Core.Program.Threads

Utility functions for running Program actions concurrently.

Haskell uses green threads: small lines of work that are scheduled down onto actual execution contexts (set by default by this library to be one per core). Haskell threads are incredibly lightweight, and you are encouraged to use them freely. Haskell provides a rich ecosystem of tools to do work concurrently and to communicate safely between threads.

This module provides wrappers around some of these primatives so you can use them easily from the Program monad.

Note that when you fire off a new thread the top-level application state is shared; it's the same τ inherited from the parent Program.

Concurrency

createScope :: Program τ α -> Program τ α

Create a scope to enclose any subsequently spawned threads as a single group. Ordinarily threads launched in Haskell are completely indepedent. Creating a scope allows you to operate on a set of threads as a single group with bi-directional exception passing. This is the basis of an approach called structured concurrency.

When the execution flow exits the scope, any threads that were spawned within it that are still running will be killed.

If any of the child threads within the scope throws an exception, the other remaining threads will be killed and then the original exception will be propegated to this parent thread and re-thrown.

Since: 0.6.0

forkThread :: Program τ α -> Program τ (Thread α)

Fork a thread. The child thread will run in the same Context as the calling Program, including sharing the user-defined application state value.

If you want to find out what the result of a thread was use waitThread on the Thread object returned from this function. If you don't need the result, use forkThread_ instead.

Threads that are launched off as children are on their own! If the code in the child thread throws an exception that is not caught within that thread, the exception will kill the thread. Threads dying without telling anyone is a bit of an anti-pattern, so this library logs a warning-level log message if this happens.

(this wraps base's forkIO)

Since: 0.2.7

forkThread_ :: Program τ α -> Program τ ()

Fork a thread with forkThread but do not wait for a result. This is on the assumption that the sub program will either be a side-effect and over quickly, or long-running daemon thread (presumably containing a forever loop in it), never returning.

Since: 0.5.2

waitThread :: Thread α -> Program τ α

Wait for the completion of a thread, returning the result. This is a blocking operation.

If the thread you are waiting on throws an exception it will be rethrown by waitThread.

If the current thread making this call is cancelled (as a result of being on the losing side of concurrentThreads or raceThreads for example, or due to the current scope exiting), then the thread you are waiting on will be cancelled too. This is necessary to ensure that child threads are not leaked if you nest forkThreads.

Since: 0.2.7

waitThread_ :: Thread α -> Program τ ()

Wait for the completion of a thread, discarding its result. This is particularly useful at the end of a do-block if you're waiting on a worker thread to finish but don't need its return value, if any; otherwise you have to explicily deal with the unused return value:

    _ <- waitThread t1
    return ()

which is a bit tedious. Instead, you can just use this convenience function:

    waitThread_ t1

The trailing underscore in the name of this function follows the same convetion as found in Control.Monad, which has mapM_ which does the same as mapM but which likewise discards the return value.

Since: 0.2.7

waitThread' :: Thread α -> Program τ (Either SomeException α)

Wait for a thread to complete, returning the result if the computation was successful or the exception if one was thrown by the child thread.

This basically is convenience for calling waitThread and putting catch around it, but as with all the other wait* functions this ensures that if the thread waiting is killed the cancellation is propagated to the thread being watched as well.

Since: 0.4.5

waitThreads' :: [Thread α] -> Program τ [Either SomeException α]

Wait for many threads to complete. This function is intended for the scenario where you fire off a number of worker threads with forkThread but rather than leaving them to run independantly, you need to wait for them all to complete.

The results of the threads that complete successfully will be returned as Right values. Should any of the threads being waited upon throw an exception, those exceptions will be returned as Left values.

If you don't need to analyse the failures individually, then you can just collect the successes using Data.Either's rights:

    responses <- waitThreads'

    info "Aggregating results..."
    combineResults (rights responses)

Likewise, if you do want to do something with all the failures, you might find lefts useful:

    mapM_ (warn . intoRope . displayException) (lefts responses)

If the thread calling waitThreads' is cancelled, then all the threads being waited upon will also be cancelled. This often occurs within a timeout or similar control measure implemented using raceThreads_. Should the thread that spawned all the workers and is waiting for their results be told to cancel because it lost the "race", the child threads need to be told in turn to cancel so as to avoid those threads being leaked and continuing to run as zombies. This function takes care of that.

(this extends waitThread' to work across a list of Threads, taking care to ensure the cancellation behaviour described throughout this module)

Since: 0.4.5

cancelThread :: Thread α -> Program τ ()

Cancel a thread.

Be careful when using this. If you are planning cancel a worker thread then the main thread that is waitThreading on it will throw an exception, specifically ThreadCancelled (unless something else has already thrown an exception in which case that will be thrown instead). In this scenario you will need to catch around your waiting function otherwise the uncaught exception will continue to unwind your execution stack and probably end your program.

(this wraps base's killThread. The underlying mechanism used is to throw the ThreadKilled exception to the other thread. That exception is asynchronous, so will not be trapped by a catch block and will indeed cause the thread receiving the exception to come to an end)

Since: 0.4.5

Helper functions

concurrentThreads :: Program τ α -> Program τ β -> Program τ (α, β)

Fork two threads and wait for both to finish. The return value is the pair of each action's return types.

This is the same as calling forkThread and waitThread twice, except that if either sub-program fails with an exception the other program which is still running will be cancelled and the original exception is then re-thrown.

    (a,b) <- concurrentThreads one two

    -- continue, doing something with both results.

For a variant that ingores the return values and just waits for both see concurrentThreads_ below.

Since: 0.4.0

concurrentThreads_ :: Program τ α -> Program τ β -> Program τ ()

Fork two threads and wait for both to finish.

This is the same as calling forkThread and waitThread_ twice, except that if either sub-program fails with an exception the other program which is still running will be cancelled and the original exception is then re-thrown.

Since: 0.4.0

raceThreads :: Program τ α -> Program τ β -> Program τ (Either α β)

Fork two threads and race them against each other. This blocks until one or the other of the threads finishes. The return value will be Left α if the first program (one) completes first, and Right β if it is the second program (two) which finishes first. The sub program which is still running will be cancelled with an exception.

    result <- raceThreads one two
    case result of
        Left a -> do
            -- one finished first
        Right b -> do
            -- two finished first

For a variant that ingores the return value and just races the threads see raceThreads_ below.

Since: 0.4.0

raceThreads_ :: Program τ α -> Program τ β -> Program τ ()

Fork two threads and race them against each other. When one action completes the other will be cancelled with an exception. This is useful for enforcing timeouts:

    raceThreads_
        (sleepThread 300)
        (do
            -- We expect this to complete within 5 minutes.
            performAction
        )

Since: 0.4.0

Internals

data Thread α

A thread for concurrent computation.

(this wraps base's ThreadId along with a holder for the result of the thread)

Since: 0.6.0

data Terminator

When a thread is aborted with cancelThread this value is used to mark a failed computation inside the Thread. Although it is not the mechanism used internally to kill the computation, it is the exception that is subsequently rethrown from waitThread if you are waiting on that thread to finish, allowing you to catch the case of a thread being cancelled if necessary.

This is mostly here to differentiate from ThreadKilled, giving you some knowledge as to whether it was your explicit cancelThread that ended the thread, or something else. You need to handle it either way, but sometimes you want to know the difference.

Since: 0.6.8

Constructors

ThreadCancelled

Instances

Exception Terminator

Defined in Core.Program.Threads

Methods

toException :: Terminator -> SomeException

fromException :: SomeException -> Maybe Terminator

displayException :: Terminator -> String

Show Terminator

Defined in Core.Program.Threads

Methods

showsPrec :: Int -> Terminator -> ShowS

show :: Terminator -> String

showList :: [Terminator] -> ShowS