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{-# LANGUAGE CPP
, NoImplicitPrelude
, RankNTypes
, TypeFamilies
, FunctionalDependencies
, FlexibleInstances
, UndecidableInstances
, MultiParamTypeClasses #-}
#if __GLASGOW_HASKELL__ >= 702
{-# LANGUAGE Safe #-}
#endif
#if MIN_VERSION_transformers(0,4,0)
-- Hide warnings for the deprecated ErrorT transformer:
{-# OPTIONS_GHC -fno-warn-warnings-deprecations #-}
#endif
{- |
Module : Control.Monad.Trans.Control
Copyright : Bas van Dijk, Anders Kaseorg
License : BSD-style
Maintainer : Bas van Dijk <v.dijk.bas@gmail.com>
Stability : experimental
-}
module Control.Monad.Trans.Control
( -- * MonadTransControl
MonadTransControl(..), Run
-- ** Defaults for MonadTransControl
-- $MonadTransControlDefaults
, RunDefault, defaultLiftWith, defaultRestoreT
-- * MonadBaseControl
, MonadBaseControl (..), RunInBase
-- ** Defaults for MonadBaseControl
-- $MonadBaseControlDefaults
, ComposeSt, RunInBaseDefault, defaultLiftBaseWith, defaultRestoreM
-- * Utility functions
, control, embed, embed_
, liftBaseOp, liftBaseOp_
, liftBaseDiscard, liftBaseOpDiscard
) where
--------------------------------------------------------------------------------
-- Imports
--------------------------------------------------------------------------------
-- from base:
import Data.Function ( (.), ($), const )
import Data.Monoid ( Monoid, mempty )
import Control.Monad ( Monad, (>>=), return, liftM )
import System.IO ( IO )
import Data.Maybe ( Maybe )
import Data.Either ( Either )
#if MIN_VERSION_base(4,4,0)
import Control.Monad.ST.Lazy.Safe ( ST )
import qualified Control.Monad.ST.Safe as Strict ( ST )
#endif
-- from stm:
import Control.Monad.STM ( STM )
-- from transformers:
import Control.Monad.Trans.Class ( MonadTrans )
import Control.Monad.Trans.Identity ( IdentityT(IdentityT), runIdentityT )
import Control.Monad.Trans.List ( ListT (ListT), runListT )
import Control.Monad.Trans.Maybe ( MaybeT (MaybeT), runMaybeT )
import Control.Monad.Trans.Error ( ErrorT (ErrorT), runErrorT, Error )
import Control.Monad.Trans.Reader ( ReaderT (ReaderT), runReaderT )
import Control.Monad.Trans.State ( StateT (StateT), runStateT )
import Control.Monad.Trans.Writer ( WriterT (WriterT), runWriterT )
import Control.Monad.Trans.RWS ( RWST (RWST), runRWST )
import Control.Monad.Trans.Except ( ExceptT (ExceptT), runExceptT )
import qualified Control.Monad.Trans.RWS.Strict as Strict ( RWST (RWST), runRWST )
import qualified Control.Monad.Trans.State.Strict as Strict ( StateT (StateT), runStateT )
import qualified Control.Monad.Trans.Writer.Strict as Strict ( WriterT(WriterT), runWriterT )
import Data.Functor.Identity ( Identity )
-- from transformers-base:
import Control.Monad.Base ( MonadBase )
#if MIN_VERSION_base(4,3,0)
import Control.Monad ( void )
#else
import Data.Functor (Functor, fmap)
void :: Functor f => f a -> f ()
void = fmap (const ())
#endif
import Prelude (id)
--------------------------------------------------------------------------------
-- MonadTransControl type class
--------------------------------------------------------------------------------
class MonadTrans t => MonadTransControl t where
-- | Monadic state of @t@.
type StT t 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.
liftWith :: Monad m => (Run t -> m 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@
restoreT :: Monad m => m (StT t a) -> t m a
-- | 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'.
type Run t = forall n b. Monad n => t n b -> n (StT t b)
--------------------------------------------------------------------------------
-- Defaults for MonadTransControl
--------------------------------------------------------------------------------
-- $MonadTransControlDefaults
--
-- 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
-- @
-- | A function like 'Run' that runs a monad transformer @t@ which wraps the
-- monad transformer @t'@. This is used in 'defaultLiftWith'.
type RunDefault t t' = forall n b. Monad n => t n b -> n (StT t' b)
-- | Default definition for the 'liftWith' method.
defaultLiftWith :: (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
defaultLiftWith t unT = \f -> t $ liftWith $ \run -> f $ run . unT
{-# INLINABLE defaultLiftWith #-}
-- | Default definition for the 'restoreT' method.
defaultRestoreT :: (Monad m, MonadTransControl n)
=> (n m a -> t m a) -- ^ Monad constructor
-> m (StT n a)
-> t m a
defaultRestoreT t = t . restoreT
{-# INLINABLE defaultRestoreT #-}
--------------------------------------------------------------------------------
-- MonadTransControl instances
--------------------------------------------------------------------------------
instance MonadTransControl IdentityT where
type StT IdentityT a = a
liftWith f = IdentityT $ f $ runIdentityT
restoreT = IdentityT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl MaybeT where
type StT MaybeT a = Maybe a
liftWith f = MaybeT $ liftM return $ f $ runMaybeT
restoreT = MaybeT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance Error e => MonadTransControl (ErrorT e) where
type StT (ErrorT e) a = Either e a
liftWith f = ErrorT $ liftM return $ f $ runErrorT
restoreT = ErrorT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl (ExceptT e) where
type StT (ExceptT e) a = Either e a
liftWith f = ExceptT $ liftM return $ f $ runExceptT
restoreT = ExceptT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl ListT where
type StT ListT a = [a]
liftWith f = ListT $ liftM return $ f $ runListT
restoreT = ListT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl (ReaderT r) where
type StT (ReaderT r) a = a
liftWith f = ReaderT $ \r -> f $ \t -> runReaderT t r
restoreT = ReaderT . const
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl (StateT s) where
type StT (StateT s) a = (a, s)
liftWith f = StateT $ \s ->
liftM (\x -> (x, s))
(f $ \t -> runStateT t s)
restoreT = StateT . const
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance MonadTransControl (Strict.StateT s) where
type StT (Strict.StateT s) a = (a, s)
liftWith f = Strict.StateT $ \s ->
liftM (\x -> (x, s))
(f $ \t -> Strict.runStateT t s)
restoreT = Strict.StateT . const
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance Monoid w => MonadTransControl (WriterT w) where
type StT (WriterT w) a = (a, w)
liftWith f = WriterT $ liftM (\x -> (x, mempty))
(f $ runWriterT)
restoreT = WriterT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance Monoid w => MonadTransControl (Strict.WriterT w) where
type StT (Strict.WriterT w) a = (a, w)
liftWith f = Strict.WriterT $ liftM (\x -> (x, mempty))
(f $ Strict.runWriterT)
restoreT = Strict.WriterT
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance Monoid w => MonadTransControl (RWST r w s) where
type StT (RWST r w s) a = (a, s, w)
liftWith f = RWST $ \r s -> liftM (\x -> (x, s, mempty))
(f $ \t -> runRWST t r s)
restoreT mSt = RWST $ \_ _ -> mSt
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
instance Monoid w => MonadTransControl (Strict.RWST r w s) where
type StT (Strict.RWST r w s) a = (a, s, w)
liftWith f =
Strict.RWST $ \r s -> liftM (\x -> (x, s, mempty))
(f $ \t -> Strict.runRWST t r s)
restoreT mSt = Strict.RWST $ \_ _ -> mSt
{-# INLINABLE liftWith #-}
{-# INLINABLE restoreT #-}
--------------------------------------------------------------------------------
-- MonadBaseControl type class
--------------------------------------------------------------------------------
class MonadBase b m => MonadBaseControl b m | m -> b where
-- | Monadic state of @m@.
type StM 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.
liftBaseWith :: (RunInBase m b -> b 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@
restoreM :: StM m a -> 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'.
type RunInBase m b = forall a. m a -> b (StM m a)
--------------------------------------------------------------------------------
-- MonadBaseControl instances for all monads in the base library
--------------------------------------------------------------------------------
#define BASE(M) \
instance MonadBaseControl (M) (M) where { \
type StM (M) a = a; \
liftBaseWith f = f id; \
restoreM = return; \
{-# INLINABLE liftBaseWith #-}; \
{-# INLINABLE restoreM #-}}
BASE(IO)
BASE(Maybe)
BASE(Either e)
BASE([])
BASE((->) r)
BASE(Identity)
BASE(STM)
#if MIN_VERSION_base(4,4,0)
BASE(Strict.ST s)
BASE( ST s)
#endif
#undef BASE
--------------------------------------------------------------------------------
-- Defaults for MonadBaseControl
--------------------------------------------------------------------------------
-- $MonadBaseControlDefaults
--
-- 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'
-- @
-- | 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 ComposeSt t m a = StM m (StT t a)
-- | A function like 'RunInBase' that runs a monad transformer @t@ in its base
-- monad @b@. It is used in 'defaultLiftBaseWith'.
type RunInBaseDefault t m b = forall a. t m a -> b (ComposeSt 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
-- @
defaultLiftBaseWith :: (MonadTransControl t, MonadBaseControl b m)
=> (RunInBaseDefault t m b -> b a) -> t m a
defaultLiftBaseWith = \f -> liftWith $ \run ->
liftBaseWith $ \runInBase ->
f $ runInBase . run
{-# INLINABLE defaultLiftBaseWith #-}
-- | Default definition for the 'restoreM' method.
--
-- Note that: @defaultRestoreM = 'restoreT' . 'restoreM'@
defaultRestoreM :: (MonadTransControl t, MonadBaseControl b m)
=> ComposeSt t m a -> t m a
defaultRestoreM = restoreT . restoreM
{-# INLINABLE defaultRestoreM #-}
--------------------------------------------------------------------------------
-- MonadBaseControl transformer instances
--------------------------------------------------------------------------------
#define BODY(T) { \
type StM (T m) a = ComposeSt (T) m a; \
liftBaseWith = defaultLiftBaseWith; \
restoreM = defaultRestoreM; \
{-# INLINABLE liftBaseWith #-}; \
{-# INLINABLE restoreM #-}}
#define TRANS( T) \
instance ( MonadBaseControl b m) => MonadBaseControl b (T m) where BODY(T)
#define TRANS_CTX(CTX, T) \
instance (CTX, MonadBaseControl b m) => MonadBaseControl b (T m) where BODY(T)
TRANS(IdentityT)
TRANS(MaybeT)
TRANS(ListT)
TRANS(ReaderT r)
TRANS(Strict.StateT s)
TRANS( StateT s)
TRANS(ExceptT e)
TRANS_CTX(Error e, ErrorT e)
TRANS_CTX(Monoid w, Strict.WriterT w)
TRANS_CTX(Monoid w, WriterT w)
TRANS_CTX(Monoid w, Strict.RWST r w s)
TRANS_CTX(Monoid w, RWST r w s)
--------------------------------------------------------------------------------
-- * Utility functions
--------------------------------------------------------------------------------
-- | An often used composition: @control f = 'liftBaseWith' f >>= 'restoreM'@
control :: MonadBaseControl b m => (RunInBase m b -> b (StM m a)) -> m a
control f = liftBaseWith f >>= restoreM
{-# INLINABLE control #-}
-- | Embed a transformer function as an function in the base monad returning a
-- mutated transformer state.
embed :: MonadBaseControl b m => (a -> m c) -> m (a -> b (StM m c))
embed f = liftBaseWith $ \runInBase -> return (runInBase . f)
{-# INLINABLE embed #-}
-- | Performs the same function as 'embed', but discards transformer state
-- from the embedded function.
embed_ :: MonadBaseControl b m => (a -> m ()) -> m (a -> b ())
embed_ f = liftBaseWith $ \runInBase -> return (void . runInBase . f)
{-# INLINABLE embed_ #-}
-- | @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
=> ((a -> b (StM m c)) -> b (StM m d))
-> ((a -> m c) -> m d)
liftBaseOp f = \g -> control $ \runInBase -> f $ runInBase . g
{-# INLINABLE liftBaseOp #-}
-- | @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@
liftBaseOp_ :: MonadBaseControl b m
=> (b (StM m a) -> b (StM m c))
-> ( m a -> m c)
liftBaseOp_ f = \m -> control $ \runInBase -> f $ runInBase m
{-# INLINABLE liftBaseOp_ #-}
-- | @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@
liftBaseDiscard :: MonadBaseControl b m => (b () -> b a) -> (m () -> m a)
liftBaseDiscard f = \m -> liftBaseWith $ \runInBase -> f $ void $ runInBase m
{-# INLINABLE liftBaseDiscard #-}
-- | @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 ()@
liftBaseOpDiscard :: MonadBaseControl b m
=> ((a -> b ()) -> b c)
-> (a -> m ()) -> m c
liftBaseOpDiscard f g = liftBaseWith $ \runInBase -> f $ void . runInBase . g
{-# INLINABLE liftBaseOpDiscard #-}
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