monad-control-0.3.2.1: Lift control operations, like exception catching, through monad transformers

Stabilityexperimental
MaintainerBas van Dijk <v.dijk.bas@gmail.com>
Safe HaskellTrustworthy

Control.Monad.Trans.Control

Contents

Description

(TODO: It would be nicer if the associated data types StT and StM were associated type synonyms instead. This would simplify a lot of code and could make some definitions more efficient because there'll be no need to wrap the monadic state in a data type. Unfortunately GHC has a bug which prevents this: http://hackage.haskell.org/trac/ghc/ticket/5595. I will switch to associated type synonyms when that bug is fixed.)

Synopsis

MonadTransControl

class MonadTrans t => MonadTransControl t where

Associated Types

data StT t :: * -> *

Monadic state of t.

Methods

liftWith :: Monad m => (Run t -> 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 t 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

type Run t = forall n b. Monad n => t n b -> n (StT t 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 for MonadTransControl

Following functions can be used to define MonadTransControl instances for newtypes. 

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

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

instance MonadTransControl CounterT where
     newtype StT CounterT a = StCounter {unStCounter :: StT (StateT Int) a}
     liftWith = defaultLiftWith CounterT unCounterT StCounter
     restoreT = defaultRestoreT CounterT unStCounter

defaultLiftWith

Arguments

:: (Monad m, MonadTransControl n) 
=> (forall b. n m b -> t m b)

Monad constructor

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

Monad deconstructor

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

StT constructor

-> (Run t -> m a) 
-> t m a 

Default definition for the liftWith method.

defaultRestoreT

Arguments

:: (Monad m, MonadTransControl n) 
=> (n m a -> t m a)

Monad constructor

-> (StT t a -> StT n a)

StT deconstructor

-> m (StT t a) 
-> t m a 

MonadBaseControl

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

Associated Types

data StM m :: * -> *

Monadic state of m.

Methods

liftBaseWith :: (RunInBase m b -> 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 m 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

type RunInBase m b = forall a. m a -> b (StM m 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 for MonadBaseControl

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
     newtype StM (T m) a = StMT {unStMT :: ComposeSt T m a}
     liftBaseWith = defaultLiftBaseWith StMT
     restoreM     = defaultRestoreM   unStMT

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

 instance MonadBaseControl B B where
     newtype StM B a = StMB {unStMB :: a}
     liftBaseWith f  = f $ liftM  StMB
     restoreM        = return . unStMB

type ComposeSt t m a = StM m (StT t a)

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

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

defaultLiftBaseWith

Arguments

:: (MonadTransControl t, MonadBaseControl b m) 
=> (forall c. ComposeSt t m c -> StM (t m) c)

StM constructor

-> (RunInBase (t m) b -> 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 stM = \f -> liftWith $ \run ->
                                   liftBaseWith $ \runInBase ->
                                     f $ liftM stM . runInBase . run

defaultRestoreM

Arguments

:: (MonadTransControl t, MonadBaseControl b m) 
=> (StM (t m) a -> ComposeSt t m a)

StM deconstructor

-> StM (t m) a -> t m a 

Default definition for the restoreM method.

Note that: defaultRestoreM unStM = restoreT . restoreM . unStM

Utility functions

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

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

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