The MonadTransControl type class is a stronger version of MonadTrans:

Instances of MonadTrans know how to lift actions in the base monad to the transformed monad. These lifted actions, however, are completely unaware of the monadic state added by the transformer.

MonadTransControl instances are aware of the monadic state of the transformer and allow to save and restore this state.

This allows to lift functions that have a monad transformer in both positive and negative position. Take, for example, the function

withFile :: FilePath -> IOMode -> (Handle -> IO r) -> IO r

MonadTrans instances can only lift the return type of the withFile function:

withFileLifted :: MonadTrans t => FilePath -> IOMode -> (Handle -> IO r) -> t IO r
withFileLifted file mode action = lift (withFile file mode action)

However, MonadTrans is not powerful enough to make withFileLifted accept a function that returns t IO. The reason is that we need to take away the transformer layer in order to pass the function to withFile. MonadTransControl allows us to do this:

withFileLifted' :: (Monad (t IO), MonadTransControl t) => FilePath -> IOMode -> (Handle -> t IO r) -> t IO r
withFileLifted' file mode action = liftWith (\run -> withFile file mode (run . action)) >>= restoreT . return

A function that runs a transformed monad t n on the monadic state that was captured by liftWith

A Run t function yields a computation in n that returns the monadic state of t. This state can later be used to restore a t computation using restoreT.

Example type equalities:

Run IdentityT    ~ forall n b. Monad n             => IdentityT  n b -> n b
Run MaybeT       ~ forall n b. Monad n             => MaybeT     n b -> n (Maybe b)
Run (ErrorT e)   ~ forall n b. (Monad n, Error e)  => ErrorT e   n b -> n (Either e b)
Run (ExceptT e)  ~ forall n b. Monad n             => ExceptT e  n b -> n (Either e b)
Run ListT        ~ forall n b. Monad n             => ListT      n b -> n [b]
Run (ReaderT r)  ~ forall n b. Monad n             => ReaderT r  n b -> n b
Run (StateT s)   ~ forall n b. Monad n             => StateT s   n b -> n (a, s)
Run (WriterT w)  ~ forall n b. (Monad n, Monoid w) => WriterT w  n b -> n (a, w)
Run (RWST r w s) ~ forall n b. (Monad n, Monoid w) => RWST r w s n b -> n (a, s, w)

This type is usually satisfied by the run function of a transformer:

flip runReaderT :: r -> Run (ReaderT r)
flip runStateT  :: s -> Run (StateT s)
runMaybeT       ::      Run MaybeT

A function like Run that runs a monad transformer t which wraps the monad transformer t'. This is used in defaultLiftWith.

Default definition for the liftWith method.

Default definition for the restoreT method.

A function like Run that runs a monad transformer t which wraps the monad transformers n and n'. This is used in defaultLiftWith2.

Default definition for the liftWith method.

Default definition for the restoreT method for double MonadTransControl.

Writing instances

The usual way to write a MonadBaseControl instance for a transformer stack over a base monad B is to write an instance MonadBaseControl B B for the base monad, and MonadTransControl T instances for every transformer T. Instances for MonadBaseControl are then simply implemented using ComposeSt, defaultLiftBaseWith, defaultRestoreM.

A function that runs a m computation on the monadic state that was captured by liftBaseWith

A RunInBase m function yields a computation in the base monad of m that returns the monadic state of m. This state can later be used to restore the m computation using restoreM.

Example type equalities:

RunInBase (IdentityT  m) b ~ forall a.             IdentityT  m a -> b (StM m a)
RunInBase (MaybeT     m) b ~ forall a.             MaybeT     m a -> b (StM m (Maybe a))
RunInBase (ErrorT e   m) b ~ forall a. Error e =>  ErrorT e   m a -> b (StM m (Either e a))
RunInBase (ExceptT e  m) b ~ forall a.             ExceptT e  m a -> b (StM m (Either e a))
RunInBase (ListT      m) b ~ forall a.             ListT      m a -> b (StM m [a])
RunInBase (ReaderT r  m) b ~ forall a.             ReaderT    m a -> b (StM m a)
RunInBase (StateT s   m) b ~ forall a.             StateT s   m a -> b (StM m (a, s))
RunInBase (WriterT w  m) b ~ forall a. Monoid w => WriterT w  m a -> b (StM m (a, w))
RunInBase (RWST r w s m) b ~ forall a. Monoid w => RWST r w s m a -> b (StM m (a, s, w))

For a transformed base monad m ~ t b, 'RunInBase m b' ~ Run t.

Handy type synonym that composes the monadic states of t and m.

It can be used to define the StM for new MonadBaseControl instances.

A function like RunInBase that runs a monad transformer t in its base monad b. It is used in defaultLiftBaseWith.

Default definition for the liftBaseWith method.

Note that it composes a liftWith of t with a liftBaseWith of m to give a liftBaseWith of t m:

defaultLiftBaseWith = \f -> liftWith $ \run ->
                              liftBaseWith $ \runInBase ->
                                f $ runInBase . run

Default definition for the restoreM method.

Note that: defaultRestoreM = restoreT . restoreM

An often used composition: control f = liftBaseWith f >>= restoreM

Example:

liftedBracket :: MonadBaseControl IO m => m a -> (a -> m b) -> (a -> m c) -> m c
liftedBracket acquire release action = control $ \runInBase ->
    bracket (runInBase acquire)
            (\saved -> runInBase (restoreM saved >>= release))
            (\saved -> runInBase (restoreM saved >>= action))

Embed a transformer function as an function in the base monad returning a mutated transformer state.

Performs the same function as embed, but discards transformer state from the embedded function.

Capture the current state of a transformer

Capture the current state above the base monad

liftBaseOp is a particular application of liftBaseWith that allows lifting control operations of type:

((a -> b c) -> b c)

to:

(MonadBaseControl b m => (a -> m c) -> m c)

For example:

liftBaseOp alloca :: (Storable a, MonadBaseControl IO m) => (Ptr a -> m c) -> m c

liftBaseOp_ is a particular application of liftBaseWith that allows lifting control operations of type:

(b a -> b a)

to:

(MonadBaseControl b m => m a -> m a)

For example:

liftBaseOp_ mask_ :: MonadBaseControl IO m => m a -> m a

liftBaseDiscard is a particular application of liftBaseWith that allows lifting control operations of type:

(b () -> b a)

to:

(MonadBaseControl b m => m () -> m a)

Note that, while the argument computation m () has access to the captured state, all its side-effects in m are discarded. It is run only for its side-effects in the base monad b.

For example:

liftBaseDiscard forkIO :: MonadBaseControl IO m => m () -> m ThreadId

liftBaseOpDiscard is a particular application of liftBaseWith that allows lifting control operations of type:

((a -> b ()) -> b c)

to:

(MonadBaseControl b m => (a -> m ()) -> m c)

Note that, while the argument computation m () has access to the captured state, all its side-effects in m are discarded. It is run only for its side-effects in the base monad b.

For example:

liftBaseDiscard (runServer addr port) :: MonadBaseControl IO m => m () -> m ()

Transform an action in t m using a transformer that operates on the underlying monad m