9. Concurrent

This library provides the Concurrent Haskell extensions [concurrentHaskell:popl96].

We are grateful to the Glasgow Haskell Project for allowing us to redistribute their implementation of this module.

module Concurrent where

data ThreadId    -- thread identifiers
instance Eq ThreadId

forkIO           :: IO () -> IO ThreadId
killThread       :: ThreadId -> IO ()

data MVar a      -- Synchronisation variables
newEmptyMVar     :: IO (MVar a)
newMVar          :: a -> IO (MVar a)
takeMVar         :: MVar a -> IO a
putMVar          :: MVar a -> a -> IO ()
swapMVar         :: MVar a -> a -> IO a
readMVar         :: MVar a -> IO a 
instance Eq (MVar a)

data Chan a      -- channels
newChan          :: IO (Chan a)
writeChan        :: Chan a -> a -> IO ()
readChan         :: Chan a -> IO a
dupChan          :: Chan a -> IO (Chan a)
unReadChan       :: Chan a -> a -> IO ()
readChanContents :: Chan a -> IO [a]
writeList2Chan   :: Chan a -> [a] -> IO ()
                      
data CVar a       -- one element channels
newCVar          :: IO (CVar a)
putCVar          :: CVar a -> a -> IO ()
getCVar          :: CVar a -> IO a
                      
data QSem        -- General/quantity semaphores
newQSem          :: Int  -> IO QSem
waitQSem         :: QSem -> IO ()
signalQSem       :: QSem -> IO ()
                      
data QSemN       -- General/quantity semaphores
newQSemN         :: Int   -> IO QSemN
waitQSemN        :: QSemN -> Int -> IO ()
signalQSemN      :: QSemN -> Int -> IO ()

type SampleVar a -- Sample variables 
newEmptySampleVar:: IO (SampleVar a)
newSampleVar     :: a -> IO (SampleVar a)
emptySampleVar   :: SampleVar a -> IO ()
readSampleVar    :: SampleVar a -> IO a
writeSampleVar   :: SampleVar a -> a -> IO ()

Notes:

• GHC uses preemptive multitasking: Context switches can occur at any time, except if you call a C function (like \verb"getchar") that blocks waiting for input. Hugs uses cooperative multitasking: Context switches only occur when you use one of the primitives defined in this module. This means that programs such as:

  main = forkIO (write 'a') >> write 'b'
   where write c = putChar c >> write c
  

  will print either aaaaaaaaaaaaaa... or bbbbbbbbbbbb..., instead of some random interleaving of as and bs. In practice, cooperative multitasking is sufficient for writing simple graphical user interfaces.
• Hugs does not provide the functions mergeIO or nmergeIO since these require preemptive multitasking.
• killThread has not been implemented yet on either system. The plan is that killThread will raise an IO exception in the killed thread which it can catch --- perhaps allowing it to kill its children before exiting.
• The Ord instance for ThreadIds provides an arbitrary total ordering which might be used to build an ordered binary tree, say.