Copyright Bas van Dijk, Anders Kaseorg BSD-style Bas van Dijk experimental Safe Haskell98

Description

Synopsis

Minimal complete definition

Associated Types

type StT t a :: * Source #

Monadic state of t.

Methods

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

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 Source #

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) Source #

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}

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


type RunDefault t t' = forall n b. Monad n => t n b -> n (StT t' b) Source #

A function like Run that runs a monad transformer t which wraps the monad transformer t'. This is used in 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 -> (RunDefault t n -> m a) -> t m a

Default definition for the liftWith method.

Arguments

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

Default definition for the restoreT method.

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

Minimal complete definition

Associated Types

type StM m a :: * Source #

Monadic state of m.

Methods

liftBaseWith :: (RunInBase m b -> b a) -> m a Source #

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 Source #

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) Source #

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 ComposeSt t m a = StM m (StT t a) Source #

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

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

type RunInBaseDefault t m b = forall a. t m a -> b (ComposeSt t m a) Source #

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

defaultLiftBaseWith :: (MonadTransControl t, MonadBaseControl b m) => (RunInBaseDefault t m b -> b a) -> t m a Source #

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, MonadBaseControl b m) => ComposeSt t m a -> t m a Source #

Default definition for the restoreM method.

Note that: defaultRestoreM = restoreT . restoreM

# Utility functions

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

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

embed :: MonadBaseControl b m => (a -> m c) -> m (a -> b (StM m c)) Source #

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

embed_ :: MonadBaseControl b m => (a -> m ()) -> m (a -> b ()) Source #

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

captureT :: (MonadTransControl t, Monad (t m), Monad m) => t m (StT t ()) Source #

Capture the current state of a transformer

captureM :: MonadBaseControl b m => m (StM m ()) Source #

Capture the current state above the base monad

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

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 Source #

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 Source #

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 => ((a -> b ()) -> b c) -> (a -> m ()) -> m c Source #

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 ()

Arguments

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

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