Maintainer | Bas van Dijk <v.dijk.bas@gmail.com> |
---|
This modules implements a technique called "Lightweight monadic regions" invented by Oleg Kiselyov and Chung-chieh Shan
- data RegionT s pr α
- runRegionT :: RegionControlIO pr => (forall s. RegionT s pr α) -> pr α
- class MonadIO m => RegionControlIO m
- class Dup h where
- class InternalAncestorRegion pr cr => AncestorRegion pr cr
- data RootRegion α
- data LocalRegion sl s α
- data Local s
- liftCallCC :: (((α -> pr β) -> pr α) -> pr α) -> ((α -> RegionT s pr β) -> RegionT s pr α) -> RegionT s pr α
- mapRegionT :: (m α -> n β) -> RegionT s m α -> RegionT s n β
- liftCatch :: (pr α -> (e -> pr α) -> pr α) -> RegionT s pr α -> (e -> RegionT s pr α) -> RegionT s pr α
Regions
A monad transformer in which scarce resources can be opened. When the region terminates, all opened resources will be closed automatically. It's a type error to return an opened resource from the region. The latter ensures no I/O with closed resources is possible.
InternalAncestorRegion pr cr => InternalAncestorRegion pr (RegionT s cr) | |
InternalAncestorRegion RootRegion (RegionT s m) | |
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) | |
Alternative pr => Alternative (RegionT s pr) | |
MonadIO pr => MonadIO (RegionT s pr) | |
RegionControlIO pr => RegionControlIO (RegionT s pr) | |
InternalAncestorRegion (LocalRegion sf s) (RegionT (Local s) m) | |
InternalAncestorRegion (RegionT s m) (RegionT (Local s) m) | |
InternalAncestorRegion (RegionT s m) (RegionT s m) | |
InternalAncestorRegion (RegionT (Local s) m) (RegionT s m) |
Running regions
runRegionT :: RegionControlIO pr => (forall s. RegionT s pr α) -> pr αSource
Execute a region inside its parent region pr
.
All resources which have been opened in the given region 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.)
Note that it is possible to run a region inside another region.
class MonadIO m => RegionControlIO m Source
Regions do not have an instance for MonadControlIO
since that would break the
safety guarantees. (Think about lifting forkIO
into a region!)
However runRegionT
and other operations on regions do need the ability to lift
control operations. This is where the RegionControlIO
class comes in. This
class is identical to MonadControlIO
but its unsafeLiftControlIO
method is
not exported by this module. So user can't accidentally break the safety.
Note that a RegionT
is an instance of this class. For the rest there is a
catch-all instance
.
MonadControlIO
m => RegionControlIO
m
MonadControlIO m => RegionControlIO m | |
RegionControlIO pr => RegionControlIO (RegionT s pr) |
Duplication
Duplicate an h
in the parent region. This h
will usually be some type of
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 (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 (RegionT ps ppr)
Back in the parent region you can safely operate on r1hDup
.
Ancestor relation between regions
class InternalAncestorRegion pr cr => AncestorRegion pr cr Source
The AncestorRegion
class is used to relate the region in which a resource
was opened to the region in which it is used. Take the following operation from
the safer-file-handles
package as an example:
hFileSize :: (pr `AncestorRegion` cr, MonadIO cr) => RegionalFileHandle ioMode pr -> cr Integer
The AncestorRegion
class defines the parent / child relationship between regions.
The constraint
pr `AncestorRegion` cr
is satisfied if and only if cr
is a sequence of zero or more "
"
(with varying RegionT
ss
) applied to pr
, in other words, if cr
is an (improper)
nested subregion of pr
.
The class constraint InternalAncestorRegion pr cr
serves two purposes. First, the
instances of InternalAncestorRegion
do the type-level recursion that implements
the relation specified above. Second, since it is not exported, user code cannot
add new instances of AncestorRegion
(as these would have to be made instances of
InternalAncestorRegion
, too), effectively turning it into a closed class.
InternalAncestorRegion pr cr => AncestorRegion pr cr |
Special regions
The root region
data RootRegion α Source
The RootRegion
is the ancestor of any region.
It's primary purpose is to tag regional handles which don't have an associated
finalizer. For example the standard file handles stdin
, stdout
and stderr
which are opened on program startup and which shouldn't be closed when a region
terminates. Another example is the nullPtr
which is a memory pointer which
doesn't point to any allocated memory so doesn't need to be freed.
InternalAncestorRegion RootRegion (RegionT s m) |
Local regions
data LocalRegion sl s α Source
A LocalRegion
is used to tag regional handles which are created locally.
An example is the LocalPtr
in the alloca
function from the
regional-pointers
package:
alloca :: (Storable a, MonadControlIO pr) => (forall sl. LocalPtr a (LocalRegion
sl s) -> RegionT (Local
s) pr b) -> RegionT s pr b
The finalisation of the LocalPtr
is not performed by the regions
library but
is handled locally by alloca
instead.
The type variable sl
, which is only quantified over the continuation, ensures
that locally opened resources don't escape.
InternalAncestorRegion (LocalRegion sf s) (RegionT (Local s) m) |
A type used to tag regions in which locally created handles (handles tagged with
LocalRegion
) can be used.
Note than any handle which is created in a RegionT (Local s)
can be used
outside that region (RegionT s
) and visa versa
(except for LocalRegion
-tagged handles).
Utilities for writing monadic instances
liftCallCC :: (((α -> pr β) -> pr α) -> pr α) -> ((α -> RegionT s pr β) -> RegionT s pr α) -> RegionT s pr αSource
Lift a callCC
operation to the new monad.
mapRegionT :: (m α -> n β) -> RegionT s m α -> RegionT s n βSource
Transform the computation inside a region.