Control.Monad.Trans.Resource

resourcet-1.1.7: Deterministic allocation and freeing of scarce resources.

Safe Haskell | Safe
Language | Haskell98

Control.Monad.Trans.Resource

Contents

- Data types
- Unwrap
- Special actions
- Monad transformation
- Registering/releasing
- Type class/associated types
  - Low-level
- Re-exports
- Internal state
- Backwards compatibility

Description

Allocate resources which are guaranteed to be released.

For more information, see https://www.fpcomplete.com/user/snoyberg/library-documentation/resourcet.

One point to note: all register cleanup actions live in the `IO` monad, not the main monad. This allows both more efficient code, and for monads to be transformed.

Synopsis

- data ResourceT m a
- type ResIO a = ResourceT IO a
- data ReleaseKey
- runResourceT :: MonadBaseControl IO m => ResourceT m a -> m a
- resourceForkIO :: MonadBaseControl IO m => ResourceT m () -> ResourceT m ThreadId
- transResourceT :: (m a -> n b) -> ResourceT m a -> ResourceT n b
- joinResourceT :: ResourceT (ResourceT m) a -> ResourceT m a
- allocate :: MonadResource m => IO a -> (a -> IO ()) -> m (ReleaseKey, a)
- register :: MonadResource m => IO () -> m ReleaseKey
- release :: MonadIO m => ReleaseKey -> m ()
- unprotect :: MonadIO m => ReleaseKey -> m (Maybe (IO ()))
- resourceMask :: MonadResource m => ((forall a. ResourceT IO a -> ResourceT IO a) -> ResourceT IO b) -> m b
- class (MonadThrow m, MonadIO m, Applicative m, MonadBase IO m) => MonadResource m where
  - liftResourceT :: ResourceT IO a -> m a
- type MonadResourceBase m = (MonadBaseControl IO m, MonadThrow m, MonadBase IO m, MonadIO m, Applicative m)
- data InvalidAccess = InvalidAccess {
  - functionName :: String
  }
- class MonadBase b m => MonadBaseControl b m | m -> b
- type InternalState = IORef ReleaseMap
- getInternalState :: Monad m => ResourceT m InternalState
- runInternalState :: ResourceT m a -> InternalState -> m a
- withInternalState :: (InternalState -> m a) -> ResourceT m a
- createInternalState :: MonadBase IO m => m InternalState
- closeInternalState :: MonadBase IO m => InternalState -> m ()
- type ExceptionT = CatchT
- runExceptionT :: ExceptionT m a -> m (Either SomeException a)
- runExceptionT_ :: Monad m => ExceptionT m a -> m a
- runException :: ExceptionT Identity a -> Either SomeException a
- runException_ :: ExceptionT Identity a -> a
- class Monad m => MonadThrow m where
  - throwM :: Exception e => e -> m a
- monadThrow :: (Exception e, MonadThrow m) => e -> m a

# Data types

data ResourceT m a

The Resource transformer. This transformer keeps track of all registered actions, and calls them upon exit (via `runResourceT`). Actions may be registered via `register`, or resources may be allocated atomically via `allocate`. `allocate` corresponds closely to `bracket`.

Releasing may be performed before exit via the `release` function. This is a highly recommended optimization, as it will ensure that scarce resources are freed early. Note that calling `release` will deregister the action, so that a release action will only ever be called once.

Since 0.3.0

Instances

MFunctor ResourceT | Since 0.4.7
MMonad ResourceT | Since 0.4.7
MonadTrans ResourceT |
MonadTransControl ResourceT |
MonadRWS r w s m => MonadRWS r w s (ResourceT m) |
MonadBase b m => MonadBase b (ResourceT m) |
MonadBaseControl b m => MonadBaseControl b (ResourceT m) |
MonadError e m => MonadError e (ResourceT m) |
MonadReader r m => MonadReader r (ResourceT m) |
MonadState s m => MonadState s (ResourceT m) |
MonadWriter w m => MonadWriter w (ResourceT m) |
Monad m => Monad (ResourceT m) |
Functor m => Functor (ResourceT m) |
Applicative m => Applicative (ResourceT m) |
Alternative m => Alternative (ResourceT m) | Since 1.1.5
MonadPlus m => MonadPlus (ResourceT m) | Since 1.1.5
MonadThrow m => MonadThrow (ResourceT m) |
MonadCatch m => MonadCatch (ResourceT m) |
MonadMask m => MonadMask (ResourceT m) |
MonadIO m => MonadIO (ResourceT m) |
MonadCont m => MonadCont (ResourceT m) |
(MonadThrow m, MonadBase IO m, MonadIO m, Applicative m) => MonadResource (ResourceT m) |
type StT ResourceT a = a |
type StM (ResourceT m) a = StM m a |

type ResIO a = ResourceT IO a

Convenient alias for `ResourceT IO`.

data ReleaseKey

A lookup key for a specific release action. This value is returned by `register` and `allocate`, and is passed to `release`.

Since 0.3.0

# Unwrap

runResourceT :: MonadBaseControl IO m => ResourceT m a -> m a

Unwrap a `ResourceT` transformer, and call all registered release actions.

Note that there is some reference counting involved due to `resourceForkIO`. If multiple threads are sharing the same collection of resources, only the last call to `runResourceT` will deallocate the resources.

Since 0.3.0

# Special actions

resourceForkIO :: MonadBaseControl IO m => ResourceT m () -> ResourceT m ThreadId

Introduce a reference-counting scheme to allow a resource context to be shared by multiple threads. Once the last thread exits, all remaining resources will be released.

Note that abuse of this function will greatly delay the deallocation of registered resources. This function should be used with care. A general guideline:

If you are allocating a resource that should be shared by multiple threads, and will be held for a long time, you should allocate it at the beginning of a new `ResourceT` block and then call `resourceForkIO` from there.

Since 0.3.0

# Monad transformation

transResourceT :: (m a -> n b) -> ResourceT m a -> ResourceT n b

Transform the monad a `ResourceT` lives in. This is most often used to strip or add new transformers to a stack, e.g. to run a `ReaderT`.

Note that this function is a slight generalization of `hoist`.

Since 0.3.0

joinResourceT :: ResourceT (ResourceT m) a -> ResourceT m a

This function mirrors `join` at the transformer level: it will collapse two levels of `ResourceT` into a single `ResourceT`.

Since 0.4.6

# Registering/releasing

allocate

Arguments

:: MonadResource m |
=> IO a | allocate
-> (a -> IO ()) | free resource
-> m (ReleaseKey, a) |

Perform some allocation, and automatically register a cleanup action.

This is almost identical to calling the allocation and then `register`ing the release action, but this properly handles masking of asynchronous exceptions.

Since 0.3.0

register :: MonadResource m => IO () -> m ReleaseKey

Register some action that will be called precisely once, either when `runResourceT` is called, or when the `ReleaseKey` is passed to `release`.

Since 0.3.0

release :: MonadIO m => ReleaseKey -> m ()

Call a release action early, and deregister it from the list of cleanup actions to be performed.

Since 0.3.0

unprotect :: MonadIO m => ReleaseKey -> m (Maybe (IO ()))

Unprotect resource from cleanup actions, this allowes you to send resource into another resourcet process and reregister it there. It returns an release action that should be run in order to clean resource or Nothing in case if resource is already freed.

Since 0.4.5

resourceMask :: MonadResource m => ((forall a. ResourceT IO a -> ResourceT IO a) -> ResourceT IO b) -> m b

Perform asynchronous exception masking.

This is more general then `Control.Exception.mask`, yet more efficient than `Control.Exception.Lifted.mask`.

Since 0.3.0

# Type class/associated types

class (MonadThrow m, MonadIO m, Applicative m, MonadBase IO m) => MonadResource m where

A `Monad` which allows for safe resource allocation. In theory, any monad transformer stack included a `ResourceT` can be an instance of `MonadResource`.

Note: `runResourceT` has a requirement for a `MonadBaseControl IO m` monad, which allows control operations to be lifted. A `MonadResource` does not have this requirement. This means that transformers such as `ContT` can be an instance of `MonadResource`. However, the `ContT` wrapper will need to be unwrapped before calling `runResourceT`.

Since 0.3.0

Methods

liftResourceT :: ResourceT IO a -> m a

Lift a `ResourceT IO` action into the current `Monad`.

Since 0.4.0

Instances

MonadResource m => MonadResource (ListT m) |
MonadResource m => MonadResource (MaybeT m) |
MonadResource m => MonadResource (IdentityT m) |
(MonadThrow m, MonadBase IO m, MonadIO m, Applicative m) => MonadResource (ResourceT m) |
MonadResource m => MonadResource (ContT r m) |
MonadResource m => MonadResource (ReaderT r m) |
MonadResource m => MonadResource (StateT s m) |
MonadResource m => MonadResource (StateT s m) |
MonadResource m => MonadResource (ExceptT e m) |
(Error e, MonadResource m) => MonadResource (ErrorT e m) |
(Monoid w, MonadResource m) => MonadResource (WriterT w m) |
(Monoid w, MonadResource m) => MonadResource (WriterT w m) |
(Monoid w, MonadResource m) => MonadResource (RWST r w s m) |
(Monoid w, MonadResource m) => MonadResource (RWST r w s m) |

type MonadResourceBase m = (MonadBaseControl IO m, MonadThrow m, MonadBase IO m, MonadIO m, Applicative m)

A `Monad` which can be used as a base for a `ResourceT`.

A `ResourceT` has some restrictions on its base monad:

- `runResourceT` requires an instance of `MonadBaseControl IO`.
- `MonadResource` requires an instance of `MonadThrow`, `MonadIO`, and `Applicative`.

While any instance of `MonadBaseControl IO` should be an instance of the other classes, this is not guaranteed by the type system (e.g., you may have a transformer in your stack with does not implement `MonadThrow`). Ideally, we would like to simply create an alias for the five type classes listed, but this is not possible with GHC currently.

Instead, this typeclass acts as a proxy for the other five. Its only purpose is to make your type signatures shorter.

Note that earlier versions of `conduit` had a typeclass `ResourceIO`. This fulfills much the same role.

Since 0.3.2

## Low-level

data InvalidAccess

Indicates either an error in the library, or misuse of it (e.g., a `ResourceT`'s state is accessed after being released).

Since 0.3.0

Constructors

InvalidAccess |
Fields functionName :: String

Instances

Show InvalidAccess |
Exception InvalidAccess |

# Re-exports

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

Minimal complete definition

liftBaseWith, restoreM

Instances

MonadBaseControl [] [] |
MonadBaseControl IO IO |
MonadBaseControl Identity Identity |
MonadBaseControl STM STM |
MonadBaseControl Maybe Maybe |
MonadBaseControl b m => MonadBaseControl b (MaybeT m) |
MonadBaseControl b m => MonadBaseControl b (ListT m) |
MonadBaseControl b m => MonadBaseControl b (IdentityT m) |
MonadBaseControl b m => MonadBaseControl b (ResourceT m) |
(Monoid w, MonadBaseControl b m) => MonadBaseControl b (WriterT w m) |
(Monoid w, MonadBaseControl b m) => MonadBaseControl b (WriterT w m) |
MonadBaseControl b m => MonadBaseControl b (StateT s m) |
MonadBaseControl b m => MonadBaseControl b (StateT s m) |
MonadBaseControl b m => MonadBaseControl b (ReaderT r m) |
MonadBaseControl b m => MonadBaseControl b (ExceptT e m) |
(Error e, MonadBaseControl b m) => MonadBaseControl b (ErrorT e m) |
(Monoid w, MonadBaseControl b m) => MonadBaseControl b (RWST r w s m) |
(Monoid w, MonadBaseControl b m) => MonadBaseControl b (RWST r w s m) |
MonadBaseControl ((->) r) ((->) r) |
MonadBaseControl (Either e) (Either e) |
MonadBaseControl (ST s) (ST s) |
MonadBaseControl (ST s) (ST s) |

# Internal state

A `ResourceT` internally is a modified `ReaderT` monad transformer holding onto a mutable reference to all of the release actions still remaining to be performed. If you are building up a custom application monad, it may be more efficient to embed this `ReaderT` functionality directly in your own monad instead of wrapping around `ResourceT` itself. This section provides you the means of doing so.

type InternalState = IORef ReleaseMap

The internal state held by a `ResourceT` transformer.

Since 0.4.6

getInternalState :: Monad m => ResourceT m InternalState

Get the internal state of the current `ResourceT`.

Since 0.4.6

runInternalState :: ResourceT m a -> InternalState -> m a

Unwrap a `ResourceT` using the given `InternalState`.

Since 0.4.6

withInternalState :: (InternalState -> m a) -> ResourceT m a

Run an action in the underlying monad, providing it the `InternalState`.

Since 0.4.6

createInternalState :: MonadBase IO m => m InternalState

Create a new internal state. This state must be closed with `closeInternalState`. It is your responsibility to ensure exception safety. Caveat emptor!

Since 0.4.9

closeInternalState :: MonadBase IO m => InternalState -> m ()

Close an internal state created by `createInternalState`.

Since 0.4.9

# Backwards compatibility

type ExceptionT = CatchT

For backwards compatibility.

runExceptionT :: ExceptionT m a -> m (Either SomeException a)

For backwards compatibility.

runExceptionT_ :: Monad m => ExceptionT m a -> m a

Same as `runExceptionT`, but immediately `throw` any exception returned.

Since 0.3.0

runException :: ExceptionT Identity a -> Either SomeException a

Run an `ExceptionT Identity` stack.

Since 0.4.2

runException_ :: ExceptionT Identity a -> a

Run an `ExceptionT Identity` stack, but immediately `throw` any exception returned.

Since 0.4.2

class Monad m => MonadThrow m where

A class for monads in which exceptions may be thrown.

Instances should obey the following law:

    throwM e >> x = throwM e

In other words, throwing an exception short-circuits the rest of the monadic computation.

Methods

throwM :: Exception e => e -> m a

Throw an exception. Note that this throws when this action is run in the monad `m`, not when it is applied. It is a generalization of Control.Exception's `throwIO`.

Should satisfy the law:

    throwM e >> f = throwM e

Instances

MonadThrow [] |
MonadThrow IO |
MonadThrow STM |
MonadThrow Maybe |
(~) * e SomeException => MonadThrow (Either e) |
Monad m => MonadThrow (CatchT m) |
MonadThrow m => MonadThrow (ListT m) |
MonadThrow m => MonadThrow (MaybeT m) | Throws exceptions into the base monad.
MonadThrow m => MonadThrow (IdentityT m) |
MonadThrow m => MonadThrow (ResourceT m) |
MonadThrow m => MonadThrow (ContT r m) |
MonadThrow m => MonadThrow (ReaderT r m) |
MonadThrow m => MonadThrow (StateT s m) |
MonadThrow m => MonadThrow (StateT s m) |
MonadThrow m => MonadThrow (ExceptT e m) | Throws exceptions into the base monad.
(Error e, MonadThrow m) => MonadThrow (ErrorT e m) | Throws exceptions into the base monad.
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) |
(MonadThrow m, Monoid w) => MonadThrow (WriterT w m) |
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) |
(MonadThrow m, Monoid w) => MonadThrow (RWST r w s m) |

monadThrow :: (Exception e, MonadThrow m) => e -> m a

Backwards compatibility