Class of scarce resources. A scarce resource is a resource that only one user can use at a time. (like a file, memory pointer or USB device for example).

Because of the scarcity, these resources need to be opened to grant temporary sole access to the resource. When the resource is no longer needed it should be closed a.s.a.p to grant others access to the resource.

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)

Duplicate an α in the parent region. This α will usually be a (RegionalHandle resource) but it can be any value "derived" from this regional handle.

For example, suppose you run the following region:

runRegionT $ do

Inside this region you run a nested child region like:

    r1hDup <- runRegionT $ do

Now in this child region you open the resource r1:

        r1h <- open r1

...yielding the regional handle r1h. Note that:

r1h :: RegionalHandle resource (RegionT resource cs (RegionT resource ps ppr))

where cs is bound by the inner (child) runRegionT and ps is bound by the outer (parent) runRegionT.

Suppose you want to use the resulting regional handle r1h in the parent device region. You can't simply return r1h because then the type variable cs, escapes the inner region.

However, if you duplicate the regional handle you can safely return it.

        r1hDup <- dup r1h
        return r1hDup

Note that r1hDup :: RegionalHandle resource (RegionT resource ps ppr)

Back in the parent region you can safely operate on r1hDup.

The ParentOf class declares the parent/child relationship between regions.

A region is the parent of another region if they're either equivalent like:

RegionT resource ps pr  `ParentOf`  RegionT resource ps pr

or if it is the parent of the parent of the child like:

RegionT resource ps ppr `ParentOf` RegionT resource cs
                                     (RegionT resource pcs
                                       (RegionT resource ppcs
                                         (RegionT resource ps ppr)))