The async package provides functionality for performing actions asynchronously, across multiple threads. While it's built on top of the forkIO function from base (in Control.Concurrent), the async package improves on this in many ways:

Tutorial exercise

To help motivate learning, keep in mind this exercise while reading through the content below, and try to implement a solution. Write a helper function which allows you to pass actions to worker threads, and which properly handles exceptions for all of these actions.

Concepts

There is little cognitive overhead to using this package. The primary datatype it exposes is Async. An Async a value represents a separate thread which will ultimately generate a value of type a. The package follows some pretty standard naming conventions:

Concurrently

To warm up, we'll start with examples of using concurrently, race, and the Concurrently newtype. These are simpler (and more efficient) variants of the more general Async-based interface. The general rule with this library is: if you can get away with concurrently/race/Concurrently, you should.

Basics

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent
import Control.Concurrent.Async

action1 :: IO Int
action1 = do
    threadDelay 500000 -- just to make it interesting
    return 5

action2 :: IO String
action2 = do
    threadDelay 1000000
    return "action2 result"

main :: IO ()
main = do
    res <- concurrently action1 action2
    print (res :: (Int, String))

As you can see, the concurrently function waits until both operations complete, and then returns both results in a tuple. In contrast, the race function returns only the first one to complete:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent
import Control.Concurrent.Async

action1 :: IO Int
action1 = do
    threadDelay 500000 -- just to make it interesting
    return 5

action2 :: IO String
action2 = do
    threadDelay 1000000
    return "action2 result"

main :: IO ()
main = do
    res <- race action1 action2
    print (res :: Either Int String)

Exercise: what will this program output?

The Concurrently newtype wrapper uses the concurrently function to implement its Applicative instance, and race to implement its Alternative instance. We can demonstrate that, though the code will be quite a bit more verbose:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Applicative
import Control.Concurrent
import Control.Concurrent.Async

action1 :: IO Int
action1 = do
    threadDelay 500000 -- just to make it interesting
    return 5

action2 :: IO String
action2 = do
    threadDelay 1000000
    return "action2 result"

main :: IO ()
main = do
    res1 <- runConcurrently $ (,)
        <$> Concurrently action1
        <*> Concurrently action2
    print (res1 :: (Int, String))

    res2 <- runConcurrently
          $ (Left <$> Concurrently action1)
        <|> (Right <$> Concurrently action2)
    print (res2 :: Either Int String)

While this seems tedious for such an example, the Concurrently newtype can be great for larger scale cases, such as when we want to discard some results.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent.Async
import Data.Foldable (traverse_)

type Score = Int
data Person = Person FilePath Score

people :: [Person]
people =
    [ Person "alice.txt" 50
    , Person "bob.txt" 60
    , Person "charlie.txt" 70
    ]

-- | This function returns a unit value that we don't care about. Using
-- concurrently on two such actions would give us ((), ()).
writePerson :: Person -> IO ()
writePerson (Person fp score) = writeFile fp (show score)

-- | Let's write lots of people to their respective files in parallel, instead
-- of sequentially.
writePeople :: [Person] -> IO ()
writePeople = runConcurrently . traverse_ (Concurrently . writePerson)

-- Note: traverse_ is just mapM_ for Applicative instead instead of Monad.
-- Remember, Concurrently is _not_ a Monad instance.

main :: IO ()
main = writePeople people

Exceptions

When either child thread throws an exception, that exception is thrown to the other thread:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent
import Control.Concurrent.Async
import Control.Exception

action1 :: IO Int
action1 = error "action1 errored"

action2 :: IO String
action2 = handle onErr $ do
    threadDelay 500000
    return "action2 completed"
  where
    onErr e = do
        putStrLn $ "action2 was killed by: " ++ displayException e
        throwIO (e :: SomeException)

main :: IO ()
main = do
    res <- concurrently action1 action2
    print res

You'll get some interleaving of output most likely since string-based I/O work character-by-character, but you should get the idea from running this.

Exercises:

Companion infinite threads

There's a neat trick you can accomplish with race when you want a companion thread to continue running as long as the main thread is in operation:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent
import Control.Concurrent.Async
import Control.Exception

-- | Print successive numbers to stdout. Notice how it returns @a@ instead of
-- @()@. This lets the type system know that, under normal circumstances, this
-- function will never exit.
counter :: IO a
counter =
    let loop i = do
            putStrLn $ "counter: " ++ show i
            threadDelay 1000000
            loop $! i + 1
     in loop 1

-- | This function will continue to run counter with whatever action you've
-- provided, and stop running counter once that action exits. If by some chance
-- counter throws an exception, it will take down your thread as well.
withCounter :: IO a -> IO a
withCounter inner = do
    res <- race counter inner
    case res of
        Left x -> assert False x
        Right x -> return x

-- More succintly
-- withCounter = fmap (either id id) . race counter

main :: IO ()
main = do
    putStrLn "Before withCounter"
    threadDelay 2000000
    withCounter $ do
        threadDelay 2000000
        putStrLn "Inside withCounter"
        threadDelay 2000000
    threadDelay 2000000
    putStrLn "After withCounter"
    threadDelay 2000000
    putStrLn "Exiting!"

Exercises:

Advanced While it's nice to be able to run companion threads, it can be restricting to require that your main thread live in IO. Perhaps you want to have your main thread live in some monad transformer stack on top of IO instead. Using the powerful (and complex) monad-control package, we can capture the monadic state to make this work.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE FlexibleContexts #-}
import Control.Concurrent
import Control.Concurrent.Async
import Control.Exception
import Control.Monad.Reader
import Control.Monad.Trans.Control

-- | Print successive numbers to stdout. Notice how it returns @a@ instead of
-- @()@. This lets the type system know that, under normal circumstances, this
-- function will never exit.
counter :: IO a
counter =
    let loop i = do
            putStrLn $ "counter: " ++ show i
            threadDelay 1000000
            loop $! i + 1
     in loop 1

-- | This function will continue to run counter with whatever action you've
-- provided, and stop running counter once that action exits. If by some chance
-- counter throws an exception, it will take down your thread as well.
withCounter :: MonadBaseControl IO m => m a -> m a
withCounter inner = control $ \runInIO -> do
    res <- race counter (runInIO inner)
    case res of
        Left x -> assert False x
        Right x -> return x

-- More succintly
-- withCounter = fmap (either id id) . race counter

main :: IO ()
main = do
    putStrLn "Before withCounter"
    threadDelay 2000000
    flip runReaderT "some string" $ withCounter $ do
        liftIO $ threadDelay 2000000
        str <- ask
        liftIO $ putStrLn $ "Inside withCounter, str == " ++ str
        liftIO $ threadDelay 2000000
    threadDelay 2000000
    putStrLn "After withCounter"
    threadDelay 2000000
    putStrLn "Exiting!"

Async

Sometimes you need a bit more control than is offered by the Concurrently family. In those cases, you'll want to use the Async type and its associated functions. The core type for all of this is:

data Async a

This represents an action running in a different thread which, if successful, will give a result of type a. There are lots of things you can do to interact with that thread:

Most of this is pretty straightforward, but there are three possibly surprising things worth pointing out right off the bat:

  1. I used the phrase "if successful" above. It's possible that the action will instead fail with a runtime exception. Therefore, any function which gets a result also needs to deal with a possible SomeException. As you'll see, a number of the functions here deal with that case by simply rethrowing that exception in the calling thread.
  2. In order to allow for more composability (as we'll see later), the ability to query a thread's status can be performed from within an STM transaction. Most (if not all) of these STM functions also have IO variants, but that's just for user convenience.
  3. Killing a thread early, or canceling it, involves throwing it an async exception. If your action misbehaves with async exceptions, canceling may fail. We'll demonstrate that below, but I recommend checking out the safe-exceptions tutorial for advice on doing this right.

But before we can get into details, let's just see the launching of some basic Asyncs.

Launching

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Monad
import Say

talker :: String -> IO ()
talker str = forever $ do
  sayString str
  threadDelay 500000

getResult :: IO Int
getResult = do
  sayString "Doing some big computation..."
  threadDelay 2000000
  sayString "Done!"
  return 42

main :: IO ()
main = do
  async1 <- async $ talker "async"
  withAsync (talker "withAsync") $ \async2 -> do
    async3 <- async getResult

    res <- poll async3
    case res of
      Nothing -> sayString "getResult still running"
      Just (Left e) -> sayString $ "getResult failed: " ++ show e
      Just (Right x) -> sayString $ "getResult finished: " ++ show x

    res <- waitCatch async3
    case res of
      Left e -> sayString $ "getResult failed: " ++ show e
      Right x -> sayString $ "getResult finished: " ++ show x

    res <- wait async3
    sayString $ "getResult finished: " ++ show (res :: Int)

  sayString "withAsync talker should be dead, but not async"
  threadDelay 2000000

  sayString "Now killing async talker"
  cancel async1

  threadDelay 2000000
  sayString "Goodbye!"

This demonstrates the two most common ways of launching an Async:

The advantage of withAsync is that it is exception safe.

Exercise Implement your own withAsync using bracket, async, and cancel.

This also demonstrated three of the most popular functions for querying an Async's status:

Exercise Implement wait in terms of waitCatch. Can you efficiently implement waitCatch in terms of poll?

In general, you should prefer withAsync in place of async to avoid having unneeded threads running. If you specifically need a thread to outlive a certain block, then async is the right function to use.

Composing in STM

Let's simplify our example above a bit, and instead of using the IO variants of the functions, use the STM functions. This should look pretty familiar:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Concurrent.STM
import Control.Monad
import Say

getResult :: IO Int
getResult = do
  sayString "Doing some big computation..."
  threadDelay 2000000
  sayString "Done!"
  return 42

main :: IO ()
main = withAsync getResult $ \a -> do
  res <- atomically $ pollSTM a

  case res of
    Nothing -> sayString "getResult still running"
    Just (Left e) -> sayString $ "getResult failed: " ++ show e
    Just (Right x) -> sayString $ "getResult finished: " ++ show x

  res <- atomically $ waitCatchSTM a
  case res of
    Left e -> sayString $ "getResult failed: " ++ show e
    Right x -> sayString $ "getResult finished: " ++ show x

  res <- atomically $ waitSTM a
  sayString $ "getResult finished: " ++ show (res :: Int)

All we've done is append STM to the function names and stick an atomically at the front. On its own, this isn't too exciting, just informative.

Exercise Implement both waitCatchSTM and waitSTM in terms of pollSTM

Exercise Now implement pollSTM in terms of waitCatchSTM.

So what's so great about this STM business? For one, it can allow us to do more sophisticated queries, like racing two Asyncs:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Applicative ((<|>))
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Concurrent.STM
import Say

getResult1 :: IO Int
getResult1 = do
  sayString "Doing some big computation..."
  threadDelay 2000000
  sayString "Done!"
  return 42

getResult2 :: IO Int
getResult2 = do
  sayString "Doing some smaller computation..."
  threadDelay 1000000
  sayString "Done!"
  return 41

main :: IO ()
main = do
  res <- withAsync getResult1 $ \a1 ->
         withAsync getResult2 $ \a2 ->
         atomically $ waitSTM a1 <|> waitSTM a2

  sayString $ "getResult finished: " ++ show (res :: Int)

Yes, in this case using the race function would make more sense, but as problems grow in complexity, this flexibility will be important.

Exercise Identify the behavior of this program if getResult2 throws an exception. Can you modify the code so that, if either of the threads completes successfully, we get a result?

Breaking async exceptions

Look at how easy it is to break our program completely:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Exception
import Control.Monad
import Say

evil :: IO ()
evil = forever $ do
  eres <- try $ threadDelay 1000000
  sayShow (eres :: Either SomeException ())

main :: IO ()
main = withAsync evil $ const $ return ()

This code will loop forever, since the cancel call's exception is caught by the try and execution continues indefinitely. This is very bad code, don't do this!

Exercise Switch the import from Control.Exception to UnliftIO.Exception. What happens? Why?

Linking

Let's say you want to run some kind of a background processing thread. You want to kick it off, leave it running, and essentially forget all about it. However, if that thread goes down for some reason, you need to ensure that your main thread goes down too. Such a situation makes a lot of sense, for example, with a job queue.

To make this work, you can use the link function, which ensures that if your Async ends with an exception, that exception is rethrown to the main thread.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE OverloadedStrings #-}
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Concurrent.STM
import Control.Monad (forever)
import Say (say)
import Data.Text (Text)

data Work = Work Text -- intentionally lazy, you'll see why below

jobQueue :: TChan Work -> IO a
jobQueue chan = forever $ do
  Work t <- atomically $ readTChan chan
  say t

main :: IO ()
main = do
  chan <- newTChanIO
  a <- async $ jobQueue chan
  link a
  forever $ do
    atomically $ do
      writeTChan chan $ Work "Hello"
      writeTChan chan $ Work undefined
      writeTChan chan $ Work "World"
    threadDelay 1000000

Question What's the behavior without the link call?

Question What's the behavior if Work was strict in its Text? How about if we forced evaluation with writeTChan chan $! Work $! undefined?

Exercise Rewrite this program to use race instead of async and link.

Lifted

All of the functions in Control.Concurrent.Async live in either the STM or IO types. Many of the IO functions—like cancel—could be lifted to MonadIO with a simple liftIO call. However, many others, like async, cannot (since they have the IO type appearing as an input, otherwise known as negative position). That would seem to exclude the possibility of using monad transformers.

We can make usage of a transformer like ReaderT work by manually wrapping/unwrapping its constructor. Let's adapt the jobQueue example from above to use a ReaderT and see how that goes:

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE OverloadedStrings #-}
import Control.Concurrent (threadDelay)
import Control.Concurrent.Async
import Control.Concurrent.STM
import Control.Monad (forever)
import Say (say)
import Data.Text (Text)
import Control.Monad.Reader

data Work = Work Text

jobQueue :: ReaderT (TChan Work) IO a
jobQueue = forever $ do
  chan <- ask
  Work t <- liftIO $ atomically $ readTChan chan
  say t

inner :: ReaderT (TChan Work) IO ()
inner = ReaderT $ \chan -> do
  race_ (runReaderT jobQueue chan) $ flip runReaderT chan $ do
    forever $ do
      chan <- ask
      liftIO $ atomically $ do
        writeTChan chan $ Work "Hello"
        writeTChan chan $ Work undefined
        writeTChan chan $ Work "World"
      liftIO $ threadDelay 1000000

main :: IO ()
main = do
  chan <- newTChanIO
  runReaderT inner chan

Workable, but tedious. Fortunately, the unliftio package provides a version of the async API which is lifted to many more monads. Let's see how a simple change in import lets us write much nicer code.

#!/usr/bin/env stack
-- stack script --resolver lts-12.21
{-# LANGUAGE OverloadedStrings #-}
import UnliftIO.Async
import UnliftIO.Concurrent (threadDelay)
import UnliftIO.STM
import Control.Monad (forever)
import Say (say)
import Data.Text (Text)
import Control.Monad.Reader

data Work = Work Text

jobQueue :: ReaderT (TChan Work) IO a
jobQueue = forever $ do
  chan <- ask
  Work t <- liftIO $ atomically $ readTChan chan
  say t

inner :: ReaderT (TChan Work) IO ()
inner = do
  race_ jobQueue $ do
    forever $ do
      chan <- ask
      liftIO $ atomically $ do
        writeTChan chan $ Work "Hello"
        writeTChan chan $ Work undefined
        writeTChan chan $ Work "World"
      liftIO $ threadDelay 1000000

main :: IO ()
main = do
  chan <- newTChanIO
  runReaderT inner chan

All of the runReaderT/ReaderT mess in inner completely disappeared.

This will work for any monad stack which is an instance of MonadUnliftIO, which allows for transformers like ReaderT or IdentityT, but disallows transformers like StateT and WriterT as they will result in discarded monadic state. Imagine a function like concurrently (put 5) (put 6). Which state will survive? It's frankly arbitrary, and there are three valid options to consider:

Using UnliftIO.Async as a drop in replacement for Control.Concurrent.Async is straightforward, the only API change to be aware of is that the Async data type will take an extra type paramter for the underlying monad. This API is also exported directly from the RIO module, so if you're using rio, you're all set!

Section exercises

Modify the program below so that it completes in under 10 seconds, without modifying expensiveComputation:

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
import RIO
import Test.Hspec
import Control.Monad (replicateM)

data App = App
  { appCounter :: !(TVar Int)
  }

expensiveComputation :: RIO App Int
expensiveComputation = do
  app <- ask
  -- I'm not actually an expensive computation, but I play one on TV
  delayVar <- registerDelay 1000000
  atomically $ do
    readTVar delayVar >>= checkSTM
    let var = appCounter app
    modifyTVar' var (+ 1)
    readTVar var

main :: IO ()
main = hspec $ it "works" $ do
  app <- App <$> newTVarIO 0
  res <- runRIO app $ replicateM 10 expensiveComputation
  sum res `shouldBe` sum [1..10]

QUESTION Why is the sum necessary to get the tests to pass?

Next, modify the following similar program so that the test passes in under 10 seconds, by only changing the line specified:

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
import RIO
import Test.Hspec
import Control.Monad (replicateM)

expensiveComputation :: MonadIO m => Int -> m Int
expensiveComputation input = do
  threadDelay 1000000
  pure input

main :: IO ()
main = hspec $ it "works" $ do
  -- Modify only the following line
  res <- for [1..10] expensiveComputation

  -- Don't change this, it's cheating! :)
  res `shouldBe` [1..10]

QUESTION Why is sum not necessary in this code?

Next, implement the sumConcurrently function below.

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
import RIO
import Test.Hspec
import Control.Monad (replicateM)

expensiveComputation :: MonadIO m => Int -> m Int
expensiveComputation input = do
  threadDelay 1000000
  pure input

sumConcurrently
  :: (a -> IO Int)
  -> [a]
  -> IO Int
sumConcurrently f list = _

main :: IO ()
main = hspec $ it "works" $ do
  res <- sumConcurrently expensiveComputation [1..10]
  res `shouldBe` sum [1..10]

SOLUTION There are a few ways of implementing sumConcurrently. However, one way that avoids unnecessary lists by leveraging commutativity of addition is:

sumConcurrently
  :: (a -> IO Int)
  -> [a]
  -> IO Int
sumConcurrently f list = do
  totalVar <- newTVarIO 0
  forConcurrently_ list $ \a -> do
    int <- f a
    atomically $ modifyTVar' totalVar (+ int)
  atomically $ readTVar totalVar

Another set of exercises: implement the replicateConcurrently_ function using:

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE NoImplicitPrelude #-}
import RIO
import Test.Hspec

-- Use mapConcurrently_ here
rc1 :: Int -> IO () -> IO ()
rc1 = undefined

-- Use the Concurrently newtype here
rc2 :: Int -> IO () -> IO ()
rc2 = undefined

-- Use concurrently_ here
rc3 :: Int -> IO () -> IO ()
rc3 = undefined

-- Use withAsync here
rc4 :: Int -> IO () -> IO ()
rc4 = undefined

main :: IO ()
main = hspec $ do
  let rcs =
        [ ("replicateConcurrently_", replicateConcurrently_)
        , ("using mapConcurrently_", rc1)
        , ("using Concurrently (the newtype)", rc2)
        , ("using concurrently_ (the function)", rc3)
        , ("using withAsync", rc4)
        ]
  for_ rcs $ \(name, rc) -> it name $ do
    resVar <- newTVarIO 0
    rc 10 $ atomically $ modifyTVar' resVar (+ 1)
    atomically (readTVar resVar) `shouldReturn` 10

SOLUTION

#!/usr/bin/env stack
-- stack --resolver lts-12.21 script
{-# LANGUAGE NoImplicitPrelude #-}
import RIO
import Test.Hspec

-- Use mapConcurrently_ here
rc1 :: Int -> IO () -> IO ()
rc1 count action = mapConcurrently_ (const action) [1..count]

-- Use the Concurrently newtype here
rc2 :: Int -> IO () -> IO ()
rc2 count action = runConcurrently $ sequenceA_ $ replicate count (Concurrently action)

-- Use concurrently_ here
rc3 :: Int -> IO () -> IO ()
rc3 count0 action =
    let loop 1 = action
        loop count = concurrently_ action $ loop $ count - 1
     in loop count0

-- Use withAsync here
rc4 :: Int -> IO () -> IO ()
rc4 count0 action =
    let loop 1 = action
        loop count = withAsync action $ \thread -> do
          loop $ count - 1
          wait thread
     in loop count0

main :: IO ()
main = hspec $ do
  let rcs =
        [ ("replicateConcurrently_", replicateConcurrently_)
        , ("using mapConcurrently_", rc1)
        , ("using Concurrently (the newtype)", rc2)
        , ("using concurrently_ (the function)", rc3)
        , ("using withAsync", rc4)
        ]
  for_ rcs $ \(name, rc) -> it name $ do
    resVar <- newTVarIO 0
    rc 10 $ atomically $ modifyTVar' resVar (+ 1)
    atomically (readTVar resVar) `shouldReturn` 10

Final exercise: implement mapConcurrently using mapConcurrently_.

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