Safe Haskell | Safe |
---|---|

Language | Haskell98 |

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.

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

# Data types

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

data ReleaseKey Source

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 Source

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 Source

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 Source

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 Source

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

:: 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 Source

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 () Source

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 ())) Source

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 Source

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 Source

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

liftResourceT :: ResourceT IO a -> m a Source

Lift a `ResourceT IO`

action into the current `Monad`

.

Since 0.4.0

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) Source

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 Source

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

# Re-exports

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

# 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 Source

The internal state held by a `ResourceT`

transformer.

Since 0.4.6

getInternalState :: Monad m => ResourceT m InternalState Source

Get the internal state of the current `ResourceT`

.

Since 0.4.6

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

Unwrap a `ResourceT`

using the given `InternalState`

.

Since 0.4.6

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

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

.

Since 0.4.6

createInternalState :: MonadBase IO m => m InternalState Source

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 () Source

Close an internal state created by `createInternalState`

.

Since 0.4.9

# Backwards compatibility

type ExceptionT = CatchT Source

For backwards compatibility.

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

For backwards compatibility.

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

Same as `runExceptionT`

, but immediately `throw`

any exception returned.

Since 0.3.0

runException :: ExceptionT Identity a -> Either SomeException a Source

Run an `ExceptionT Identity`

stack.

Since 0.4.2

runException_ :: ExceptionT Identity a -> a Source

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.

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

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 Source

Backwards compatibility