    \begin{code}
    {-# OPTIONS_GHC -fno-implicit-prelude -funbox-strict-fields #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.IOBase
    -- 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)
    --
    -- Definitions for the 'IO' monad and its friends.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.IOBase(
        IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
        unsafePerformIO, unsafeInterleaveIO,
        unsafeDupablePerformIO, unsafeDupableInterleaveIO,
        noDuplicate,

    	-- To and from from ST
        stToIO, ioToST, unsafeIOToST, unsafeSTToIO,

    	-- References
        IORef(..), newIORef, readIORef, writeIORef,
        IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
        MVar(..),

        	-- Handles, file descriptors,
        FilePath,
        Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
        isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,

        	-- Buffers
        Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
        bufferIsWritable, bufferEmpty, bufferFull,

        	-- Exceptions
        Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
        stackOverflow, heapOverflow, throw, throwIO, ioException,
        IOError, IOException(..), IOErrorType(..), ioError, userError,
        ExitCode(..)
      ) where

    import GHC.ST
    import GHC.Arr	-- to derive Ix class
    import GHC.Enum -- to derive Enum class
    import GHC.STRef
    import GHC.Base
    --  import GHC.Num	-- To get fromInteger etc, needed because of -fno-implicit-prelude
    import Data.Maybe  ( Maybe(..) )
    import GHC.Show
    import GHC.List
    import GHC.Read
    import Foreign.C.Types (CInt)

    #ifndef __HADDOCK__
    import {-# SOURCE #-} Data.Typeable	( showsTypeRep )
    import {-# SOURCE #-} Data.Dynamic	( Dynamic, dynTypeRep )
    #endif

    -- ---------------------------------------------------------------------------
    -- The IO Monad

    {-
    The IO Monad is just an instance of the ST monad, where the state is
    the real world.  We use the exception mechanism (in GHC.Exception) to
    implement IO exceptions.

    NOTE: The IO representation is deeply wired in to various parts of the
    system.  The following list may or may not be exhaustive:

    Compiler  - types of various primitives in PrimOp.lhs

    RTS 	  - forceIO (StgMiscClosures.hc)
    	  - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
    	    (Exceptions.hc)
    	  - raiseAsync (Schedule.c)

    Prelude   - GHC.IOBase.lhs, and several other places including
    	    GHC.Exception.lhs.

    Libraries - parts of hslibs/lang.

    --SDM
    -}

    {-|
    A value of type @'IO' a@ is a computation which, when performed,
    does some I\/O before returning a value of type @a@.

    There is really only one way to \"perform\" an I\/O action: bind it to
    @Main.main@ in your program.  When your program is run, the I\/O will
    be performed.  It isn't possible to perform I\/O from an arbitrary
    function, unless that function is itself in the 'IO' monad and called
    at some point, directly or indirectly, from @Main.main@.

    'IO' is a monad, so 'IO' actions can be combined using either the do-notation
    or the '>>' and '>>=' operations from the 'Monad' class.
    -}
    newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

    unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
    unIO (IO a) = a

    instance  Functor IO where
       fmap f x = x >>= (return . f)

    instance  Monad IO  where
        {-# INLINE return #-}
        {-# INLINE (>>)   #-}
        {-# INLINE (>>=)  #-}
        m >> k      =  m >>= \ _ -> k
        return x	= returnIO x

        m >>= k     = bindIO m k
        fail s	= failIO s

    failIO :: String -> IO a
    failIO s = ioError (userError s)

    liftIO :: IO a -> State# RealWorld -> STret RealWorld a
    liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r

    bindIO :: IO a -> (a -> IO b) -> IO b
    bindIO (IO m) k = IO ( \ s ->
      case m s of
        (# new_s, a #) -> unIO (k a) new_s
      )

    thenIO :: IO a -> IO b -> IO b
    thenIO (IO m) k = IO ( \ s ->
      case m s of
        (# new_s, a #) -> unIO k new_s
      )

    returnIO :: a -> IO a
    returnIO x = IO (\ s -> (# s, x #))

    -- ---------------------------------------------------------------------------
    -- Coercions between IO and ST

    -- | A monad transformer embedding strict state transformers in the 'IO'
    -- monad.  The 'RealWorld' parameter indicates that the internal state
    -- used by the 'ST' computation is a special one supplied by the 'IO'
    -- monad, and thus distinct from those used by invocations of 'runST'.
    stToIO	      :: ST RealWorld a -> IO a
    stToIO (ST m) = IO m

    ioToST	      :: IO a -> ST RealWorld a
    ioToST (IO m) = (ST m)

    -- This relies on IO and ST having the same representation modulo the
    -- constraint on the type of the state
    --
    unsafeIOToST        :: IO a -> ST s a
    unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s

    unsafeSTToIO :: ST s a -> IO a
    unsafeSTToIO (ST m) = IO (unsafeCoerce# m)

    -- ---------------------------------------------------------------------------
    -- Unsafe IO operations

    {-|
    This is the \"back door\" into the 'IO' monad, allowing
    'IO' computation to be performed at any time.  For
    this to be safe, the 'IO' computation should be
    free of side effects and independent of its environment.

    If the I\/O computation wrapped in 'unsafePerformIO'
    performs side effects, then the relative order in which those side
    effects take place (relative to the main I\/O trunk, or other calls to
    'unsafePerformIO') is indeterminate.  You have to be careful when
    writing and compiling modules that use 'unsafePerformIO':

      * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
    	that calls 'unsafePerformIO'.  If the call is inlined,
    	the I\/O may be performed more than once.

      * Use the compiler flag @-fno-cse@ to prevent common sub-expression
    	elimination being performed on the module, which might combine
    	two side effects that were meant to be separate.  A good example
    	is using multiple global variables (like @test@ in the example below).

      * Make sure that the either you switch off let-floating, or that the
    	call to 'unsafePerformIO' cannot float outside a lambda.  For example,
    	if you say:
    	@
    	   f x = unsafePerformIO (newIORef [])
    	@
    	you may get only one reference cell shared between all calls to @f@.
    	Better would be
    	@
    	   f x = unsafePerformIO (newIORef [x])
    	@
    	because now it can't float outside the lambda.

    It is less well known that
    'unsafePerformIO' is not type safe.  For example:

    >     test :: IORef [a]
    >     test = unsafePerformIO $ newIORef []
    >
    >     main = do
    >     	      writeIORef test [42]
    >     	      bang <- readIORef test
    >     	      print (bang :: [Char])

    This program will core dump.  This problem with polymorphic references
    is well known in the ML community, and does not arise with normal
    monadic use of references.  There is no easy way to make it impossible
    once you use 'unsafePerformIO'.  Indeed, it is
    possible to write @coerce :: a -> b@ with the
    help of 'unsafePerformIO'.  So be careful!
    -}
    unsafePerformIO	:: IO a -> a
    unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)

    {-|
    This version of 'unsafePerformIO' is slightly more efficient,
    because it omits the check that the IO is only being performed by a
    single thread.  Hence, when you write 'unsafeDupablePerformIO',
    there is a possibility that the IO action may be performed multiple
    times (on a multiprocessor), and you should therefore ensure that
    it gives the same results each time.
    -}
    {-# NOINLINE unsafeDupablePerformIO #-}
    unsafeDupablePerformIO	:: IO a -> a
    unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)

    -- Why do we NOINLINE unsafeDupablePerformIO?  See the comment with
    -- GHC.ST.runST.  Essentially the issue is that the IO computation
    -- inside unsafePerformIO must be atomic: it must either all run, or
    -- not at all.  If we let the compiler see the application of the IO
    -- to realWorld#, it might float out part of the IO.

    -- Why is there a call to 'lazy' in unsafeDupablePerformIO?
    -- If we don't have it, the demand analyser discovers the following strictness
    -- for unsafeDupablePerformIO:  C(U(AV))
    -- But then consider
    --	unsafeDupablePerformIO (\s -> let r = f x in
    --			       case writeIORef v r s of (# s1, _ #) ->
    --			       (# s1, r #)
    -- The strictness analyser will find that the binding for r is strict,
    -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
    -- doing the writeIORef.  This actually makes tests/lib/should_run/memo002
    -- get a deadlock!
    --
    -- Solution: don't expose the strictness of unsafeDupablePerformIO,
    --	     by hiding it with 'lazy'

    {-|
    'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
    When passed a value of type @IO a@, the 'IO' will only be performed
    when the value of the @a@ is demanded.  This is used to implement lazy
    file reading, see 'System.IO.hGetContents'.
    -}
    {-# INLINE unsafeInterleaveIO #-}
    unsafeInterleaveIO :: IO a -> IO a
    unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)

    -- We believe that INLINE on unsafeInterleaveIO is safe, because the
    -- state from this IO thread is passed explicitly to the interleaved
    -- IO, so it cannot be floated out and shared.

    {-# INLINE unsafeDupableInterleaveIO #-}
    unsafeDupableInterleaveIO :: IO a -> IO a
    unsafeDupableInterleaveIO (IO m)
      = IO ( \ s -> let
    		   r = case m s of (# _, res #) -> res
    		in
    		(# s, r #))

    {-|
    Ensures that the suspensions under evaluation by the current thread
    are unique; that is, the current thread is not evaluating anything
    that is also under evaluation by another thread that has also executed
    'noDuplicate'.

    This operation is used in the definition of 'unsafePerformIO' to
    prevent the IO action from being executed multiple times, which is usually
    undesirable.
    -}
    noDuplicate :: IO ()
    noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)

    -- ---------------------------------------------------------------------------
    -- Handle type

    data MVar a = MVar (MVar# RealWorld a)
    {- ^
    An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
    for communication between concurrent threads.  It can be thought of
    as a a box, which may be empty or full.
    -}

    -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
    instance Eq (MVar a) where
    	(MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#

    --  A Handle is represented by (a reference to) a record
    --  containing the state of the I/O port/device. We record
    --  the following pieces of info:

    --    * type (read,write,closed etc.)
    --    * the underlying file descriptor
    --    * buffering mode
    --    * buffer, and spare buffers
    --    * user-friendly name (usually the
    --	FilePath used when IO.openFile was called)

    -- Note: when a Handle is garbage collected, we want to flush its buffer
    -- and close the OS file handle, so as to free up a (precious) resource.

    -- | Haskell defines operations to read and write characters from and to files,
    -- represented by values of type @Handle@.  Each value of this type is a
    -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
    -- with file system objects.  A handle has at least the following properties:
    --
    --  * whether it manages input or output or both;
    --
    --  * whether it is /open/, /closed/ or /semi-closed/;
    --
    --  * whether the object is seekable;
    --
    --  * whether buffering is disabled, or enabled on a line or block basis;
    --
    --  * a buffer (whose length may be zero).
    --
    -- Most handles will also have a current I\/O position indicating where the next
    -- input or output operation will occur.  A handle is /readable/ if it
    -- manages only input or both input and output; likewise, it is /writable/ if
    -- it manages only output or both input and output.  A handle is /open/ when
    -- first allocated.
    -- Once it is closed it can no longer be used for either input or output,
    -- though an implementation cannot re-use its storage while references
    -- remain to it.  Handles are in the 'Show' and 'Eq' classes.  The string
    -- produced by showing a handle is system dependent; it should include
    -- enough information to identify the handle for debugging.  A handle is
    -- equal according to '==' only to itself; no attempt
    -- is made to compare the internal state of different handles for equality.
    --
    -- GHC note: a 'Handle' will be automatically closed when the garbage
    -- collector detects that it has become unreferenced by the program.
    -- However, relying on this behaviour is not generally recommended:
    -- the garbage collector is unpredictable.  If possible, use explicit
    -- an explicit 'hClose' to close 'Handle's when they are no longer
    -- required.  GHC does not currently attempt to free up file
    -- descriptors when they have run out, it is your responsibility to
    -- ensure that this doesn't happen.

    data Handle
      = FileHandle				-- A normal handle to a file
    	FilePath			-- the file (invariant)
    	!(MVar Handle__)

      | DuplexHandle			-- A handle to a read/write stream
    	FilePath			-- file for a FIFO, otherwise some
    					--   descriptive string.
    	!(MVar Handle__)		-- The read side
    	!(MVar Handle__)		-- The write side

    -- NOTES:
    --    * A 'FileHandle' is seekable.  A 'DuplexHandle' may or may not be
    --      seekable.

    instance Eq Handle where
     (FileHandle _ h1)     == (FileHandle _ h2)     = h1 == h2
     (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
     _ == _ = False

    type FD = CInt

    data Handle__
      = Handle__ {
          haFD	    :: !FD,		     -- file descriptor
          haType        :: HandleType,	     -- type (read/write/append etc.)
          haIsBin       :: Bool,		     -- binary mode?
          haIsStream    :: Bool,		     -- Windows : is this a socket?
                                                 -- Unix    : is O_NONBLOCK set?
          haBufferMode  :: BufferMode,	     -- buffer contains read/write data?
          haBuffer	    :: !(IORef Buffer),	     -- the current buffer
          haBuffers     :: !(IORef BufferList),  -- spare buffers
          haOtherSide   :: Maybe (MVar Handle__) -- ptr to the write side of a
    					     -- duplex handle.
        }

    -- ---------------------------------------------------------------------------
    -- Buffers

    -- The buffer is represented by a mutable variable containing a
    -- record, where the record contains the raw buffer and the start/end
    -- points of the filled portion.  We use a mutable variable so that
    -- the common operation of writing (or reading) some data from (to)
    -- the buffer doesn't need to modify, and hence copy, the handle
    -- itself, it just updates the buffer.

    -- There will be some allocation involved in a simple hPutChar in
    -- order to create the new Buffer structure (below), but this is
    -- relatively small, and this only has to be done once per write
    -- operation.

    -- The buffer contains its size - we could also get the size by
    -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
    -- to be rounded up to the nearest Word.

    type RawBuffer = MutableByteArray# RealWorld

    -- INVARIANTS on a Buffer:
    --
    --   * A handle *always* has a buffer, even if it is only 1 character long
    --     (an unbuffered handle needs a 1 character buffer in order to support
    --      hLookAhead and hIsEOF).
    --   * r <= w
    --   * if r == w, then r == 0 && w == 0
    --   * if state == WriteBuffer, then r == 0
    --   * a write buffer is never full.  If an operation
    --     fills up the buffer, it will always flush it before
    --     returning.
    --   * a read buffer may be full as a result of hLookAhead.  In normal
    --     operation, a read buffer always has at least one character of space.

    data Buffer
      = Buffer {
    	bufBuf   :: RawBuffer,
    	bufRPtr  :: !Int,
    	bufWPtr  :: !Int,
    	bufSize  :: !Int,
    	bufState :: BufferState
      }

    data BufferState = ReadBuffer | WriteBuffer deriving (Eq)

    -- we keep a few spare buffers around in a handle to avoid allocating
    -- a new one for each hPutStr.  These buffers are *guaranteed* to be the
    -- same size as the main buffer.
    data BufferList
      = BufferListNil
      | BufferListCons RawBuffer BufferList

    bufferIsWritable :: Buffer -> Bool
    bufferIsWritable Buffer{ bufState=WriteBuffer } = True
    bufferIsWritable _other = False

    bufferEmpty :: Buffer -> Bool
    bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w

    -- only makes sense for a write buffer
    bufferFull :: Buffer -> Bool
    bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b

    --  Internally, we classify handles as being one
    --  of the following:

    data HandleType
     = ClosedHandle
     | SemiClosedHandle
     | ReadHandle
     | WriteHandle
     | AppendHandle
     | ReadWriteHandle

    isReadableHandleType ReadHandle         = True
    isReadableHandleType ReadWriteHandle    = True
    isReadableHandleType _	       	        = False

    isWritableHandleType AppendHandle    = True
    isWritableHandleType WriteHandle     = True
    isWritableHandleType ReadWriteHandle = True
    isWritableHandleType _	       	     = False

    isReadWriteHandleType ReadWriteHandle{} = True
    isReadWriteHandleType _                 = False

    -- | File and directory names are values of type 'String', whose precise
    -- meaning is operating system dependent. Files can be opened, yielding a
    -- handle which can then be used to operate on the contents of that file.

    type FilePath = String

    -- ---------------------------------------------------------------------------
    -- Buffering modes

    -- | Three kinds of buffering are supported: line-buffering,
    -- block-buffering or no-buffering.  These modes have the following
    -- effects. For output, items are written out, or /flushed/,
    -- from the internal buffer according to the buffer mode:
    --
    --  * /line-buffering/: the entire output buffer is flushed
    --    whenever a newline is output, the buffer overflows,
    --    a 'System.IO.hFlush' is issued, or the handle is closed.
    --
    --  * /block-buffering/: the entire buffer is written out whenever it
    --    overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
    --
    --  * /no-buffering/: output is written immediately, and never stored
    --    in the buffer.
    --
    -- An implementation is free to flush the buffer more frequently,
    -- but not less frequently, than specified above.
    -- The output buffer is emptied as soon as it has been written out.
    --
    -- Similarly, input occurs according to the buffer mode for the handle:
    --
    --  * /line-buffering/: when the buffer for the handle is not empty,
    --    the next item is obtained from the buffer; otherwise, when the
    --    buffer is empty, characters up to and including the next newline
    --    character are read into the buffer.  No characters are available
    --    until the newline character is available or the buffer is full.
    --
    --  * /block-buffering/: when the buffer for the handle becomes empty,
    --    the next block of data is read into the buffer.
    --
    --  * /no-buffering/: the next input item is read and returned.
    --    The 'System.IO.hLookAhead' operation implies that even a no-buffered
    --    handle may require a one-character buffer.
    --
    -- The default buffering mode when a handle is opened is
    -- implementation-dependent and may depend on the file system object
    -- which is attached to that handle.
    -- For most implementations, physical files will normally be block-buffered
    -- and terminals will normally be line-buffered.

    data BufferMode
     = NoBuffering	-- ^ buffering is disabled if possible.
     | LineBuffering
    		-- ^ line-buffering should be enabled if possible.
     | BlockBuffering (Maybe Int)
    		-- ^ block-buffering should be enabled if possible.
    		-- The size of the buffer is @n@ items if the argument
    		-- is 'Just' @n@ and is otherwise implementation-dependent.
       deriving (Eq, Ord, Read, Show)

    -- ---------------------------------------------------------------------------
    -- IORefs

    -- |A mutable variable in the 'IO' monad
    newtype IORef a = IORef (STRef RealWorld a)

    -- explicit instance because Haddock can't figure out a derived one
    instance Eq (IORef a) where
      IORef x == IORef y = x == y

    -- |Build a new 'IORef'
    newIORef    :: a -> IO (IORef a)
    newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)

    -- |Read the value of an 'IORef'
    readIORef   :: IORef a -> IO a
    readIORef  (IORef var) = stToIO (readSTRef var)

    -- |Write a new value into an 'IORef'
    writeIORef  :: IORef a -> a -> IO ()
    writeIORef (IORef var) v = stToIO (writeSTRef var v)

    -- ---------------------------------------------------------------------------
    -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
    -- The type arguments are as follows:
    --
    --  * @i@: the index type of the array (should be an instance of 'Ix')
    --
    --  * @e@: the element type of the array.
    --
    --

    newtype IOArray i e = IOArray (STArray RealWorld i e)

    -- explicit instance because Haddock can't figure out a derived one
    instance Eq (IOArray i e) where
      IOArray x == IOArray y = x == y

    -- |Build a new 'IOArray'
    newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
    {-# INLINE newIOArray #-}
    newIOArray lu init  = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}

    -- | Read a value from an 'IOArray'
    unsafeReadIOArray  :: Ix i => IOArray i e -> Int -> IO e
    {-# INLINE unsafeReadIOArray #-}
    unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)

    -- | Write a new value into an 'IOArray'
    unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
    {-# INLINE unsafeWriteIOArray #-}
    unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)

    -- | Read a value from an 'IOArray'
    readIOArray  :: Ix i => IOArray i e -> i -> IO e
    readIOArray (IOArray marr) i = stToIO (readSTArray marr i)

    -- | Write a new value into an 'IOArray'
    writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
    writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)

    -- ---------------------------------------------------------------------------
    -- Show instance for Handles

    -- handle types are 'show'n when printing error msgs, so
    -- we provide a more user-friendly Show instance for it
    -- than the derived one.

    instance Show HandleType where
      showsPrec p t =
        case t of
          ClosedHandle      -> showString "closed"
          SemiClosedHandle  -> showString "semi-closed"
          ReadHandle        -> showString "readable"
          WriteHandle       -> showString "writable"
          AppendHandle      -> showString "writable (append)"
          ReadWriteHandle   -> showString "read-writable"

    instance Show Handle where
      showsPrec p (FileHandle   file _)   = showHandle file
      showsPrec p (DuplexHandle file _ _) = showHandle file

    showHandle file = showString "{handle: " . showString file . showString "}"

    -- ------------------------------------------------------------------------
    -- Exception datatype and operations

    -- |The type of exceptions.  Every kind of system-generated exception
    -- has a constructor in the 'Exception' type, and values of other
    -- types may be injected into 'Exception' by coercing them to
    -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
    -- "Control.Exception\#DynamicExceptions").
    data Exception
      = ArithException  	ArithException
    	-- ^Exceptions raised by arithmetic
    	-- operations.  (NOTE: GHC currently does not throw
    	-- 'ArithException's except for 'DivideByZero').
      | ArrayException	ArrayException
    	-- ^Exceptions raised by array-related
    	-- operations.  (NOTE: GHC currently does not throw
    	-- 'ArrayException's).
      | AssertionFailed	String
    	-- ^This exception is thrown by the
    	-- 'assert' operation when the condition
    	-- fails.  The 'String' argument contains the
    	-- location of the assertion in the source program.
      | AsyncException	AsyncException
    	-- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
      | BlockedOnDeadMVar
    	-- ^The current thread was executing a call to
    	-- 'Control.Concurrent.MVar.takeMVar' that could never return,
    	-- because there are no other references to this 'MVar'.
      | BlockedIndefinitely
    	-- ^The current thread was waiting to retry an atomic memory transaction
    	-- that could never become possible to complete because there are no other
    	-- threads referring to any of the TVars involved.
      | NestedAtomically
    	-- ^The runtime detected an attempt to nest one STM transaction
    	-- inside another one, presumably due to the use of
    	-- 'unsafePeformIO' with 'atomically'.
      | Deadlock
    	-- ^There are no runnable threads, so the program is
    	-- deadlocked.  The 'Deadlock' exception is
    	-- raised in the main thread only (see also: "Control.Concurrent").
      | DynException	Dynamic
    	-- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
      | ErrorCall		String
    	-- ^The 'ErrorCall' exception is thrown by 'error'.  The 'String'
    	-- argument of 'ErrorCall' is the string passed to 'error' when it was
    	-- called.
      | ExitException	ExitCode
    	-- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
    	-- 'System.Exit.exitFailure').  The 'ExitCode' argument is the value passed
    	-- to 'System.Exit.exitWith'.  An unhandled 'ExitException' exception in the
    	-- main thread will cause the program to be terminated with the given
    	-- exit code.
      | IOException 	IOException
    	-- ^These are the standard IO exceptions generated by
      	-- Haskell\'s @IO@ operations.  See also "System.IO.Error".
      | NoMethodError       String
    	-- ^An attempt was made to invoke a class method which has
    	-- no definition in this instance, and there was no default
    	-- definition given in the class declaration.  GHC issues a
    	-- warning when you compile an instance which has missing
    	-- methods.
      | NonTermination
    	-- ^The current thread is stuck in an infinite loop.  This
    	-- exception may or may not be thrown when the program is
    	-- non-terminating.
      | PatternMatchFail	String
    	-- ^A pattern matching failure.  The 'String' argument should contain a
    	-- descriptive message including the function name, source file
    	-- and line number.
      | RecConError		String
    	-- ^An attempt was made to evaluate a field of a record
    	-- for which no value was given at construction time.  The
    	-- 'String' argument gives the location of the
    	-- record construction in the source program.
      | RecSelError		String
    	-- ^A field selection was attempted on a constructor that
    	-- doesn\'t have the requested field.  This can happen with
    	-- multi-constructor records when one or more fields are
    	-- missing from some of the constructors.  The
    	-- 'String' argument gives the location of the
    	-- record selection in the source program.
      | RecUpdError		String
    	-- ^An attempt was made to update a field in a record,
    	-- where the record doesn\'t have the requested field.  This can
    	-- only occur with multi-constructor records, when one or more
    	-- fields are missing from some of the constructors.  The
    	-- 'String' argument gives the location of the
    	-- record update in the source program.

    -- |The type of arithmetic exceptions
    data ArithException
      = Overflow
      | Underflow
      | LossOfPrecision
      | DivideByZero
      | Denormal
      deriving (Eq, Ord)

    -- |Asynchronous exceptions
    data AsyncException
      = StackOverflow
    	-- ^The current thread\'s stack exceeded its limit.
    	-- Since an exception has been raised, the thread\'s stack
    	-- will certainly be below its limit again, but the
    	-- programmer should take remedial action
    	-- immediately.
      | HeapOverflow
    	-- ^The program\'s heap is reaching its limit, and
    	-- the program should take action to reduce the amount of
    	-- live data it has. Notes:
    	--
    	-- 	* It is undefined which thread receives this exception.
    	--
    	-- 	* GHC currently does not throw 'HeapOverflow' exceptions.
      | ThreadKilled
    	-- ^This exception is raised by another thread
    	-- calling 'Control.Concurrent.killThread', or by the system
    	-- if it needs to terminate the thread for some
    	-- reason.
      deriving (Eq, Ord)

    -- | Exceptions generated by array operations
    data ArrayException
      = IndexOutOfBounds  	String
    	-- ^An attempt was made to index an array outside
    	-- its declared bounds.
      | UndefinedElement	String
    	-- ^An attempt was made to evaluate an element of an
    	-- array that had not been initialized.
      deriving (Eq, Ord)

    stackOverflow, heapOverflow :: Exception -- for the RTS
    stackOverflow = AsyncException StackOverflow
    heapOverflow  = AsyncException HeapOverflow

    instance Show ArithException where
      showsPrec _ Overflow        = showString "arithmetic overflow"
      showsPrec _ Underflow       = showString "arithmetic underflow"
      showsPrec _ LossOfPrecision = showString "loss of precision"
      showsPrec _ DivideByZero    = showString "divide by zero"
      showsPrec _ Denormal        = showString "denormal"

    instance Show AsyncException where
      showsPrec _ StackOverflow   = showString "stack overflow"
      showsPrec _ HeapOverflow    = showString "heap overflow"
      showsPrec _ ThreadKilled    = showString "thread killed"

    instance Show ArrayException where
      showsPrec _ (IndexOutOfBounds s)
    	= showString "array index out of range"
    	. (if not (null s) then showString ": " . showString s
    			   else id)
      showsPrec _ (UndefinedElement s)
    	= showString "undefined array element"
    	. (if not (null s) then showString ": " . showString s
    			   else id)

    instance Show Exception where
      showsPrec _ (IOException err)	         = shows err
      showsPrec _ (ArithException err)       = shows err
      showsPrec _ (ArrayException err)       = shows err
      showsPrec _ (ErrorCall err)	         = showString err
      showsPrec _ (ExitException err)        = showString "exit: " . shows err
      showsPrec _ (NoMethodError err)        = showString err
      showsPrec _ (PatternMatchFail err)     = showString err
      showsPrec _ (RecSelError err)	         = showString err
      showsPrec _ (RecConError err)	         = showString err
      showsPrec _ (RecUpdError err)	         = showString err
      showsPrec _ (AssertionFailed err)      = showString err
      showsPrec _ (DynException err)         = showString "exception :: " . showsTypeRep (dynTypeRep err)
      showsPrec _ (AsyncException e)	 = shows e
      showsPrec _ (BlockedOnDeadMVar)	 = showString "thread blocked indefinitely"
      showsPrec _ (BlockedIndefinitely)	 = showString "thread blocked indefinitely"
      showsPrec _ (NestedAtomically)	 = showString "Control.Concurrent.STM.atomically was nested"
      showsPrec _ (NonTermination)           = showString "<<loop>>"
      showsPrec _ (Deadlock)                 = showString "<<deadlock>>"

    instance Eq Exception where
      IOException e1      == IOException e2      = e1 == e2
      ArithException e1   == ArithException e2   = e1 == e2
      ArrayException e1   == ArrayException e2   = e1 == e2
      ErrorCall e1        == ErrorCall e2	     = e1 == e2
      ExitException	e1    == ExitException e2    = e1 == e2
      NoMethodError e1    == NoMethodError e2    = e1 == e2
      PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
      RecSelError e1      == RecSelError e2      = e1 == e2
      RecConError e1      == RecConError e2      = e1 == e2
      RecUpdError e1      == RecUpdError e2      = e1 == e2
      AssertionFailed e1  == AssertionFailed e2  = e1 == e2
      DynException _      == DynException _      = False -- incomparable
      AsyncException e1   == AsyncException e2   = e1 == e2
      BlockedOnDeadMVar   == BlockedOnDeadMVar   = True
      NonTermination      == NonTermination      = True
      NestedAtomically    == NestedAtomically    = True
      Deadlock            == Deadlock            = True
      _                   == _                   = False

    -- -----------------------------------------------------------------------------
    -- The ExitCode type

    -- We need it here because it is used in ExitException in the
    -- Exception datatype (above).

    data ExitCode
      = ExitSuccess	-- ^ indicates successful termination;
      | ExitFailure Int
    		-- ^ indicates program failure with an exit code.
    		-- The exact interpretation of the code is
    		-- operating-system dependent.	In particular, some values
    		-- may be prohibited (e.g. 0 on a POSIX-compliant system).
      deriving (Eq, Ord, Read, Show)

    -- --------------------------------------------------------------------------
    -- Primitive throw

    -- | Throw an exception.  Exceptions may be thrown from purely
    -- functional code, but may only be caught within the 'IO' monad.
    throw :: Exception -> a
    throw exception = raise# exception

    -- | A variant of 'throw' that can be used within the 'IO' monad.
    --
    -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
    -- two functions are subtly different:
    --
    -- > throw e   `seq` x  ===> throw e
    -- > throwIO e `seq` x  ===> x
    --
    -- The first example will cause the exception @e@ to be raised,
    -- whereas the second one won\'t.  In fact, 'throwIO' will only cause
    -- an exception to be raised when it is used within the 'IO' monad.
    -- The 'throwIO' variant should be used in preference to 'throw' to
    -- raise an exception within the 'IO' monad because it guarantees
    -- ordering with respect to other 'IO' operations, whereas 'throw'
    -- does not.
    throwIO         :: Exception -> IO a
    throwIO err	=  IO $ raiseIO# err

    ioException	:: IOException -> IO a
    ioException err =  IO $ raiseIO# (IOException err)

    -- | Raise an 'IOError' in the 'IO' monad.
    ioError         :: IOError -> IO a
    ioError		=  ioException

    -- ---------------------------------------------------------------------------
    -- IOError type

    -- | The Haskell 98 type for exceptions in the 'IO' monad.
    -- Any I\/O operation may raise an 'IOError' instead of returning a result.
    -- For a more general type of exception, including also those that arise
    -- in pure code, see 'Control.Exception.Exception'.
    --
    -- In Haskell 98, this is an opaque type.
    type IOError = IOException

    -- |Exceptions that occur in the @IO@ monad.
    -- An @IOException@ records a more specific error type, a descriptive
    -- string and maybe the handle that was used when the error was
    -- flagged.
    data IOException
     = IOError {
         ioe_handle   :: Maybe Handle,   -- the handle used by the action flagging
    				     -- the error.
         ioe_type     :: IOErrorType,    -- what it was.
         ioe_location :: String,	     -- location.
         ioe_description :: String,      -- error type specific information.
         ioe_filename :: Maybe FilePath  -- filename the error is related to.
       }

    instance Eq IOException where
      (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
        e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2

    -- | An abstract type that contains a value for each variant of 'IOError'.
    data IOErrorType
      -- Haskell 98:
      = AlreadyExists
      | NoSuchThing
      | ResourceBusy
      | ResourceExhausted
      | EOF
      | IllegalOperation
      | PermissionDenied
      | UserError
      -- GHC only:
      | UnsatisfiedConstraints
      | SystemError
      | ProtocolError
      | OtherError
      | InvalidArgument
      | InappropriateType
      | HardwareFault
      | UnsupportedOperation
      | TimeExpired
      | ResourceVanished
      | Interrupted
      | DynIOError Dynamic -- cheap&cheerful extensible IO error type.

    instance Eq IOErrorType where
       x == y =
         case x of
           DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
           _ -> getTag x ==# getTag y

    instance Show IOErrorType where
      showsPrec _ e =
        showString $
        case e of
          AlreadyExists	-> "already exists"
          NoSuchThing       -> "does not exist"
          ResourceBusy      -> "resource busy"
          ResourceExhausted -> "resource exhausted"
          EOF		-> "end of file"
          IllegalOperation	-> "illegal operation"
          PermissionDenied  -> "permission denied"
          UserError		-> "user error"
          HardwareFault	-> "hardware fault"
          InappropriateType -> "inappropriate type"
          Interrupted       -> "interrupted"
          InvalidArgument   -> "invalid argument"
          OtherError        -> "failed"
          ProtocolError     -> "protocol error"
          ResourceVanished  -> "resource vanished"
          SystemError	-> "system error"
          TimeExpired       -> "timeout"
          UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
          UnsupportedOperation -> "unsupported operation"
          DynIOError{}      -> "unknown IO error"

    -- | Construct an 'IOError' value with a string describing the error.
    -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
    -- 'userError', thus:
    --
    -- > instance Monad IO where
    -- >   ...
    -- >   fail s = ioError (userError s)
    --
    userError       :: String  -> IOError
    userError str	=  IOError Nothing UserError "" str Nothing

    -- ---------------------------------------------------------------------------
    -- Showing IOErrors

    instance Show IOException where
        showsPrec p (IOError hdl iot loc s fn) =
          (case fn of
    	 Nothing -> case hdl of
    		        Nothing -> id
    			Just h  -> showsPrec p h . showString ": "
    	 Just name -> showString name . showString ": ") .
          (case loc of
             "" -> id
    	 _  -> showString loc . showString ": ") .
          showsPrec p iot .
          (case s of
    	 "" -> id
    	 _  -> showString " (" . showString s . showString ")")

    -- -----------------------------------------------------------------------------
    -- IOMode type

    data IOMode      =  ReadMode | WriteMode | AppendMode | ReadWriteMode
                        deriving (Eq, Ord, Ix, Enum, Read, Show)
    \end{code}