigated.
No polymorphism, no lifting
All of the pitfalls of
MonadBaseControl stem from the same root problem, and that’s the particular technique it uses to save and restore monadic state. We’ll start by considering one of the simplest ways that technique is thwarted, and that’s monomorphism. Consider the following two functions:
poly :: IO a -> IO a mono :: IO X -> IO X
Even after all we’ve covered, it may surprise you to learn that although
poly can be easily lifted to
MonadBaseControl IO m => m a -> m a, it’s impossible to lift
MonadBaseControl IO m => m X -> m X. It’s a little unintuitive, as we often think of polymorphic types as being more complicated (so surely lifting polymorphic functions ought to be harder), but in fact, it’s the flexibility of polymorphism that allows
MonadBaseControl to work in the first place.
To understand the problem, remember that when we lift a function of type
forall a. b a -> b a using
MonadBaseControl, we actually instantiate
(StM m c). That produces a function of type
b (StM m c) -> b (StM m c), which is isomorphic to the
m c -> m c type we want. The instantiation step is easily overlooked, but it’s crucial, since otherwise we have no way to thread the state through the otherwise opaque function we’re trying to lift!
In the case of
mono, that’s exactly the problem we’re faced with.
mono will not accept an
IO (StM m X) as an argument, only precisely an
IO X, so we can’t pass along the monadic state. For all its machinery,
MonadBaseControl is no help at all if no polymorphism is involved. Trying to generalize
mono without modifying its implementation is a lost cause.
The dangers of discarded state
Our inability to lift
mono is frustrating, but at least it’s conclusively impossible. In practice, however, many functions lie in an insidious in-between: polymorphic enough to be lifted, but not without compromises. The simplest of these functions have types such as the following:
sideEffect :: IO a -> IO ()
mono, it’s entirely possible to lift
sideEffect' :: MonadBaseControl IO m => m a -> m () sideEffect' m = liftBaseWith $ \runInBase -> sideEffect (runInBase m)
This definition typechecks, but you may very well prefer it didn’t, since it has a serious problem: any changes made by
m to the monadic state are completely discarded once
sideEffect' returns! Since
sideEffect' never calls
restoreM, there’s no way the state of
m can be any different from the original state, but it’s impossible to call
restoreM since we don’t actually get an
StM m () result from
Sometimes this may be acceptable, since some monad transformers don’t actually have any output state anyway, such as
ReaderT r. In other cases, however,
sideEffect' could be a bug waiting to happen. One way to make
sideEffect' safe would be to add a
StM m a ~ a constraint to its context, since that guarantees the monad transformers being lifted through are stateless, and nothing is actually being discarded. Of course, that significantly restricts the set of monad transformers that can be lifted through.
One scenario where state discarding can actually be useful is operations with so-called rewindable or transactional state. The most common example of such an operation is
catch :: Exception e => IO a -> (e -> IO a) -> IO a
When lifted, state changes from the action or from the exception handler will be “committed,” but never both. If an exception is raised during the computation, those state changes are discarded (“rewound”), giving
catch a kind of backtracking semantics. This behavior arises naturally from the way a lifted version of
catch must be implemented:
catch' :: (Exception e, MonadBaseControl IO m) => m a -> (e -> m a) -> m a catch' m f = do s <- liftBaseWith $ \runInBase -> catch (runInBase m) (runInBase . f) restoreM s
m raises an exception, it will never return an
StM m a value, so there’s no way to get ahold of any of the state changes that happened before the exception. Therefore, the only option is to discard that state.
This behavior is actually quite useful, and it’s definitely not unreasonable. However, useful or not, it’s inconsistent with state changes to mutable values like
MVars (they stay modified whether an exception is raised or not), so it can still be a gotcha. Either way, it’s worth being aware of.
Partially discarded state
The next function we’re going to examine is
finally :: IO a -> IO b -> IO a
This function has a similar type to
catch, and it even has similar semantics. Like
finally can be lifted, but unlike
catch, its state can’t be given any satisfying treatment. The only way to implement a lifted version is
finally' :: MonadBaseControl IO m => m a -> m b -> m a finally' ma mb = do s <- liftBaseWith $ \runInBase -> finally (runInBase ma) (runInBase mb) restoreM s
which always discards all state changes made by the second argument. This is clear just from looking at
finally’s type: since
b doesn’t appear anywhere in the return type, there’s simply no way to access that action’s result, and therefore no way to access its modified state.
However, don’t despair: there actually is a way to produce a lifted version of
finally that preserves all state changes. It can’t be done by lifting
finally directly, but if we reimplement
finally in terms of simpler lifted functions that are more amenable to lifting, we can produce a lifted version of
finally that preserves all the state:4
finally' :: MonadBaseControl IO m => m a -> m b -> m a finally' ma mb = mask' $ \restore -> do a <- liftBaseWith $ \runInBase -> try (runInBase (restore ma)) case a of Left e -> mb *> liftBase (throwIO (e :: SomeException)) Right s -> restoreM s <* mb
This illustrates an important (and interesting) point about
MonadBaseControl: whether or not an operation can be made state-preserving is not a fundamental property of the operation’s type, but rather a property of the types of the exposed primitives. There is sometimes a way to implement a state-preserving variant of operations that might otherwise seem unliftable given the right primitives and a bit of cleverness.
As a final example, I want to provide an example where the state may not actually be discarded per se, just inaccessible. Consider the type of
forkIO :: IO () -> IO ThreadId
forkIO isn’t actually polymorphic in its argument, we can convert any
IO action to one that produces
void, so it might as well be. Therefore, we can lift
forkIO in much the same way we did with
forkIO' :: MonadBaseControl IO m => m () -> m ThreadId forkIO' m = liftBaseWith $ \runInBase -> forkIO (void $ runInBase m)
sideEffect, we can’t recover the output state, but in this case, there’s a fundamental reason that goes deeper than the types: we’ve forked off a concurrent computation! We’ve therefore split the state in two, which might be what we want… but it also might not.
forkIO is yet another illustration that it’s important to think about the state-preservation semantics when using
MonadBaseControl, or you may end up with a bug!
MonadBaseControl in context
Congratulations: you’ve made it through most of this blog post. If you’ve followed everything so far, you now understand
MonadBaseControl. All the tricky parts are over. However, before wrapping up, I’d like to add a little extra information about how
MonadBaseControl relates to various other parts of the Haskell ecosystem. In practice, that information can be as important as understanding
The remainder of
If you look at the documentation for
monad-control, you’ll find that it provides more than just the
MonadBaseControl typeclass. I’m not going to cover everything else in detail in this blog post, but I do want to touch upon it briefly.
First off, you should definitely take a look at the handful of helper functions provided by
monad-control, such as
liftBaseOp_. These functions provide support for lifting common function types without having to use
liftBaseWith directly. It’s useful to understand
liftBaseWith, since it’s the most general way to use
MonadBaseControl, but in practice, it is simpler and more readable to use the more specialized functions wherever possible. Many of the examples in this very blog post could be simplified using them, and I only stuck to
liftBaseWith to introduce as few new concepts at a time as possible.
Second, I’d like to mention the related
MonadTransControl typeclass. You hopefully remember from earlier in the blog post how we defined
MonadBaseControl instances inductively so that we could lift all the way down to the base monad.
MonadTransControl is like
MonadBaseControl if it intentionally did not do that—it allows lifting through a single transformer at a time, rather than through all of them at once.
MonadTransControl is not terribly useful to use directly (though I did use it once in a previous blog post of mine to help derive instances of mtl-style classes), but it is useful for implementing
MonadBaseControl instances for your own transformers. If you define a
MonadTransControl instance for your monad transformer, you can get a
MonadBaseControl implementation for free using the provided
defaultRestoreM bindings; see the documentation for more details.
If you’re going to use
lifted-async packages are good to know about. As their names imply, they provide lifted versions of bindings in the
async packages, so you can use them directly without needing to lift them yourself. For example, if you needed a lifted version of
Control.Exception, you could swap it for the
mask export from
Control.Exception.Lifted, and everything would mostly just work (though always be sure to check the documentation for any caveats on state discarding).
Recently, FP Complete has developed the
unliftio package as an alternative to
monad-control. It provides the
MonadUnliftIO typeclass, which is similar in spirit to
MonadBaseControl, but heavily restricted: it is specialized to
IO as the base monad, and it only allows instances for stateless monads, such as
ReaderT. This is designed to encourage the so-called
ReaderT design pattern, which avoids ever using stateful monads like
IO, encouraging the use of
IO exceptions and mutable variables (e.g.
I should be clear: I really like most of what FP Complete has done—to this day, I still use
stack as my Haskell build tool of choice—and I think the suggestions given in the aforementioned “
ReaderT design pattern” blog post have real weight to them. I have a deep respect for Michael Snoyman’s commitment to opinionated, user-friendly tools and libraries. But truthfully, I can’t stand
MonadUnliftIO is designed to avoid all the complexity around state discarding that
MonadBaseControl introduces, and on its own, that’s a noble goal. Safety first, after all. The problem is that
MonadUnliftIO really is extremely limiting, and what’s more, it can actually be trivially encoded in terms of
MonadBaseControl as follows:
type MonadUnliftIO m = (MonadBaseControl IO m, forall a. StM m a ~ a)
This alias can be used to define safe, lifted functions that never discard state while still allowing functions that can be safely lifted through stateful transformers to do so. Indeed, the
Control.Concurrent.Async.Lifted.Safe module from
lifted-async does exactly that (albeit with a slightly different formulation than the above alias).
To be fair, the
unliftio README does address this in its comparison section:
monad-controlallows us to unlift both styles. In theory, we could write a variant of
lifted-basethat never does state discards […] In other words, this is an advantage of
MonadUnliftIO. We’ve avoided providing any such extra typeclass in this package though, for two reasons:
MonadUnliftIOis a simple typeclass, easy to explain. We don’t want to complicated [sic] matters […]
Having this kind of split would be confusing in user code, when suddenly [certain operations are] not available to us.
In other words, the authors of
unliftio felt that
MonadBaseControl was simply not worth the complexity, and they could get away with
MonadUnliftIO. Frankly, if you feel the same way, by all means, use
unliftio. I just found it too limiting given the way I write Haskell, plain and simple.
So ends another long blog post. As often seems the case, I set out to write something short, but I ended up writing well over 5,000 words. I suppose that means I learned something from this experience, too:
MonadBaseControl is more complicated than I had anticipated! Maybe there’s something to take away from that.
In any case, it’s over now, so I’d like to briefly summarize what we’ve covered:
MonadBaseControlallows us to lift higher-order monadic operations.
It operates by capturing the current monadic state and explicitly threading it through the action in the base monad before restoring it.
That technique works well for polymorphic operations for the type
forall a. b a -> b a, but it can be tricky or even impossible for more complex operations, sometimes leading to discarded state.
This can sometimes be mitigated by restricting certain operations to stateless monads using a
StM m a ~ aconstraint, or by reimplementing the operation in terms of simpler primitives.
lifted-asyncpackages provide lifted versions of existing operations, avoiding the need to lift them yourself.
As with many abstractions in Haskell, don’t worry too much if you don’t have a completely firm grasp of
MonadBaseControl at first. Insight often comes with repeated experience, and
monad-control can still be used in useful ways even without a perfect understanding. My hope is that this blog post has helped you build intuitions about
MonadBaseControl even if some of the underlying machinery remains a little fuzzy, and I hope it can also serve as a reference for those who want or need to understand (or just be reminded of) all the little details.
Finally, I’ll admit
MonadBaseControl isn’t especially elegant or beautiful as Haskell abstractions go. In fact, in many ways, it’s a bit of a kludge! Perhaps, in time, effect systems will evolve and mature so that it and its ilk are no longer necessary, and they may become distant relics of an inferior past. But in the meantime, it’s here, it’s useful, and I think it’s worth embracing. If you’ve shied away from it in the past, I hope I’ve illuminated it enough to make you consider giving it another try.
One example of a function with that type is
Types with polymorphic types under type constructors are called impredicative. GHC technically has limited support for impredicativity via the
ImpredicativeTypeslanguage extension, but as of GHC 8.8, it has been fairly broken for some time. A fix is apparently being worked on, but even if that effort is successful, I don’t know what impact it will have on type inference. ↩
askRunInBase = liftBaseWith (pure . RunInBase)does not typecheck, as it would require impredicative polymorphism: it would require instantiating the type of
(.)with polymorphic types. The version using
($)works because GHC actually has special typechecking rules for
f $ xis really syntax in GHC. ↩
mask'is a suitably lifted version of
mask(which can in fact be made state-preserving). ↩