chp-1.1.1: An implementation of concurrency ideas from Communicating Sequential ProcessesSource codeContentsIndex
Control.Concurrent.CHP.Barriers
Description

A module containing barriers.

A barrier is a synchronisation primitive. When N processes are enrolled on a barrier, all N must synchronise on the barrier before any synchronisations may complete, at which point they all complete. That is, when a single process synchronises on a barrier, it must then wait until all the other enrolled processes also synchronise before it can finish.

Only processes enrolled on a barrier may synchronise on it. Enrolled barriers should not be passed around between processes, or used twice in a parallel composition. Instead, each process should enroll on the barrier itself.

Barriers support choice (alting). This can lead to a lot of non-determinism and some confusion. Consider these two processes, both enrolled on barriers a and b:

 (sync a <-> sync b)
 (sync b <-> sync a)

Which barrier completes is determined by the run-time, and will be an arbitrary choice. This is even the case when priority is involved:

 (sync a </> sync b)
 (sync b </> sync a)

Clearly there is no way to resolve this to satisfy both priorities; the run-time will end up choosing.

Barrier poison can be detected when syncing, enrolling or resigning. You may only poison a barrier that you are currently enrolled on.

Barriers can also support phases. The idea behind a phased barrier is that a barrier is always on a certain phase P. Whenever a barrier successfully completes, the phase is incremented (but it does not have to be an integer). Everyone is told the new phase once they complete a synchronisation, and may query the current phase for any barrier that they are currently enrolled on.

Synopsis
type Barrier = PhasedBarrier ()
type EnrolledBarrier = Enrolled PhasedBarrier ()
newBarrier :: CHP Barrier
newBarrierWithLabel :: String -> CHP Barrier
data (Enum phase, Bounded phase, Eq phase) => PhasedBarrier phase
newPhasedBarrier :: (Enum phase, Bounded phase, Eq phase) => phase -> CHP (PhasedBarrier phase)
newPhasedBarrierWithLabel :: (Enum phase, Bounded phase, Eq phase) => String -> phase -> CHP (PhasedBarrier phase)
currentPhase :: (Enum phase, Bounded phase, Eq phase) => Enrolled PhasedBarrier phase -> CHP phase
waitForPhase :: (Enum phase, Bounded phase, Eq phase) => phase -> Enrolled PhasedBarrier phase -> CHP ()
syncBarrier :: (Enum phase, Bounded phase, Eq phase) => Enrolled PhasedBarrier phase -> CHP phase
getBarrierIdentifier :: (Enum ph, Bounded ph, Eq ph) => PhasedBarrier ph -> Unique
Documentation
type Barrier = PhasedBarrier ()Source
A special case of the PhasedBarrier that has no useful phases, i.e. a standard barrier.
type EnrolledBarrier = Enrolled PhasedBarrier ()Source

A useful type synonym for enrolled barriers with no phases

Added in 1.1.0

newBarrier :: CHP BarrierSource
Creates a new barrier with no processes enrolled
newBarrierWithLabel :: String -> CHP BarrierSource
Creates a new barrier with no processes enrolled and labels it in traces using the given label
data (Enum phase, Bounded phase, Eq phase) => PhasedBarrier phase Source

A phased barrier that is capable of being poisoned and throwing poison. You will need to enroll on it to do anything useful with it. For the phases you can use any type that satisfies Enum, Bounded and Eq. The phase increments every time the barrier completes. Incrementing consists of: if p == maxBound then minBound else succ p. Examples of things that make sense for phases:

  • The () type (see the Barrier type). This effectively has a single repeating phase, and acts like a non-phased barrier.
  • A bounded integer type. This increments the count every time the barrier completes. But don't forget that the count will wrap round when it reaches the end. You cannot use Integer for a phase because it is unbounded. If you really want to have an infinitely increasing count, you can wrap Integer in a newtype and provide a Bounded instance for it (with minBound and maxBound set to -1, if you start on 0).
  • A boolean. This implements a simple black-white barrier, where the state flips on each iteration.
  • A custom data type that has only constructors. For example, data MyPhases = Discover | Plan | Move. Haskell supports deriving Enum, Bounded and Eq automatically on such types.
show/hide Instances
(Enum phase, Bounded phase, Eq phase) => Enrollable PhasedBarrier phase
newPhasedBarrier :: (Enum phase, Bounded phase, Eq phase) => phase -> CHP (PhasedBarrier phase)Source
Creates a new barrier with no processes enrolled, that will be on the given phase. You will often want to pass in the last value in your phase cycle, so that the first synchronisation moves it on to the first
newPhasedBarrierWithLabel :: (Enum phase, Bounded phase, Eq phase) => String -> phase -> CHP (PhasedBarrier phase)Source
Creates a new barrier with no processes enrolled and labels it in traces using the given label
currentPhase :: (Enum phase, Bounded phase, Eq phase) => Enrolled PhasedBarrier phase -> CHP phaseSource
Finds out the current phase a barrier is on.
waitForPhase :: (Enum phase, Bounded phase, Eq phase) => phase -> Enrolled PhasedBarrier phase -> CHP ()Source
If the barrier is not in the given phase, synchronises on the barrier repeatedly until it is in the given phase
syncBarrier :: (Enum phase, Bounded phase, Eq phase) => Enrolled PhasedBarrier phase -> CHP phaseSource
Synchronises on the given barrier. You must be enrolled on a barrier in order to synchronise on it. Returns the new phase, following the synchronisation.
getBarrierIdentifier :: (Enum ph, Bounded ph, Eq ph) => PhasedBarrier ph -> UniqueSource
Gets the identifier of a Barrier. Useful if you want to identify it in the trace later on.
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