Control.Monad.Trans.Region

This modules implements a technique called "Lightweight monadic regions" invented by Oleg Kiselyov and Chung-chieh Shan

See: http://okmij.org/ftp/Haskell/regions.html#light-weight

Regions

data RegionT s pr α

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.

Instances

MonadTrans (RegionT s)

Monad pr => Monad (RegionT s pr)

Functor pr => Functor (RegionT s pr)

MonadFix pr => MonadFix (RegionT s pr)

MonadPlus pr => MonadPlus (RegionT s pr)

Applicative pr => Applicative (RegionT s pr)

MonadCatchIO pr => MonadCatchIO (RegionT s pr)

Alternative pr => Alternative (RegionT s pr)

MonadIO pr => MonadIO (RegionT s pr)

Running regions

runRegionT :: MonadCatchIO pr => (forall s. RegionT s pr α) -> pr α

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.

type TopRegion s = RegionT s IO

A region which has IO as its parent region which enables it to be:

• directly executed in IO by runTopRegion,
• concurrently executed in a new thread by forkTopRegion.

runTopRegion :: (forall s. TopRegion s α) -> IO α

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

Note that: runTopRegion = runRegionT

forkTopRegion :: MonadIO pr => TopRegion s () -> RegionT s pr ThreadId

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.

Opening resources

data RegionalHandle resource r

A handle to an opened resource parameterized by the resource and the region r in which it was created.

Instances

Resource resource => Dup (RegionalHandle resource)

open :: (Resource resource, MonadCatchIO pr) => resource -> RegionT s pr (RegionalHandle resource (RegionT s pr))

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.

with :: (Resource resource, MonadCatchIO pr) => resource -> (forall s. RegionalHandle resource (RegionT s pr) -> RegionT s pr α) -> pr α

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 resource f = runRegionT (open resource >>= f).

Duplication

class Dup α where

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 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
        return r1hDup

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

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

Methods

dup :: MonadCatchIO ppr => α (RegionT cs (RegionT ps ppr)) -> RegionT cs (RegionT ps ppr) (α (RegionT ps ppr))

Instances

Resource resource => Dup (RegionalHandle resource)

Parent/child relationship between regions.

class (Monad pr, Monad cr) => ParentOf pr cr

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 pcs
                                (RegionT ppcs
                                  (RegionT ps ppr)))

Instances

(Monad cr, TypeCast2 cr (RegionT s pcr), ParentOf pr pcr) => ParentOf pr cr

Monad r => ParentOf r r

Handy functions for writing monadic instances

liftCallCC :: (((α -> pr β) -> pr α) -> pr α) -> ((α -> RegionT s pr β) -> RegionT s pr α) -> RegionT s pr α

Lift a callCC operation to the new monad.

mapRegionT :: (m α -> n β) -> RegionT s m α -> RegionT s n β

Transform the computation inside a region.

liftCatch

Arguments

:: (pr α -> (e -> pr α) -> pr α)

catch on the argument monad.

-> RegionT s pr α

Computation to attempt.

-> (e -> RegionT s pr α)

Exception handler.

-> RegionT s pr α

Lift a catchError operation to the new monad.