content: \begin{code}
  {-# LANGUAGE Unsafe #-}
  {-# LANGUAGE CPP
             , NoImplicitPrelude
             , BangPatterns
             , MagicHash
             , UnboxedTuples
             , UnliftedFFITypes
             , ForeignFunctionInterface
             , DeriveDataTypeable
             , StandaloneDeriving
             , RankNTypes
    #-}
  {-# OPTIONS_GHC -fno-warn-missing-signatures #-}
  {-# OPTIONS_HADDOCK not-home #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- Module      :  GHC.Conc.Sync
  -- Copyright   :  (c) The University of Glasgow, 1994-2002
  -- License     :  see libraries/base/LICENSE
  --
  -- Maintainer  :  cvs-ghc@haskell.org
  -- Stability   :  internal
  -- Portability :  non-portable (GHC extensions)
  --
  -- Basic concurrency stuff.
  --
  -----------------------------------------------------------------------------
  
  -- No: #hide, because bits of this module are exposed by the stm package.
  -- However, we don't want this module to be the home location for the
  -- bits it exports, we'd rather have Control.Concurrent and the other
  -- higher level modules be the home.  Hence:
  
  #include "Typeable.h"
  
  -- #not-home
  module GHC.Conc.Sync
          ( ThreadId(..)
  
          -- * Forking and suchlike
          , forkIO        -- :: IO a -> IO ThreadId
          , forkIOUnmasked
          , forkIOWithUnmask
          , forkOn      -- :: Int -> IO a -> IO ThreadId
          , forkOnIO    -- DEPRECATED
          , forkOnIOUnmasked
          , forkOnWithUnmask
          , numCapabilities -- :: Int
          , getNumCapabilities -- :: IO Int
          , setNumCapabilities -- :: Int -> IO ()
          , getNumProcessors   -- :: IO Int
          , numSparks      -- :: IO Int
          , childHandler  -- :: Exception -> IO ()
          , myThreadId    -- :: IO ThreadId
          , killThread    -- :: ThreadId -> IO ()
          , throwTo       -- :: ThreadId -> Exception -> IO ()
          , par           -- :: a -> b -> b
          , pseq          -- :: a -> b -> b
          , runSparks
          , yield         -- :: IO ()
          , labelThread   -- :: ThreadId -> String -> IO ()
  
          , ThreadStatus(..), BlockReason(..)
          , threadStatus  -- :: ThreadId -> IO ThreadStatus
          , threadCapability
  
          -- * TVars
          , STM(..)
          , atomically    -- :: STM a -> IO a
          , retry         -- :: STM a
          , orElse        -- :: STM a -> STM a -> STM a
          , throwSTM      -- :: Exception e => e -> STM a
          , catchSTM      -- :: Exception e => STM a -> (e -> STM a) -> STM a
          , alwaysSucceeds -- :: STM a -> STM ()
          , always        -- :: STM Bool -> STM ()
          , TVar(..)
          , newTVar       -- :: a -> STM (TVar a)
          , newTVarIO     -- :: a -> STM (TVar a)
          , readTVar      -- :: TVar a -> STM a
          , readTVarIO    -- :: TVar a -> IO a
          , writeTVar     -- :: a -> TVar a -> STM ()
          , unsafeIOToSTM -- :: IO a -> STM a
  
          -- * Miscellaneous
          , withMVar
          , modifyMVar_
  
          , setUncaughtExceptionHandler      -- :: (Exception -> IO ()) -> IO ()
          , getUncaughtExceptionHandler      -- :: IO (Exception -> IO ())
  
          , reportError, reportStackOverflow
  
          , sharedCAF
          ) where
  
  import Foreign hiding (unsafePerformIO)
  import Foreign.C
  
  #ifdef mingw32_HOST_OS
  import Data.Typeable
  #endif
  
  #ifndef mingw32_HOST_OS
  import Data.Dynamic
  #endif
  import Control.Monad
  import Data.Maybe
  
  import GHC.Base
  import {-# SOURCE #-} GHC.IO.Handle ( hFlush )
  import {-# SOURCE #-} GHC.IO.Handle.FD ( stdout )
  import GHC.IO
  import GHC.IO.Exception
  import GHC.Exception
  import GHC.IORef
  import GHC.MVar
  import GHC.Real         ( fromIntegral )
  import GHC.Pack         ( packCString# )
  import GHC.Show         ( Show(..), showString )
  
  infixr 0 `par`, `pseq`
  \end{code}
  
  %************************************************************************
  %*                                                                      *
  \subsection{@ThreadId@, @par@, and @fork@}
  %*                                                                      *
  %************************************************************************
  
  \begin{code}
  data ThreadId = ThreadId ThreadId# deriving( Typeable )
  -- ToDo: data ThreadId = ThreadId (Weak ThreadId#)
  -- But since ThreadId# is unlifted, the Weak type must use open
  -- type variables.
  {- ^
  A 'ThreadId' is an abstract type representing a handle to a thread.
  'ThreadId' is an instance of 'Eq', 'Ord' and 'Show', where
  the 'Ord' instance implements an arbitrary total ordering over
  'ThreadId's. The 'Show' instance lets you convert an arbitrary-valued
  'ThreadId' to string form; showing a 'ThreadId' value is occasionally
  useful when debugging or diagnosing the behaviour of a concurrent
  program.
  
  /Note/: in GHC, if you have a 'ThreadId', you essentially have
  a pointer to the thread itself.  This means the thread itself can\'t be
  garbage collected until you drop the 'ThreadId'.
  This misfeature will hopefully be corrected at a later date.
  
  /Note/: Hugs does not provide any operations on other threads;
  it defines 'ThreadId' as a synonym for ().
  -}
  
  instance Show ThreadId where
     showsPrec d t =
          showString "ThreadId " .
          showsPrec d (getThreadId (id2TSO t))
  
  foreign import ccall unsafe "rts_getThreadId" getThreadId :: ThreadId# -> CInt
  
  id2TSO :: ThreadId -> ThreadId#
  id2TSO (ThreadId t) = t
  
  foreign import ccall unsafe "cmp_thread" cmp_thread :: ThreadId# -> ThreadId# -> CInt
  -- Returns -1, 0, 1
  
  cmpThread :: ThreadId -> ThreadId -> Ordering
  cmpThread t1 t2 =
     case cmp_thread (id2TSO t1) (id2TSO t2) of
        -1 -> LT
        0  -> EQ
        _  -> GT -- must be 1
  
  instance Eq ThreadId where
     t1 == t2 =
        case t1 `cmpThread` t2 of
           EQ -> True
           _  -> False
  
  instance Ord ThreadId where
     compare = cmpThread
  
  {- |
  Sparks off a new thread to run the 'IO' computation passed as the
  first argument, and returns the 'ThreadId' of the newly created
  thread.
  
  The new thread will be a lightweight thread; if you want to use a foreign
  library that uses thread-local storage, use 'Control.Concurrent.forkOS' instead.
  
  GHC note: the new thread inherits the /masked/ state of the parent
  (see 'Control.Exception.mask').
  
  The newly created thread has an exception handler that discards the
  exceptions 'BlockedIndefinitelyOnMVar', 'BlockedIndefinitelyOnSTM', and
  'ThreadKilled', and passes all other exceptions to the uncaught
  exception handler.
  -}
  forkIO :: IO () -> IO ThreadId
  forkIO action = IO $ \ s ->
     case (fork# action_plus s) of (# s1, tid #) -> (# s1, ThreadId tid #)
   where
    action_plus = catchException action childHandler
  
  {-# DEPRECATED forkIOUnmasked "use forkIOWithUnmask instead" #-}
  -- | This function is deprecated; use 'forkIOWIthUnmask' instead
  forkIOUnmasked :: IO () -> IO ThreadId
  forkIOUnmasked io = forkIO (unsafeUnmask io)
  
  -- | Like 'forkIO', but the child thread is passed a function that can
  -- be used to unmask asynchronous exceptions.  This function is
  -- typically used in the following way
  --
  -- >  ... mask_ $ forkIOWithUnmask $ \unmask ->
  -- >                 catch (unmask ...) handler
  --
  -- so that the exception handler in the child thread is established
  -- with asynchronous exceptions masked, meanwhile the main body of
  -- the child thread is executed in the unmasked state.
  --
  -- Note that the unmask function passed to the child thread should
  -- only be used in that thread; the behaviour is undefined if it is
  -- invoked in a different thread.
  --
  forkIOWithUnmask :: ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
  forkIOWithUnmask io = forkIO (io unsafeUnmask)
  
  {- |
  Like 'forkIO', but lets you specify on which processor the thread
  should run.  Unlike a `forkIO` thread, a thread created by `forkOn`
  will stay on the same processor for its entire lifetime (`forkIO`
  threads can migrate between processors according to the scheduling
  policy).  `forkOn` is useful for overriding the scheduling policy when
  you know in advance how best to distribute the threads.
  
  The `Int` argument specifies a /capability number/ (see
  'getNumCapabilities').  Typically capabilities correspond to physical
  processors, but the exact behaviour is implementation-dependent.  The
  value passed to 'forkOn' is interpreted modulo the total number of
  capabilities as returned by 'getNumCapabilities'.
  
  GHC note: the number of capabilities is specified by the @+RTS -N@
  option when the program is started.  Capabilities can be fixed to
  actual processor cores with @+RTS -qa@ if the underlying operating
  system supports that, although in practice this is usually unnecessary
  (and may actually degrade perforamnce in some cases - experimentation
  is recommended).
  -}
  forkOn :: Int -> IO () -> IO ThreadId
  forkOn (I# cpu) action = IO $ \ s ->
     case (forkOn# cpu action_plus s) of (# s1, tid #) -> (# s1, ThreadId tid #)
   where
    action_plus = catchException action childHandler
  
  {-# DEPRECATED forkOnIO "renamed to forkOn" #-}
  -- | This function is deprecated; use 'forkOn' instead
  forkOnIO :: Int -> IO () -> IO ThreadId
  forkOnIO = forkOn
  
  {-# DEPRECATED forkOnIOUnmasked "use forkOnWithUnmask instead" #-}
  -- | This function is deprecated; use 'forkOnWIthUnmask' instead
  forkOnIOUnmasked :: Int -> IO () -> IO ThreadId
  forkOnIOUnmasked cpu io = forkOn cpu (unsafeUnmask io)
  
  -- | Like 'forkIOWithUnmask', but the child thread is pinned to the
  -- given CPU, as with 'forkOn'.
  forkOnWithUnmask :: Int -> ((forall a . IO a -> IO a) -> IO ()) -> IO ThreadId
  forkOnWithUnmask cpu io = forkOn cpu (io unsafeUnmask)
  
  -- | the value passed to the @+RTS -N@ flag.  This is the number of
  -- Haskell threads that can run truly simultaneously at any given
  -- time, and is typically set to the number of physical processor cores on
  -- the machine.
  --
  -- Strictly speaking it is better to use 'getNumCapabilities', because
  -- the number of capabilities might vary at runtime.
  --
  numCapabilities :: Int
  numCapabilities = unsafePerformIO $ getNumCapabilities
  
  {- |
  Returns the number of Haskell threads that can run truly
  simultaneously (on separate physical processors) at any given time.
  The number passed to `forkOn` is interpreted modulo this
  value.
  
  An implementation in which Haskell threads are mapped directly to
  OS threads might return the number of physical processor cores in
  the machine, and 'forkOn' would be implemented using the OS's
  affinity facilities.  An implementation that schedules Haskell
  threads onto a smaller number of OS threads (like GHC) would return
  the number of such OS threads that can be running simultaneously.
  
  GHC notes: this returns the number passed as the argument to the
  @+RTS -N@ flag.  In current implementations, the value is fixed
  when the program starts and never changes, but it is possible that
  in the future the number of capabilities might vary at runtime.
  -}
  getNumCapabilities :: IO Int
  getNumCapabilities = do
     n <- peek n_capabilities
     return (fromIntegral n)
  
  {- |
  Set the number of Haskell threads that can run truly simultaneously
  (on separate physical processors) at any given time.
  
  GHC notes: in the current implementation, the value may only be
  /increased/, not decreased, by calling 'setNumCapabilities'.  The
  initial value is given by the @+RTS -N@ flag, and the current value
  may be obtained using 'getNumCapabilities'.
  -}
  setNumCapabilities :: Int -> IO ()
  setNumCapabilities i = c_setNumCapabilities (fromIntegral i)
  
  foreign import ccall safe "setNumCapabilities"
    c_setNumCapabilities :: CUInt -> IO ()
  
  getNumProcessors :: IO Int
  getNumProcessors = fmap fromIntegral c_getNumberOfProcessors
  
  foreign import ccall unsafe "getNumberOfProcessors"
    c_getNumberOfProcessors :: IO CUInt
  
  -- | Returns the number of sparks currently in the local spark pool
  numSparks :: IO Int
  numSparks = IO $ \s -> case numSparks# s of (# s', n #) -> (# s', I# n #)
  
  #if defined(mingw32_HOST_OS) && defined(__PIC__)
  foreign import ccall "_imp__n_capabilities" n_capabilities :: Ptr CInt
  #else
  foreign import ccall "&n_capabilities" n_capabilities :: Ptr CInt
  #endif
  childHandler :: SomeException -> IO ()
  childHandler err = catchException (real_handler err) childHandler
  
  real_handler :: SomeException -> IO ()
  real_handler se@(SomeException ex) =
    -- ignore thread GC and killThread exceptions:
    case cast ex of
    Just BlockedIndefinitelyOnMVar        -> return ()
    _ -> case cast ex of
         Just BlockedIndefinitelyOnSTM    -> return ()
         _ -> case cast ex of
              Just ThreadKilled           -> return ()
              _ -> case cast ex of
                   -- report all others:
                   Just StackOverflow     -> reportStackOverflow
                   _                      -> reportError se
  
  {- | 'killThread' raises the 'ThreadKilled' exception in the given
  thread (GHC only).
  
  > killThread tid = throwTo tid ThreadKilled
  
  -}
  killThread :: ThreadId -> IO ()
  killThread tid = throwTo tid ThreadKilled
  
  {- | 'throwTo' raises an arbitrary exception in the target thread (GHC only).
  
  'throwTo' does not return until the exception has been raised in the
  target thread.
  The calling thread can thus be certain that the target
  thread has received the exception.  This is a useful property to know
  when dealing with race conditions: eg. if there are two threads that
  can kill each other, it is guaranteed that only one of the threads
  will get to kill the other.
  
  Whatever work the target thread was doing when the exception was
  raised is not lost: the computation is suspended until required by
  another thread.
  
  If the target thread is currently making a foreign call, then the
  exception will not be raised (and hence 'throwTo' will not return)
  until the call has completed.  This is the case regardless of whether
  the call is inside a 'mask' or not.  However, in GHC a foreign call
  can be annotated as @interruptible@, in which case a 'throwTo' will
  cause the RTS to attempt to cause the call to return; see the GHC
  documentation for more details.
  
  Important note: the behaviour of 'throwTo' differs from that described in
  the paper \"Asynchronous exceptions in Haskell\"
  (<http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm>).
  In the paper, 'throwTo' is non-blocking; but the library implementation adopts
  a more synchronous design in which 'throwTo' does not return until the exception
  is received by the target thread.  The trade-off is discussed in Section 9 of the paper.
  Like any blocking operation, 'throwTo' is therefore interruptible (see Section 5.3 of
  the paper).  Unlike other interruptible operations, however, 'throwTo'
  is /always/ interruptible, even if it does not actually block.
  
  There is no guarantee that the exception will be delivered promptly,
  although the runtime will endeavour to ensure that arbitrary
  delays don't occur.  In GHC, an exception can only be raised when a
  thread reaches a /safe point/, where a safe point is where memory
  allocation occurs.  Some loops do not perform any memory allocation
  inside the loop and therefore cannot be interrupted by a 'throwTo'.
  
  If the target of 'throwTo' is the calling thread, then the behaviour
  is the same as 'Control.Exception.throwIO', except that the exception
  is thrown as an asynchronous exception.  This means that if there is
  an enclosing pure computation, which would be the case if the current
  IO operation is inside 'unsafePerformIO' or 'unsafeInterleaveIO', that
  computation is not permanently replaced by the exception, but is
  suspended as if it had received an asynchronous exception.
  
  Note that if 'throwTo' is called with the current thread as the
  target, the exception will be thrown even if the thread is currently
  inside 'mask' or 'uninterruptibleMask'.
    -}
  throwTo :: Exception e => ThreadId -> e -> IO ()
  throwTo (ThreadId tid) ex = IO $ \ s ->
     case (killThread# tid (toException ex) s) of s1 -> (# s1, () #)
  
  -- | Returns the 'ThreadId' of the calling thread (GHC only).
  myThreadId :: IO ThreadId
  myThreadId = IO $ \s ->
     case (myThreadId# s) of (# s1, tid #) -> (# s1, ThreadId tid #)
  
  
  -- |The 'yield' action allows (forces, in a co-operative multitasking
  -- implementation) a context-switch to any other currently runnable
  -- threads (if any), and is occasionally useful when implementing
  -- concurrency abstractions.
  yield :: IO ()
  yield = IO $ \s ->
     case (yield# s) of s1 -> (# s1, () #)
  
  {- | 'labelThread' stores a string as identifier for this thread if
  you built a RTS with debugging support. This identifier will be used in
  the debugging output to make distinction of different threads easier
  (otherwise you only have the thread state object\'s address in the heap).
  
  Other applications like the graphical Concurrent Haskell Debugger
  (<http://www.informatik.uni-kiel.de/~fhu/chd/>) may choose to overload
  'labelThread' for their purposes as well.
  -}
  
  labelThread :: ThreadId -> String -> IO ()
  labelThread (ThreadId t) str = IO $ \ s ->
     let !ps  = packCString# str
         !adr = byteArrayContents# ps in
       case (labelThread# t adr s) of s1 -> (# s1, () #)
  
  --      Nota Bene: 'pseq' used to be 'seq'
  --                 but 'seq' is now defined in PrelGHC
  --
  -- "pseq" is defined a bit weirdly (see below)
  --
  -- The reason for the strange "lazy" call is that
  -- it fools the compiler into thinking that pseq  and par are non-strict in
  -- their second argument (even if it inlines pseq at the call site).
  -- If it thinks pseq is strict in "y", then it often evaluates
  -- "y" before "x", which is totally wrong.
  
  {-# INLINE pseq  #-}
  pseq :: a -> b -> b
  pseq  x y = x `seq` lazy y
  
  {-# INLINE par  #-}
  par :: a -> b -> b
  par  x y = case (par# x) of { _ -> lazy y }
  
  -- | Internal function used by the RTS to run sparks.
  runSparks :: IO ()
  runSparks = IO loop
    where loop s = case getSpark# s of
                     (# s', n, p #) ->
                        if n ==# 0# then (# s', () #)
                                    else p `seq` loop s'
  
  data BlockReason
    = BlockedOnMVar
          -- ^blocked on on 'MVar'
    | BlockedOnBlackHole
          -- ^blocked on a computation in progress by another thread
    | BlockedOnException
          -- ^blocked in 'throwTo'
    | BlockedOnSTM
          -- ^blocked in 'retry' in an STM transaction
    | BlockedOnForeignCall
          -- ^currently in a foreign call
    | BlockedOnOther
          -- ^blocked on some other resource.  Without @-threaded@,
          -- I\/O and 'threadDelay' show up as 'BlockedOnOther', with @-threaded@
          -- they show up as 'BlockedOnMVar'.
    deriving (Eq,Ord,Show)
  
  -- | The current status of a thread
  data ThreadStatus
    = ThreadRunning
          -- ^the thread is currently runnable or running
    | ThreadFinished
          -- ^the thread has finished
    | ThreadBlocked  BlockReason
          -- ^the thread is blocked on some resource
    | ThreadDied
          -- ^the thread received an uncaught exception
    deriving (Eq,Ord,Show)
  
  threadStatus :: ThreadId -> IO ThreadStatus
  threadStatus (ThreadId t) = IO $ \s ->
     case threadStatus# t s of
      (# s', stat, _cap, _locked #) -> (# s', mk_stat (I# stat) #)
     where
          -- NB. keep these in sync with includes/Constants.h
       mk_stat 0  = ThreadRunning
       mk_stat 1  = ThreadBlocked BlockedOnMVar
       mk_stat 2  = ThreadBlocked BlockedOnBlackHole
       mk_stat 6  = ThreadBlocked BlockedOnSTM
       mk_stat 10 = ThreadBlocked BlockedOnForeignCall
       mk_stat 11 = ThreadBlocked BlockedOnForeignCall
       mk_stat 12 = ThreadBlocked BlockedOnException
       mk_stat 16 = ThreadFinished
       mk_stat 17 = ThreadDied
       mk_stat _  = ThreadBlocked BlockedOnOther
  
  -- | returns the number of the capability on which the thread is currently
  -- running, and a boolean indicating whether the thread is locked to
  -- that capability or not.  A thread is locked to a capability if it
  -- was created with @forkOn@.
  threadCapability :: ThreadId -> IO (Int, Bool)
  threadCapability (ThreadId t) = IO $ \s ->
     case threadStatus# t s of
       (# s', _, cap#, locked# #) -> (# s', (I# cap#, locked# /=# 0#) #)
  \end{code}
  
  
  %************************************************************************
  %*                                                                      *
  \subsection[stm]{Transactional heap operations}
  %*                                                                      *
  %************************************************************************
  
  TVars are shared memory locations which support atomic memory
  transactions.
  
  \begin{code}
  -- |A monad supporting atomic memory transactions.
  newtype STM a = STM (State# RealWorld -> (# State# RealWorld, a #))
  
  unSTM :: STM a -> (State# RealWorld -> (# State# RealWorld, a #))
  unSTM (STM a) = a
  
  INSTANCE_TYPEABLE1(STM,stmTc,"STM")
  
  instance  Functor STM where
     fmap f x = x >>= (return . f)
  
  instance  Monad STM  where
      {-# INLINE return #-}
      {-# INLINE (>>)   #-}
      {-# INLINE (>>=)  #-}
      m >> k      = thenSTM m k
      return x    = returnSTM x
      m >>= k     = bindSTM m k
  
  bindSTM :: STM a -> (a -> STM b) -> STM b
  bindSTM (STM m) k = STM ( \s ->
    case m s of
      (# new_s, a #) -> unSTM (k a) new_s
    )
  
  thenSTM :: STM a -> STM b -> STM b
  thenSTM (STM m) k = STM ( \s ->
    case m s of
      (# new_s, _ #) -> unSTM k new_s
    )
  
  returnSTM :: a -> STM a
  returnSTM x = STM (\s -> (# s, x #))
  
  instance MonadPlus STM where
    mzero = retry
    mplus = orElse
  
  -- | Unsafely performs IO in the STM monad.  Beware: this is a highly
  -- dangerous thing to do.
  --
  --   * The STM implementation will often run transactions multiple
  --     times, so you need to be prepared for this if your IO has any
  --     side effects.
  --
  --   * The STM implementation will abort transactions that are known to
  --     be invalid and need to be restarted.  This may happen in the middle
  --     of `unsafeIOToSTM`, so make sure you don't acquire any resources
  --     that need releasing (exception handlers are ignored when aborting
  --     the transaction).  That includes doing any IO using Handles, for
  --     example.  Getting this wrong will probably lead to random deadlocks.
  --
  --   * The transaction may have seen an inconsistent view of memory when
  --     the IO runs.  Invariants that you expect to be true throughout
  --     your program may not be true inside a transaction, due to the
  --     way transactions are implemented.  Normally this wouldn't be visible
  --     to the programmer, but using `unsafeIOToSTM` can expose it.
  --
  unsafeIOToSTM :: IO a -> STM a
  unsafeIOToSTM (IO m) = STM m
  
  -- |Perform a series of STM actions atomically.
  --
  -- You cannot use 'atomically' inside an 'unsafePerformIO' or 'unsafeInterleaveIO'.
  -- Any attempt to do so will result in a runtime error.  (Reason: allowing
  -- this would effectively allow a transaction inside a transaction, depending
  -- on exactly when the thunk is evaluated.)
  --
  -- However, see 'newTVarIO', which can be called inside 'unsafePerformIO',
  -- and which allows top-level TVars to be allocated.
  
  atomically :: STM a -> IO a
  atomically (STM m) = IO (\s -> (atomically# m) s )
  
  -- |Retry execution of the current memory transaction because it has seen
  -- values in TVars which mean that it should not continue (e.g. the TVars
  -- represent a shared buffer that is now empty).  The implementation may
  -- block the thread until one of the TVars that it has read from has been
  -- udpated. (GHC only)
  retry :: STM a
  retry = STM $ \s# -> retry# s#
  
  -- |Compose two alternative STM actions (GHC only).  If the first action
  -- completes without retrying then it forms the result of the orElse.
  -- Otherwise, if the first action retries, then the second action is
  -- tried in its place.  If both actions retry then the orElse as a
  -- whole retries.
  orElse :: STM a -> STM a -> STM a
  orElse (STM m) e = STM $ \s -> catchRetry# m (unSTM e) s
  
  -- | A variant of 'throw' that can only be used within the 'STM' monad.
  --
  -- Throwing an exception in @STM@ aborts the transaction and propagates the
  -- exception.
  --
  -- Although 'throwSTM' has a type that is an instance of the type of 'throw', the
  -- two functions are subtly different:
  --
  -- > throw e    `seq` x  ===> throw e
  -- > throwSTM e `seq` x  ===> x
  --
  -- The first example will cause the exception @e@ to be raised,
  -- whereas the second one won\'t.  In fact, 'throwSTM' will only cause
  -- an exception to be raised when it is used within the 'STM' monad.
  -- The 'throwSTM' variant should be used in preference to 'throw' to
  -- raise an exception within the 'STM' monad because it guarantees
  -- ordering with respect to other 'STM' operations, whereas 'throw'
  -- does not.
  throwSTM :: Exception e => e -> STM a
  throwSTM e = STM $ raiseIO# (toException e)
  
  -- |Exception handling within STM actions.
  catchSTM :: Exception e => STM a -> (e -> STM a) -> STM a
  catchSTM (STM m) handler = STM $ catchSTM# m handler'
      where
        handler' e = case fromException e of
                       Just e' -> unSTM (handler e')
                       Nothing -> raiseIO# e
  
  -- | Low-level primitive on which always and alwaysSucceeds are built.
  -- checkInv differs form these in that (i) the invariant is not
  -- checked when checkInv is called, only at the end of this and
  -- subsequent transcations, (ii) the invariant failure is indicated
  -- by raising an exception.
  checkInv :: STM a -> STM ()
  checkInv (STM m) = STM (\s -> (check# m) s)
  
  -- | alwaysSucceeds adds a new invariant that must be true when passed
  -- to alwaysSucceeds, at the end of the current transaction, and at
  -- the end of every subsequent transaction.  If it fails at any
  -- of those points then the transaction violating it is aborted
  -- and the exception raised by the invariant is propagated.
  alwaysSucceeds :: STM a -> STM ()
  alwaysSucceeds i = do ( i >> retry ) `orElse` ( return () )
                        checkInv i
  
  -- | always is a variant of alwaysSucceeds in which the invariant is
  -- expressed as an STM Bool action that must return True.  Returning
  -- False or raising an exception are both treated as invariant failures.
  always :: STM Bool -> STM ()
  always i = alwaysSucceeds ( do v <- i
                                 if (v) then return () else ( error "Transactional invariant violation" ) )
  
  -- |Shared memory locations that support atomic memory transactions.
  data TVar a = TVar (TVar# RealWorld a)
  
  INSTANCE_TYPEABLE1(TVar,tvarTc,"TVar")
  
  instance Eq (TVar a) where
          (TVar tvar1#) == (TVar tvar2#) = sameTVar# tvar1# tvar2#
  
  -- |Create a new TVar holding a value supplied
  newTVar :: a -> STM (TVar a)
  newTVar val = STM $ \s1# ->
      case newTVar# val s1# of
           (# s2#, tvar# #) -> (# s2#, TVar tvar# #)
  
  -- |@IO@ version of 'newTVar'.  This is useful for creating top-level
  -- 'TVar's using 'System.IO.Unsafe.unsafePerformIO', because using
  -- 'atomically' inside 'System.IO.Unsafe.unsafePerformIO' isn't
  -- possible.
  newTVarIO :: a -> IO (TVar a)
  newTVarIO val = IO $ \s1# ->
      case newTVar# val s1# of
           (# s2#, tvar# #) -> (# s2#, TVar tvar# #)
  
  -- |Return the current value stored in a TVar.
  -- This is equivalent to
  --
  -- >  readTVarIO = atomically . readTVar
  --
  -- but works much faster, because it doesn't perform a complete
  -- transaction, it just reads the current value of the 'TVar'.
  readTVarIO :: TVar a -> IO a
  readTVarIO (TVar tvar#) = IO $ \s# -> readTVarIO# tvar# s#
  
  -- |Return the current value stored in a TVar
  readTVar :: TVar a -> STM a
  readTVar (TVar tvar#) = STM $ \s# -> readTVar# tvar# s#
  
  -- |Write the supplied value into a TVar
  writeTVar :: TVar a -> a -> STM ()
  writeTVar (TVar tvar#) val = STM $ \s1# ->
      case writeTVar# tvar# val s1# of
           s2# -> (# s2#, () #)
  
  \end{code}
  
  MVar utilities
  
  \begin{code}
  withMVar :: MVar a -> (a -> IO b) -> IO b
  withMVar m io =
    mask $ \restore -> do
      a <- takeMVar m
      b <- catchAny (restore (io a))
              (\e -> do putMVar m a; throw e)
      putMVar m a
      return b
  
  modifyMVar_ :: MVar a -> (a -> IO a) -> IO ()
  modifyMVar_ m io =
    mask $ \restore -> do
      a <- takeMVar m
      a' <- catchAny (restore (io a))
              (\e -> do putMVar m a; throw e)
      putMVar m a'
      return ()
  \end{code}
  
  %************************************************************************
  %*                                                                      *
  \subsection{Thread waiting}
  %*                                                                      *
  %************************************************************************
  
  \begin{code}
  
  -- Machinery needed to ensureb that we only have one copy of certain
  -- CAFs in this module even when the base package is present twice, as
  -- it is when base is dynamically loaded into GHCi.  The RTS keeps
  -- track of the single true value of the CAF, so even when the CAFs in
  -- the dynamically-loaded base package are reverted, nothing bad
  -- happens.
  --
  sharedCAF :: a -> (Ptr a -> IO (Ptr a)) -> IO a
  sharedCAF a get_or_set =
     mask_ $ do
       stable_ref <- newStablePtr a
       let ref = castPtr (castStablePtrToPtr stable_ref)
       ref2 <- get_or_set ref
       if ref==ref2
          then return a
          else do freeStablePtr stable_ref
                  deRefStablePtr (castPtrToStablePtr (castPtr ref2))
  
  reportStackOverflow :: IO ()
  reportStackOverflow = callStackOverflowHook
  
  reportError :: SomeException -> IO ()
  reportError ex = do
     handler <- getUncaughtExceptionHandler
     handler ex
  
  -- SUP: Are the hooks allowed to re-enter Haskell land?  If so, remove
  -- the unsafe below.
  foreign import ccall unsafe "stackOverflow"
          callStackOverflowHook :: IO ()
  
  {-# NOINLINE uncaughtExceptionHandler #-}
  uncaughtExceptionHandler :: IORef (SomeException -> IO ())
  uncaughtExceptionHandler = unsafePerformIO (newIORef defaultHandler)
     where
        defaultHandler :: SomeException -> IO ()
        defaultHandler se@(SomeException ex) = do
           (hFlush stdout) `catchAny` (\ _ -> return ())
           let msg = case cast ex of
                 Just Deadlock -> "no threads to run:  infinite loop or deadlock?"
                 _ -> case cast ex of
                      Just (ErrorCall s) -> s
                      _                  -> showsPrec 0 se ""
           withCString "%s" $ \cfmt ->
            withCString msg $ \cmsg ->
              errorBelch cfmt cmsg
  
  -- don't use errorBelch() directly, because we cannot call varargs functions
  -- using the FFI.
  foreign import ccall unsafe "HsBase.h errorBelch2"
     errorBelch :: CString -> CString -> IO ()
  
  setUncaughtExceptionHandler :: (SomeException -> IO ()) -> IO ()
  setUncaughtExceptionHandler = writeIORef uncaughtExceptionHandler
  
  getUncaughtExceptionHandler :: IO (SomeException -> IO ())
  getUncaughtExceptionHandler = readIORef uncaughtExceptionHandler
  
  \end{code}
  