Safe Haskell	None
Language	Haskell2010

Control.Monad.Trans.Managed

Synopsis

newtype ManagedT m a = ManagedT {
- runManagedT :: forall r. (a -> m r) -> m r
}
allocate :: MonadBaseControl IO m => m a -> (a -> m b) -> ManagedT m a
allocate_ :: MonadBaseControl IO m => m a -> m b -> ManagedT m ()
lowerManagedT :: Monad m => ManagedT m a -> m a
hoistManagedTReaderT :: Monad m => r -> ManagedT (ReaderT r m) a -> ManagedT m a

Documentation

newtype ManagedT m a Source #

This type is like a transformer version of the Managed monad from the managed library. It can be used to manage resources by pairing together their allocation with their finalizers.

The documentation for the managed library is an excellent introduction to the idea here.

We could use Codensity directly, but we'd have to define an orphan instance for MonadFix. This also gives us the opportunity to give it a slightly more friendly name.

We could also have used ResourceT, but that would have involved writing instances for MonadUnliftIO. That could still be a good option to consider later, however.

Constructors

ManagedT
Fields runManagedT :: forall r. (a -> m r) -> m r

Instances

Instances details

MonadTrans ManagedT Source #
Instance details Defined in Control.Monad.Trans.Managed Methods lift :: Monad m => m a -> ManagedT m a #
MonadReader s m => MonadState s (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods get :: ManagedT m s # put :: s -> ManagedT m () # state :: (s -> (a, s)) -> ManagedT m a #
MonadReader r m => MonadReader r (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods ask :: ManagedT m r # local :: (r -> r) -> ManagedT m a -> ManagedT m a # reader :: (r -> a) -> ManagedT m a #
Monad (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods (>>=) :: ManagedT m a -> (a -> ManagedT m b) -> ManagedT m b # (>>) :: ManagedT m a -> ManagedT m b -> ManagedT m b # return :: a -> ManagedT m a #
Functor (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods fmap :: (a -> b) -> ManagedT m a -> ManagedT m b # (<$) :: a -> ManagedT m b -> ManagedT m a #
MonadIO m => MonadFix (ManagedT m) Source #	We need this instance to tie the knot when initializing resources. It'd be nice if we could do this with a `MonadFix` constraint on the underlying monad, but here we just use `MonadIO` to tie the knot using a lazily-evaluated `MVar`-based promise for the eventual result. We need to be careful not to leak allocated resources via the use of recursively-defined monadic actions when making use of this instance.
Instance details Defined in Control.Monad.Trans.Managed Methods mfix :: (a -> ManagedT m a) -> ManagedT m a #
Applicative (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods pure :: a -> ManagedT m a # (<>) :: ManagedT m (a -> b) -> ManagedT m a -> ManagedT m b # liftA2 :: (a -> b -> c) -> ManagedT m a -> ManagedT m b -> ManagedT m c # (>) :: ManagedT m a -> ManagedT m b -> ManagedT m b # (<*) :: ManagedT m a -> ManagedT m b -> ManagedT m a #
MonadIO m => MonadIO (ManagedT m) Source #
Instance details Defined in Control.Monad.Trans.Managed Methods liftIO :: IO a -> ManagedT m a #

allocate :: MonadBaseControl IO m => m a -> (a -> m b) -> ManagedT m a Source #

Allocate a resource by providing setup and finalizer actions.

allocate_ :: MonadBaseControl IO m => m a -> m b -> ManagedT m () Source #

Allocate a resource but do not return a reference to it.

lowerManagedT :: Monad m => ManagedT m a -> m a Source #

Run the provided computation by returning its result, and run any finalizers. Watch out: this function might leak finalized resources.

hoistManagedTReaderT :: Monad m => r -> ManagedT (ReaderT r m) a -> ManagedT m a Source #