|Maintainer||Bas van Dijk <firstname.lastname@example.org>|
This modules implements a technique called "Lightweight monadic regions"
invented by Oleg Kiselyov and Chung-chieh Shan
A monad transformer in which scarce resources 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.
A region which has IO as its parent region which enables it to be:
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
For example the following is allowed:
runRegionT $ do
regionalHndl <- open resource
threadId <- forkTopRegion $ doSomethingWith 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.
|data RegionalHandle resource r ||Source|
|A handle to an opened resource parameterized by the resource and the
region r in which it was created.
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
Note that: with resource f = runRegionT (open resource >>= 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
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 cs (RegionT 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
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
Note that r1hDup :: RegionalHandle resource (RegionT ps ppr)
Back in the parent region you can safely operate on r1hDup.
|Parent/child relationship between regions.
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 ps pr `ParentOf` RegionT ps pr
or if it is the parent of the parent of the child like:
RegionT ps ppr `ParentOf` RegionT cs
(RegionT ps ppr)))
|Handy functions for writing monadic instances
|Transform the computation inside a region.
|=> pr α -> (e -> pr α) -> pr α||Computation to attempt.
|-> RegionT s pr α||Exception handler.
|-> e -> RegionT s pr α|
|-> RegionT s pr α|
|Lift a catchError operation to the new monad.
|TODO: define and export: liftCallCC
|Produced by Haddock version 2.6.0|