A monad transformer in which resources of type resource can be opened which are automatically closed when the region terminates.

Note that regions can be nested. pr (for parent region) is a monad which is usually the region which is running this region. However when you are running a TopRegion the parent region will be IO.

Execute a region inside its parent region pr.

All resources which have been opened in the given region using open, and which haven't been duplicated using dup, will be closed on exit from this function wether by normal termination or by raising an exception.

Also all resources which have been duplicated to this region from a child region are closed on exit if they haven't been duplicated themselves.

Note the type variable s of the region wich is only quantified over the region itself. This ensures that all values, having a type containing s, can not be returned from this function. (Note the similarity with the ST monad.)

An example of such a value is a RegionalHandle. Regional handles are created by opening a resource in a region using open. Regional handles are parameterized by the region in which they were created. So regional handles have this s in their type. This ensures that these regional handles, which may have been closed on exit from this function, can't be returned from this function. This ensures you can never do any IO with a closed regional handle.

Note that it is possible to run a region inside another region.

Convenience funtion for running a top-level region in IO.

Note that: runTopRegion = runRegionT

Return a region which executes the given top-level region in a new thread.

Note that the forked region has the same type variable s as the resulting region. This means that all values which can be referenced in the resulting region (like RegionalHandles for example) can also be referenced in the forked region.

For example the following is allowed:

runRegionT $ do
  regionalHndl <- open resource
  threadId <- forkTopRegion $ doSomethingWith regionalHndl
  doSomethingElseWith regionalHndl

Note that the regionalHndl and all other resources opened in the current thread are closed only when the current thread or the forked thread terminates whichever comes last.

Open the given resource in a region yielding a regional handle to it.

Note that the returned regional handle is parameterized by the region in which it was created. This ensures that regional handles can never escape their region. And it also allows operations on regional handles to be executed in a child region of the region in which the regional handle was created.

Note that if you do wish to return a regional handle from the region in which it was created you have to duplicate the handle by applying dup to it.

A convenience function which opens the given resource, applies the given continuation function to the resulting regional handle and runs the resulting region.

Note that: with dev f = runRegionT (open dev >>= f)