Parallel and Concurrent Programming in Haskell

Chapter 7. Basic Concurrency: Threads and MVars

The fundamental action in concurrency is forking a new thread of control. In Concurrent Haskell, this is achieved with the forkIO operation:

forkIO :: IO () -> IO ThreadId

The forkIO operation takes a computation of type IO () as its argument; that is, a computation in the IO monad that eventually delivers a value of type (). The computation passed to forkIO is executed in a new thread that runs concurrently with the other threads in the system. If the thread has effects, those effects will be interleaved in an indeterminate fashion with the effects from other threads.

To illustrate the interleaving of effects, let’s try a simple example with two threads, one that repeatedly prints the letter A while the other repeatedly prints B:

fork.hs

import Control.Concurrent
import Control.Monad
import System.IO

main = do
  hSetBuffering stdout NoBuffering            -- 
  forkIO (replicateM_ 100000 (putChar 'A'))   -- 
  replicateM_ 100000 (putChar 'B')            -- 

Put the output Handle into nonbuffered mode, so that we can see the interleaving more clearly.

Create a thread to ...