bracket package:base-prelude

When you want to acquire a resource, do some work with it, and then release the resource, it is a good idea to use bracket, because bracket will install the necessary exception handler to release the resource in the event that an exception is raised during the computation. If an exception is raised, then bracket will re-raise the exception (after performing the release). A common example is opening a file:
bracket
(openFile "filename" ReadMode)
(hClose)
(\fileHandle -> do { ... })
The arguments to bracket are in this order so that we can partially apply it, e.g.:
withFile name mode = bracket (openFile name mode) hClose
Bracket wraps the release action with mask, which is sufficient to ensure that the release action executes to completion when it does not invoke any interruptible actions, even in the presence of asynchronous exceptions. For example, hClose is uninterruptible when it is not racing other uses of the handle. Similarly, closing a socket (from "network" package) is also uninterruptible under similar conditions. An example of an interruptible action is killThread. Completion of interruptible release actions can be ensured by wrapping them in in uninterruptibleMask_, but this risks making the program non-responsive to Control-C, or timeouts. Another option is to run the release action asynchronously in its own thread:
void $ uninterruptibleMask_ $ forkIO $ do { ... }
The resource will be released as soon as possible, but the thread that invoked bracket will not block in an uninterruptible state.
Like bracket, but only performs the final action if there was an exception raised by the in-between computation.
A variant of bracket where the return value from the first computation is not required.