Abstracts queues like TBQueue.

For documentation on the constructors, see the module Polysemy.Conc.Data.Queue.

import Polysemy.Conc (Queue, QueueResult)
import Polysemy.Conc.Effect.Queue as Queue

prog :: Member (Queue Int) r => Sem r (QueueResult Int)
prog = do
  Queue.write 5
  Queue.write 10
  Queue.read >>= \case
    QueueResult.Success i -> fmap (i +) <$> Queue.read
    r -> pure r

Encodes failure reasons for queues.

For documentation on the constructors, see the module Polysemy.Conc.Data.QueueResult.

import qualified Polysemy.Conc.Data.QueueResult as QueueResult

Interpret Queue with a TBMQueue.

Interpret Queue with a TBQueue.

Variant of interpretQueueListReadOnlyAtomicWith that interprets the AtomicState.

Reinterpret Queue as AtomicState with a list that cannot be written to. Useful for testing.

Variant of interpretQueueListReadOnlyAtomicWith that interprets the State.

Reinterpret Queue as State with a list that cannot be written to. Useful for testing.

Turn a Success into Just.

Read from a Queue repeatedly until it is closed.

When an element is received, call action and recurse.

Read from a Queue repeatedly until it is closed.

When an element is received, call action and recurse if it returns True. When no element is available, evaluate na and recurse if it returns True.

Abstracts an MVar.

For documentation on the constructors, see the module Polysemy.Conc.Effect.Sync.

import Polysemy.Conc (Sync)
import qualified Polysemy.Conc.Effect.Sync as Sync

prog :: Member (Sync Int) r => Sem r Int
prog = do
  Sync.putTry 5
  Sync.takeBlock

An interface to a shared variable (MVar) that can only be read.

Convenience alias.

Interpret Sync with an empty MVar.

Interpret Sync with an MVar containing the specified value.

Run an action with a locally scoped Sync variable.

This avoids a dependency on Embed IO in application logic while still allowing the variable to be scoped.

Interpret Sync for locally scoped use with an empty MVar.

Interpret Sync for locally scoped use with an MVar containing the specified value.

Run SyncRead in terms of Sync.

An exclusive lock or mutex, protecting a region from concurrent access.

Run an action if the lock is available, block otherwise.

Run an action if the lock is available, block otherwise.

Run an action if the lock is available, skip and return Nothing otherwise.

Run an action if the lock is available, skip otherwise. Return ().

Interpret Lock as a reentrant lock, allowing nested calls to lock unless called from a different thread (as in, async was called in a higher-order action passed to lock.)

Interpret Lock by executing all actions unconditionally.

This effect abstracts over the concept of a quantity semaphore, a concurrency primitive that contains a number of slots that can be acquired and released.

Interpret Semaphore as a QSem.

Interpret Semaphore as a TSem.

A single-use synchronization point that blocks all consumers who called gate until signal is called.

The constructors are exported from Polysemy.Conc.Gate.

Convenience alias for scoped Gate.

Interpret Scoped_ Gate with an MVar ().

Interpret Gate with an MVar.

Abstract the concept of running two programs concurrently, aborting the other when one terminates. Timeout is a simpler variant, where one thread just sleeps for a given interval.

Run both programs concurrently, returning the result of the faster one.

Specialization of race for the case where both actions return the same type, obviating the need for Either.

Run the fallback action if the given program doesn't finish within the specified interval.

Specialization of timeout for the case where the main action returns the same type as the fallback, obviating the need for Either.

Version of timeout that takes a pure fallback value.

Specialization of timeoutAs for the case where the main action return the same type as the fallback, obviating the need for Either.

Specialization of timeout for unit actions.

Variant of timeout that returns Maybe.

Variant of timeout that calls Stop with the supplied error when the action times out.

Run an action repeatedly until it returns Right or the timout has been exceeded.

Run an action repeatedly until it returns Right or the timout has been exceeded.

If the action failed at least once, the last error will be returned in case of timeout.

Interpret Race in terms of race and timeout. Since this has to pass higher-order thunks as IO arguments, it is interpreted in terms of 'Final IO'.

The interrupt handler effect allows three kinds of interaction for interrupt signals:

• Execute a callback when a signal is received
• Block a thread until a signal is received
• Kill a thread when a signal is received

For documentation on the constructors, see the module Polysemy.Conc.Effect.Interrupt.

import qualified Polysemy.Conc.Effect.Interrupt as Interrupt

prog = do
  Interrupt.register "task 1" (putStrLn "interrupted")
  Interrupt.killOnQuit $ forever do
   doSomeWork

Interpret Interrupt by installing a signal handler.

Catches repeat invocations of SIGINT.

Interpret Interrupt by installing a signal handler.

Catches only the first invocation of SIGINT.

Eliminate Interrupt without interpreting.

An event publisher that can be consumed from multiple threads.

Consume events emitted by Events.

Publish one event.

Consume one event emitted by Events.

Create a new scope for Events, causing the nested program to get its own copy of the event stream. To be used with interpretEventsChan.

Create a new scope for Events, causing the nested program to get its own copy of the event stream.

Calls signal before running the argument to ensure that subscribe has finished creating a channel, for use with asynchronous execution.

Create a new scope for Events, causing the nested program to get its own copy of the event stream.

Executes in a new thread, ensuring that the main thread blocks until subscribe has finished creating a channel.

Pull repeatedly from the Events channel, passing the event to the supplied callback. Stop when the action returns False.

Pull repeatedly from the Events channel, passing the event to the supplied callback. Stop when the action returns False.

Signals the caller that the channel was successfully subscribed to using the Gate effect.

Start a new thread that pulls repeatedly from the Events channel, passing the event to the supplied callback and stops when the action returns False.

Pull repeatedly from the Events channel, passing the event to the supplied callback.

Pull repeatedly from the Events channel, passing the event to the supplied callback.

Signals the caller that the channel was successfully subscribed to using the Gate effect.

Start a new thread that pulls repeatedly from the Events channel, passing the event to the supplied callback.

Block until a value matching the predicate has been published to the Events channel.

Return the first value published to the Events channel for which the function produces Just.

Block until the specified value has been published to the Events channel.

Pull repeatedly from Consume, passing the event to the supplied callback. Stop when the action returns False.

Pull repeatedly from Consume, passing the event to the supplied callback.

Block until a value matching the predicate has been returned by Consume.

Return the first value returned by Consume for which the function produces Just.

Block until the specified value has been returned by Consume.

Convenience alias for the consumer effect.

Interpret Events and Consume together by connecting them to the two ends of an unagi channel. Consume is only interpreted in a Scoped manner, ensuring that a new duplicate of the channel is created so that all consumers see all events (from the moment they are connected).

This should be used in conjunction with subscribe:

interpretEventsChan do
  async $ subscribe do
    putStrLn =<< consume
  publish "hello"

Whenever subscribe creates a new scope, this interpreter calls dupChan and passes the duplicate to interpretConsumeChan.

An effect that catches exceptions.

Provides the exact functionality of fromExceptionSem, but pushes the dependency on Final IO to the interpreter, and makes it optional.

Interpret Critical in terms of Final IO.

Interpret Critical by doing nothing.

The scoped masking effect.

The scoped uninterruptible masking effect.

Mark a region as masked. Uses the Scoped_ pattern.

Mark a region as uninterruptibly masked. Uses the Scoped_ pattern.

Restore the previous masking state. Can only be called inside of an action passed to mask or uninterruptibleMask.

Resource type for the scoped Mask effect, wrapping the restore callback passed in by mask.

Interpret Mask in IO.

Interpret UninterruptibleMask in IO.

Interpret Mask by sequencing the action without masking.

Interpret UninterruptibleMask by sequencing the action without masking.

Scoped transforms a program so that an interpreter for effect may perform arbitrary actions, like resource management, before and after the computation wrapped by a call to scoped is executed.

Note: This effect has been merged to Polysemy and will be released there soon.

An application for this is Polysemy.Conc.Events from https://hackage.haskell.org/package/polysemy-conc, in which each program using the effect Polysemy.Conc.Consume is interpreted with its own copy of the event channel; or a database transaction, in which a transaction handle is created for the wrapped program and passed to the interpreter for the database effect.

For a longer exposition, see https://www.tweag.io/blog/2022-01-05-polysemy-scoped/. Note that the interface has changed since the blog post was published: The resource parameter no longer exists.

Resource allocation is performed by a function passed to interpretScoped.

The constructors are not intended to be used directly; the smart constructor scoped is used like a local interpreter for effect. scoped takes an argument of type param, which will be passed through to the interpreter, to be used by the resource allocation function.

As an example, imagine an effect for writing lines to a file:

data Write :: Effect where
  Write :: Text -> Write m ()
makeSem ''Write

If we now have the following requirements:

1. The file should be opened and closed right before and after the part of the program in which we write lines
2. The file name should be specifiable at the point in the program where writing begins
3. We don't want to commit to IO, lines should be stored in memory when running tests

Then we can take advantage of Scoped to write this program:

prog :: Member (Scoped FilePath Write) r => Sem r ()
prog = do
  scoped "file1.txt" do
    write "line 1"
    write "line 2"
  scoped "file2.txt" do
    write "line 1"
    write "line 2"

Here scoped creates a prompt for an interpreter to start allocating a resource for "file1.txt" and handling Write actions using that resource. When the scoped block ends, the resource should be freed.

The interpreter may look like this:

interpretWriteFile :: Members '[Resource, Embed IO] => InterpreterFor (Scoped FilePath Write) r
interpretWriteFile =
  interpretScoped allocator handler
  where
    allocator name use = bracket (openFile name WriteMode) hClose use
    handler fileHandle (Write line) = embed (Text.hPutStrLn fileHandle line)

Essentially, the bracket is executed at the point where scoped was called, wrapping the following block. When the second scoped is executed, another call to bracket is performed.

The effect of this is that the operation that uses Embed IO was moved from the call site to the interpreter, while the interpreter may be executed at the outermost layer of the app.

This makes it possible to use a pure interpreter for testing:

interpretWriteOutput :: Member (Output (FilePath, Text)) r => InterpreterFor (Scoped FilePath Write) r
interpretWriteOutput =
  interpretScoped (\ name use -> use name) \ name -> \case
    Write line -> output (name, line)

Here we simply pass the name to the interpreter in the resource allocation function.

Now imagine that we drop requirement 2 from the initial list – we still want the file to be opened and closed as late/early as possible, but the file name is globally fixed. For this case, the param type is unused, and the API provides some convenience aliases to make your code more concise:

prog :: Member (Scoped_ Write) r => Sem r ()
prog = do
  scoped_ do
    write "line 1"
    write "line 2"
  scoped_ do
    write "line 1"
    write "line 2"

The type Scoped_ and the constructor scoped_ simply fix param to ().

A convenience alias for a scope without parameters.

Constructor for Scoped, taking a nested program and transforming all instances of effect to Scoped param effect.

Please consult the documentation of Scoped for details and examples.

Constructor for Scoped_, taking a nested program and transforming all instances of effect to Scoped_ effect.

Please consult the documentation of Scoped for details and examples.

Transform the parameters of a Scoped program.

This allows incremental additions to the data passed to the interpreter, for example to create an API that permits different ways of running an effect with some fundamental parameters being supplied at scope creation and some optional or specific parameters being selected by the user downstream.

First-order variant of interpretScopedH.

Construct an interpreter for a higher-order effect wrapped in a Scoped, given a resource allocation function and a parameterized handler for the plain effect.

This combinator is analogous to interpretH in that it allows the handler to use the Tactical environment and transforms the effect into other effects on the stack.

Variant of interpretScopedH that allows the resource acquisition function to use Tactical.

Variant of interpretScoped in which the resource allocator is a plain action.

First-order variant of interpretScopedWithH.

Note: It is necessary to specify the list of local interpreters with a type application; GHC won't be able to figure them out from the type of withResource:

data SomeAction :: Effect where
  SomeAction :: SomeAction m ()

foo :: InterpreterFor (Scoped () SomeAction) r
foo =
  interpretScopedWith @[Reader Int, State Bool] localEffects \ () -> \case
    SomeAction -> put . (> 0) =<< ask @Int
  where
    localEffects () use = evalState False (runReader 5 (use ()))

Higher-order interpreter for Scoped that allows the handler to use additional effects that are interpreted by the resource allocator.

Note: It is necessary to specify the list of local interpreters with a type application; GHC won't be able to figure them out from the type of withResource.

As an example for a higher order effect, consider a mutexed concurrent state effect, where an effectful function may lock write access to the state while making it still possible to read it:

data MState s :: Effect where
  MState :: (s -> m (s, a)) -> MState s m a
  MRead :: MState s m s

makeSem ''MState

We can now use an AtomicState to store the current value and lock write access with an MVar. Since the state callback is effectful, we need a higher order interpreter:

withResource ::
  Member (Embed IO) r =>
  s ->
  (MVar () -> Sem (AtomicState s : r) a) ->
  Sem r a
withResource initial use = do
  tv <- embed (newTVarIO initial)
  lock <- embed (newMVar ())
  runAtomicStateTVar tv $ use lock

interpretMState ::
  ∀ s r .
  Members [Resource, Embed IO] r =>
  InterpreterFor (Scoped s (MState s)) r
interpretMState =
  interpretScopedWithH @'[AtomicState s] withResource \ lock -> \case
    MState f ->
      bracket_ (embed (takeMVar lock)) (embed (tryPutMVar lock ())) do
        s0 <- atomicGet
        res <- runTSimple (f s0)
        Inspector ins <- getInspectorT
        for_ (ins res) \ (s, _) -> atomicPut s
        pure (snd <$> res)
    MRead ->
      liftT atomicGet

Variant of interpretScopedWith in which no resource is used and the resource allocator is a plain interpreter. This is useful for scopes that only need local effects, but no resources in the handler.

See the Note on interpretScopedWithH.

Variant of interpretScoped that uses another interpreter instead of a handler.

This is mostly useful if you want to reuse an interpreter that you cannot easily rewrite (like from another library). If you have full control over the implementation, interpretScoped should be preferred.

Note: In previous versions of Polysemy, the wrapped interpreter was executed fully, including the initializing code surrounding its handler, for each action in the program. However, new and continuing discoveries regarding Scoped has allowed the improvement of having the interpreter be used only once per use of scoped, and have it cover the same scope of actions that withResource does.

This renders withResource practically redundant; for the moment, the API surrounding Scoped remains the same, but work is in progress to revamp the entire API of Scoped.

Variant of runScoped in which the resource allocator returns the resource rather tnen calling a continuation.

Combined interpreter for Scoped and Resumable. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope.

Combined higher-order interpreter for Scoped and Resumable. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope.

Combined interpreter for Scoped and Resumable. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope. In this variant, the resource allocator is a plain action.

Combined interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope.

Combined higher-order interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope.

Combined interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. This allows Stop to be sent from within the resource allocator so that the consumer receives it, terminating the entire scope. In this variant, no resource is used and the allocator is a plain interpreter.

Combined interpreter for Resumable and Scoped. In this variant, only the handler may send Stop, but this allows resumption to happen on each action inside of the scope.

Combined higher-order interpreter for Resumable and Scoped. In this variant, only the handler may send Stop, but this allows resumption to happen on each action inside of the scope.

Combined interpreter for Resumable and Scoped. In this variant: - Only the handler may send Stop, but this allows resumption to happen on each action inside of the scope. - The resource allocator is a plain action.

Combined interpreter for Resumable and Scoped that allows the handler to use additional effects that are interpreted by the resource allocator. In this variant, only the handler may send Stop, but this allows resumption to happen on each action inside of the scope.

Combined higher-order interpreter for Resumable and Scoped that allows the handler to use additional effects that are interpreted by the resource allocator. In this variant, only the handler may send Stop, but this allows resumption to happen on each action inside of the scope.

Combined interpreter for Resumable and Scoped that allows the handler to use additional effects that are interpreted by the resource allocator. In this variant: - Only the handler may send Stop, but this allows resumption to happen on each action inside of the scope. - No resource is used and the allocator is a plain interpreter.

Combined interpreter for Scoped and Resumable. In this variant, both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption.

Combined higher-order interpreter for Resumable and Scoped. In this variant, both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption.

Combined interpreter for Scoped and Resumable. In this variant: - Both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption. - The resource allocator is a plain action.

Combined interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. In this variant, both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption.

Combined higher-order interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. In this variant, both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption.

Combined interpreter for Scoped and Resumable that allows the handler to use additional effects that are interpreted by the resource allocator. - Both the handler and the scope may send different errors via Stop, encoding the concept that the resource allocation may fail to prevent the scope from being executed, and each individual scoped action may fail, continuing the scope execution on resumption. - The resource allocator is a plain action.

Mark a region as being subject to intervention by a monitoring program. This can mean that a thread is repeatedly checking a condition and cancelling this region when it is unmet. A use case could be checking whether a remote service is available, or whether the system was suspended and resumed. This should be used in a Scoped_ context, like withMonitor.

Mark a region as being subject to intervention by a monitoring program.

Start a region that can contain monitor-intervention regions.

Variant of withMonitor that uses the Restart strategy.

Marker type for the restarting action for Monitor.

Monitor specialized to the Restart action.

Convenience alias for a Scoped_ Monitor.

Interpret Scoped Monitor with the Restart strategy. This takes a check action that may put an MVar when the scoped region should be restarted. The check is executed in a loop, with an interval given in MonitorCheck.

Run Monitor as a no-op.

Check for Monitor that checks every interval whether the difference between the current time and the time at the last check is larger than interval + tolerance. Can be used to detect that the operating system suspended and resumed.

Config for monitorClockSkew.

Smart constructor for ClockSkewConfig that takes arbitrary TimeUnits.

A default basic stack with Final for _polysemy-conc_.

Interprets UninterruptibleMask, Mask and Race in terms of Final IO and runs the entire rest of the stack.

Convenience wrapper around runAtomicStateTVar that creates a new TVar.

Run the first action asynchronously while the second action executes, then cancel the first action. Passes the handle into the action to allow it to await its result.

When cancelling, this variant will wait indefinitely for the thread to be gone.

Run the first action asynchronously while the second action executes, then cancel the first action. Passes the handle into the sync action to allow it to await the async action's result.

When cancelling, this variant will wait for 500ms for the thread to be gone.

Run the first action asynchronously while the second action executes, then cancel the first action. Discards the handle, expecting the async action to either terminate or be cancelled.

When cancelling, this variant will wait for 500ms for the thread to be gone.

Run an action with async, but don't start it right away, so the thread handle can be processed before the action executes.

Takes a callback function that is invoked after spawning the thread. The callback receives the Async handle and a unit action that starts the computation.

This is helpful if the Async has to be stored in state and the same state is written when the action finishes. In that case, the race condition causes the handle to be written over the finished state.

makeRequest = put Nothing

main = scheduleAsync makeRequest  handle start -> do
  put (Just handle)
  start -- now makeRequest is executed

Variant of scheduleAsync that directly interprets the MVar used for signalling.

Run the first action asynchronously while the second action executes, then cancel the first action.

The second action will only start when the first action calls signal.

Passes the handle into the sync action to allow it to await the async action's result.

This can be used to ensure that the async action has acquired its resources before the main action starts.

Run the first action asynchronously while the second action executes, then cancel the first action.

The second action will only start when the first action calls signal.

This can be used to ensure that the async action has acquired its resources before the main action starts.