Haskell Language Concurrency


Good resources for learning about concurrent and parallel programming in Haskell are:

Spawning Threads with `forkIO`

Haskell supports many forms of concurrency and the most obvious being forking a thread using forkIO.

The function forkIO :: IO () -> IO ThreadId takes an IO action and returns its ThreadId, meanwhile the action will be run in the background.

We can demonstrate this quite succinctly using ghci:

Prelude Control.Concurrent> forkIO $ (print . sum) [1..100000000]
ThreadId 290
Prelude Control.Concurrent> forkIO $ print "hi!"
-- some time later....
Prelude Control.Concurrent> 50000005000000

Both actions will run in the background, and the second is almost guaranteed to finish before the last!

Communicating between Threads with `MVar`

It is very easy to pass information between threads using the MVar a type and its accompanying functions in Control.Concurrent:

  • newEmptyMVar :: IO (MVar a) -- creates a new MVar a
  • newMVar :: a -> IO (MVar a) -- creates a new MVar with the given value
  • takeMVar :: MVar a -> IO a -- retrieves the value from the given MVar, or blocks until one is available
  • putMVar :: MVar a -> a -> IO () -- puts the given value in the MVar, or blocks until it's empty

Let's sum the numbers from 1 to 100 million in a thread and wait on the result:

import Control.Concurrent
main = do
  m <- newEmptyMVar
  forkIO $ putMVar m $ sum [1..10000000]
  print =<< takeMVar m  -- takeMVar will block 'til m is non-empty!

A more complex demonstration might be to take user input and sum in the background while waiting for more input:

main2 = loop
    loop = do
        m <- newEmptyMVar
        n <- getLine
        putStrLn "Calculating. Please wait"
        -- In another thread, parse the user input and sum
        forkIO $ putMVar m $ sum [1..(read n :: Int)]
        -- In another thread, wait 'til the sum's complete then print it
        forkIO $ print =<< takeMVar m

As stated earlier, if you call takeMVar and the MVar is empty, it blocks until another thread puts something into the MVar, which could result in a Dining Philosophers Problem. The same thing happens with putMVar: if it's full, it'll block 'til it's empty!

Take the following function:

concurrent ma mb = do
  a <- takeMVar ma
  b <- takeMVar mb
  putMVar ma a
  putMVar mb b

We run the the two functions with some MVars

concurrent ma mb     -- new thread 1 
concurrent mb ma     -- new thread 2

What could happen is that:

  1. Thread 1 reads ma and blocks ma
  2. Thread 2 reads mb and thus blocks mb

Now Thread 1 cannot read mb as Thread 2 has blocked it, and Thread 2 cannot read ma as Thread 1 has blocked it. A classic deadlock!

Atomic Blocks with Software Transactional Memory

Another powerful & mature concurrency tool in Haskell is Software Transactional Memory, which allows for multiple threads to write to a single variable of type TVar a in an atomic manner.

TVar a is the main type associated with the STM monad and stands for transactional variable. They're used much like MVar but within the STM monad through the following functions:

atomically :: STM a -> IO a

Perform a series of STM actions atomically.

readTVar :: TVar a -> STM a

Read the TVar's value, e.g.:

value <- readTVar t

writeTVar :: TVar a -> a -> STM ()

Write a value to the given TVar.

t <- newTVar Nothing
writeTVar t (Just "Hello")

This example is taken from the Haskell Wiki:

import Control.Monad
import Control.Concurrent
import Control.Concurrent.STM
main = do 
  -- Initialise a new TVar
  shared <- atomically $ newTVar 0
  -- Read the value
  before <- atomRead shared
  putStrLn $ "Before: " ++ show before
  forkIO $ 25 `timesDo` (dispVar shared >> milliSleep 20)
  forkIO $ 10 `timesDo` (appV ((+) 2) shared >> milliSleep 50)
  forkIO $ 20 `timesDo` (appV pred shared >> milliSleep 25)
  milliSleep 800
  after <- atomRead shared
  putStrLn $ "After: " ++ show after
  where timesDo = replicateM_
       milliSleep = threadDelay . (*) 1000

atomRead = atomically . readTVar
dispVar x = atomRead x >>= print
appV fn x = atomically $ readTVar x >>= writeTVar x . fn