Control.Monad.Trans.Control.Aligned

MonadTransControl

class MonadTrans t => MonadTransControl t stT | t -> stT where

Methods

liftWith :: Monad m => (Run t stT -> m a) -> t m a

liftWith is similar to lift in that it lifts a computation from the argument monad to the constructed monad.

Instances should satisfy similar laws as the MonadTrans laws:

liftWith . const . return = return

liftWith (const (m >>= f)) = liftWith (const m) >>= liftWith . const . f

The difference with lift is that before lifting the m computation liftWith captures the state of t. It then provides the m computation with a Run function that allows running t n computations in n (for all n) on the captured state.

restoreT :: Monad m => m (stT a) -> t m a

Construct a t computation from the monadic state of t that is returned from a Run function.

Instances should satisfy:

liftWith (\run -> run t) >>= restoreT . return = t

Instances

MonadTransControl MaybeT Maybe

MonadTransControl ListT []

MonadTransControl IdentityT Identity

MonadTransControl (ReaderT r) Identity

Monoid w => MonadTransControl (WriterT w) ((,) w)

Monoid w => MonadTransControl (WriterT w) ((,) w)

Error e => MonadTransControl (ErrorT e) (Either e)

MonadTransControl (ExceptT e) (Either e)

MonadTransControl (StateT s) ((,) s)

MonadTransControl (StateT s) ((,) s)

Monoid w => MonadTransControl (RWST r w s) ((,,) w s)

Monoid w => MonadTransControl (RWST r w s) ((,,) w s)

type Run t stT = forall n b. Monad n => t n b -> n (stT b)

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.

Defaults

The following functions can be used to define a MonadTransControl instance for a monad transformer which simply wraps another monad transformer which already has a MonadTransControl instance. For example:

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

newtype CounterT m a = CounterT {unCounterT :: StateT Int m a}
  deriving (Monad, MonadTrans)

instance MonadTransControl CounterT where
    type StT CounterT a = StT (StateT Int) a
    liftWith = defaultLiftWith CounterT unCounterT
    restoreT = defaultRestoreT CounterT

type RunDefault t stT = forall n b. Monad n => t n b -> n (stT b)

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

defaultLiftWith

Arguments

:: (Monad m, MonadTransControl n stT)

=> (forall b. n m b -> t m b)

Monad constructor

-> (forall o b. t o b -> n o b)

Monad deconstructor

-> (RunDefault t stT -> m a)

-> t m a

Default definition for the liftWith method.

defaultRestoreT

Arguments

:: (Monad m, MonadTransControl n stT)

=> (n m a -> t m a)

Monad constructor

-> m (stT a)

-> t m a

Default definition for the restoreT method.

MonadBaseControl

class MonadBase b m => MonadBaseControl b m stM | m -> b stM where

Methods

liftBaseWith :: (RunInBase m b stM -> b a) -> m a

liftBaseWith is similar to liftIO and liftBase in that it lifts a base computation to the constructed monad.

Instances should satisfy similar laws as the MonadIO and MonadBase laws:

liftBaseWith . const . return = return

liftBaseWith (const (m >>= f)) = liftBaseWith (const m) >>= liftBaseWith . const . f

The difference with liftBase is that before lifting the base computation liftBaseWith captures the state of m. It then provides the base computation with a RunInBase function that allows running m computations in the base monad on the captured state.

restoreM :: stM a -> m a

Construct a m computation from the monadic state of m that is returned from a RunInBase function.

Instances should satisfy:

liftBaseWith (\runInBase -> runInBase m) >>= restoreM = m

Instances

MonadBaseControl [] [] Identity

MonadBaseControl IO IO Identity

MonadBaseControl Identity Identity Identity

MonadBaseControl STM STM Identity

MonadBaseControl Maybe Maybe Identity

MonadBaseControl b m stM => MonadBaseControl b (ListT m) (Compose stM [])

MonadBaseControl b m stM => MonadBaseControl b (MaybeT m) (Compose stM Maybe)

MonadBaseControl b m stM => MonadBaseControl b (IdentityT m) (Compose stM Identity)

(MonadBaseControl b m stM, Monoid w) => MonadBaseControl b (WriterT w m) (Compose stM ((,) w))

(MonadBaseControl b m stM, Monoid w) => MonadBaseControl b (WriterT w m) (Compose stM ((,) w))

(MonadBaseControl b m stM, Error e) => MonadBaseControl b (ErrorT e m) (Compose stM (Either e))

MonadBaseControl b m stM => MonadBaseControl b (ExceptT e m) (Compose stM (Either e))

MonadBaseControl b m stM => MonadBaseControl b (StateT s m) (Compose stM ((,) s))

MonadBaseControl b m stM => MonadBaseControl b (StateT s m) (Compose stM ((,) s))

MonadBaseControl b m stM => MonadBaseControl b (ReaderT r m) (Compose stM Identity)

(MonadBaseControl b m stM, Monoid w) => MonadBaseControl b (RWST r w s m) (Compose stM ((,,) w s))

(MonadBaseControl b m stM, Monoid w) => MonadBaseControl b (RWST r w s m) (Compose stM ((,,) w s))

MonadBaseControl ((->) r) ((->) r) Identity

MonadBaseControl (Either e) (Either e) Identity

MonadBaseControl (ST s) (ST s) Identity

MonadBaseControl (ST s) (ST s) Identity

type RunInBase m b stM = forall a. m a -> b (stM a)

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.

Defaults

Note that by using the following default definitions it's easy to make a monad transformer T an instance of MonadBaseControl:

instance MonadBaseControl b m => MonadBaseControl b (T m) where
    type StM (T m) a = ComposeSt T m a
    liftBaseWith     = defaultLiftBaseWith
    restoreM         = defaultRestoreM

Defining an instance for a base monad B is equally straightforward:

instance MonadBaseControl B B where
    type StM B a   = a
    liftBaseWith f = f id
    restoreM       = return

type RunInBaseDefault t m b stM stT = forall a. t m a -> b (Compose stM stT a)

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

defaultLiftBaseWith :: (MonadTransControl t stT, MonadBaseControl b m stM) => (RunInBaseDefault t m b stM stT -> b a) -> t m a

Default defintion 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

defaultRestoreM :: (MonadTransControl t stT, MonadBaseControl b m stM) => Compose stM stT a -> t m a

Default definition for the restoreM method.

Note that: defaultRestoreM = restoreT . restoreM

Utility functions

control :: MonadBaseControl b m stM => (RunInBase m b stM -> b (stM a)) -> m a

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

embed :: MonadBaseControl b m stM => (a -> m c) -> m (a -> b (stM c))

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

embed_ :: MonadBaseControl b m stM => (a -> m ()) -> m (a -> b ())

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

captureT :: (MonadTransControl t stT, Monad (t m), Monad m) => t m (stT ())

Capture the current state of a transformer

captureM :: MonadBaseControl b m stM => m (stM ())

Capture the current state above the base monad

liftBaseOp :: MonadBaseControl b m stM => ((a -> b (stM c)) -> b (stM d)) -> (a -> m c) -> m d

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 :: MonadBaseControl IO m => (Ptr a -> m c) -> m c

liftBaseOp_ :: MonadBaseControl b m stM => (b (stM a) -> b (stM c)) -> m a -> 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 :: MonadBaseControl b m stM => (b () -> b a) -> m () -> 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 :: MonadBaseControl b m stM => ((a -> b ()) -> b c) -> (a -> m ()) -> m c

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 ()

liftThrough

Arguments

:: (MonadTransControl t stT, Monad (t m), Monad m)

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

-> t m a

-> t m b

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