2 {-# OPTIONS_GHC -XNoImplicitPrelude -funbox-strict-fields #-}
3 {-# OPTIONS_HADDOCK hide #-}
4 -----------------------------------------------------------------------------
7 -- Copyright : (c) The University of Glasgow 1994-2002
8 -- License : see libraries/base/LICENSE
10 -- Maintainer : cvs-ghc@haskell.org
11 -- Stability : internal
12 -- Portability : non-portable (GHC Extensions)
14 -- Definitions for the 'IO' monad and its friends.
16 -----------------------------------------------------------------------------
20 IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
21 unsafePerformIO, unsafeInterleaveIO,
22 unsafeDupablePerformIO, unsafeDupableInterleaveIO,
25 -- To and from from ST
26 stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
29 IORef(..), newIORef, readIORef, writeIORef,
30 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
33 -- Handles, file descriptors,
35 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
36 isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
39 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
40 bufferIsWritable, bufferEmpty, bufferFull,
43 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
44 stackOverflow, heapOverflow, ioException,
45 IOError, IOException(..), IOErrorType(..), ioError, userError,
47 throwIO, block, unblock, blocked, catchAny, catchException,
49 ErrorCall(..), AssertionFailed(..), assertError, untangle,
50 BlockedOnDeadMVar(..), BlockedIndefinitely(..), Deadlock(..),
51 blockedOnDeadMVar, blockedIndefinitely
55 import GHC.Arr -- to derive Ix class
56 import GHC.Enum -- to derive Enum class
59 -- import GHC.Num -- To get fromInteger etc, needed because of -XNoImplicitPrelude
60 import Data.Maybe ( Maybe(..) )
64 import Foreign.C.Types (CInt)
68 import {-# SOURCE #-} Data.Typeable ( Typeable )
71 -- ---------------------------------------------------------------------------
75 The IO Monad is just an instance of the ST monad, where the state is
76 the real world. We use the exception mechanism (in GHC.Exception) to
77 implement IO exceptions.
79 NOTE: The IO representation is deeply wired in to various parts of the
80 system. The following list may or may not be exhaustive:
82 Compiler - types of various primitives in PrimOp.lhs
84 RTS - forceIO (StgMiscClosures.hc)
85 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
87 - raiseAsync (Schedule.c)
89 Prelude - GHC.IOBase.lhs, and several other places including
92 Libraries - parts of hslibs/lang.
98 A value of type @'IO' a@ is a computation which, when performed,
99 does some I\/O before returning a value of type @a@.
101 There is really only one way to \"perform\" an I\/O action: bind it to
102 @Main.main@ in your program. When your program is run, the I\/O will
103 be performed. It isn't possible to perform I\/O from an arbitrary
104 function, unless that function is itself in the 'IO' monad and called
105 at some point, directly or indirectly, from @Main.main@.
107 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
108 or the '>>' and '>>=' operations from the 'Monad' class.
110 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
112 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
115 instance Functor IO where
116 fmap f x = x >>= (return . f)
118 instance Monad IO where
119 {-# INLINE return #-}
122 m >> k = m >>= \ _ -> k
123 return x = returnIO x
128 failIO :: String -> IO a
129 failIO s = ioError (userError s)
131 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
132 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
134 bindIO :: IO a -> (a -> IO b) -> IO b
135 bindIO (IO m) k = IO ( \ s ->
137 (# new_s, a #) -> unIO (k a) new_s
140 thenIO :: IO a -> IO b -> IO b
141 thenIO (IO m) k = IO ( \ s ->
143 (# new_s, _ #) -> unIO k new_s
146 returnIO :: a -> IO a
147 returnIO x = IO (\ s -> (# s, x #))
149 -- ---------------------------------------------------------------------------
150 -- Coercions between IO and ST
152 -- | A monad transformer embedding strict state transformers in the 'IO'
153 -- monad. The 'RealWorld' parameter indicates that the internal state
154 -- used by the 'ST' computation is a special one supplied by the 'IO'
155 -- monad, and thus distinct from those used by invocations of 'runST'.
156 stToIO :: ST RealWorld a -> IO a
159 ioToST :: IO a -> ST RealWorld a
160 ioToST (IO m) = (ST m)
162 -- This relies on IO and ST having the same representation modulo the
163 -- constraint on the type of the state
165 unsafeIOToST :: IO a -> ST s a
166 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
168 unsafeSTToIO :: ST s a -> IO a
169 unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
171 -- ---------------------------------------------------------------------------
172 -- Unsafe IO operations
175 This is the \"back door\" into the 'IO' monad, allowing
176 'IO' computation to be performed at any time. For
177 this to be safe, the 'IO' computation should be
178 free of side effects and independent of its environment.
180 If the I\/O computation wrapped in 'unsafePerformIO'
181 performs side effects, then the relative order in which those side
182 effects take place (relative to the main I\/O trunk, or other calls to
183 'unsafePerformIO') is indeterminate. You have to be careful when
184 writing and compiling modules that use 'unsafePerformIO':
186 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
187 that calls 'unsafePerformIO'. If the call is inlined,
188 the I\/O may be performed more than once.
190 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
191 elimination being performed on the module, which might combine
192 two side effects that were meant to be separate. A good example
193 is using multiple global variables (like @test@ in the example below).
195 * Make sure that the either you switch off let-floating, or that the
196 call to 'unsafePerformIO' cannot float outside a lambda. For example,
199 f x = unsafePerformIO (newIORef [])
201 you may get only one reference cell shared between all calls to @f@.
204 f x = unsafePerformIO (newIORef [x])
206 because now it can't float outside the lambda.
208 It is less well known that
209 'unsafePerformIO' is not type safe. For example:
212 > test = unsafePerformIO $ newIORef []
215 > writeIORef test [42]
216 > bang <- readIORef test
217 > print (bang :: [Char])
219 This program will core dump. This problem with polymorphic references
220 is well known in the ML community, and does not arise with normal
221 monadic use of references. There is no easy way to make it impossible
222 once you use 'unsafePerformIO'. Indeed, it is
223 possible to write @coerce :: a -> b@ with the
224 help of 'unsafePerformIO'. So be careful!
226 unsafePerformIO :: IO a -> a
227 unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)
230 This version of 'unsafePerformIO' is slightly more efficient,
231 because it omits the check that the IO is only being performed by a
232 single thread. Hence, when you write 'unsafeDupablePerformIO',
233 there is a possibility that the IO action may be performed multiple
234 times (on a multiprocessor), and you should therefore ensure that
235 it gives the same results each time.
237 {-# NOINLINE unsafeDupablePerformIO #-}
238 unsafeDupablePerformIO :: IO a -> a
239 unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
241 -- Why do we NOINLINE unsafeDupablePerformIO? See the comment with
242 -- GHC.ST.runST. Essentially the issue is that the IO computation
243 -- inside unsafePerformIO must be atomic: it must either all run, or
244 -- not at all. If we let the compiler see the application of the IO
245 -- to realWorld#, it might float out part of the IO.
247 -- Why is there a call to 'lazy' in unsafeDupablePerformIO?
248 -- If we don't have it, the demand analyser discovers the following strictness
249 -- for unsafeDupablePerformIO: C(U(AV))
251 -- unsafeDupablePerformIO (\s -> let r = f x in
252 -- case writeIORef v r s of (# s1, _ #) ->
254 -- The strictness analyser will find that the binding for r is strict,
255 -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
256 -- doing the writeIORef. This actually makes tests/lib/should_run/memo002
259 -- Solution: don't expose the strictness of unsafeDupablePerformIO,
260 -- by hiding it with 'lazy'
263 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
264 When passed a value of type @IO a@, the 'IO' will only be performed
265 when the value of the @a@ is demanded. This is used to implement lazy
266 file reading, see 'System.IO.hGetContents'.
268 {-# INLINE unsafeInterleaveIO #-}
269 unsafeInterleaveIO :: IO a -> IO a
270 unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
272 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
273 -- state from this IO thread is passed explicitly to the interleaved
274 -- IO, so it cannot be floated out and shared.
276 {-# INLINE unsafeDupableInterleaveIO #-}
277 unsafeDupableInterleaveIO :: IO a -> IO a
278 unsafeDupableInterleaveIO (IO m)
280 r = case m s of (# _, res #) -> res
285 Ensures that the suspensions under evaluation by the current thread
286 are unique; that is, the current thread is not evaluating anything
287 that is also under evaluation by another thread that has also executed
290 This operation is used in the definition of 'unsafePerformIO' to
291 prevent the IO action from being executed multiple times, which is usually
295 noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)
297 -- ---------------------------------------------------------------------------
300 data MVar a = MVar (MVar# RealWorld a)
302 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
303 for communication between concurrent threads. It can be thought of
304 as a a box, which may be empty or full.
307 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
308 instance Eq (MVar a) where
309 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
311 -- A Handle is represented by (a reference to) a record
312 -- containing the state of the I/O port/device. We record
313 -- the following pieces of info:
315 -- * type (read,write,closed etc.)
316 -- * the underlying file descriptor
318 -- * buffer, and spare buffers
319 -- * user-friendly name (usually the
320 -- FilePath used when IO.openFile was called)
322 -- Note: when a Handle is garbage collected, we want to flush its buffer
323 -- and close the OS file handle, so as to free up a (precious) resource.
325 -- | Haskell defines operations to read and write characters from and to files,
326 -- represented by values of type @Handle@. Each value of this type is a
327 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
328 -- with file system objects. A handle has at least the following properties:
330 -- * whether it manages input or output or both;
332 -- * whether it is /open/, /closed/ or /semi-closed/;
334 -- * whether the object is seekable;
336 -- * whether buffering is disabled, or enabled on a line or block basis;
338 -- * a buffer (whose length may be zero).
340 -- Most handles will also have a current I\/O position indicating where the next
341 -- input or output operation will occur. A handle is /readable/ if it
342 -- manages only input or both input and output; likewise, it is /writable/ if
343 -- it manages only output or both input and output. A handle is /open/ when
345 -- Once it is closed it can no longer be used for either input or output,
346 -- though an implementation cannot re-use its storage while references
347 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
348 -- produced by showing a handle is system dependent; it should include
349 -- enough information to identify the handle for debugging. A handle is
350 -- equal according to '==' only to itself; no attempt
351 -- is made to compare the internal state of different handles for equality.
353 -- GHC note: a 'Handle' will be automatically closed when the garbage
354 -- collector detects that it has become unreferenced by the program.
355 -- However, relying on this behaviour is not generally recommended:
356 -- the garbage collector is unpredictable. If possible, use explicit
357 -- an explicit 'hClose' to close 'Handle's when they are no longer
358 -- required. GHC does not currently attempt to free up file
359 -- descriptors when they have run out, it is your responsibility to
360 -- ensure that this doesn't happen.
363 = FileHandle -- A normal handle to a file
364 FilePath -- the file (invariant)
367 | DuplexHandle -- A handle to a read/write stream
368 FilePath -- file for a FIFO, otherwise some
369 -- descriptive string.
370 !(MVar Handle__) -- The read side
371 !(MVar Handle__) -- The write side
374 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
377 instance Eq Handle where
378 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
379 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
386 haFD :: !FD, -- file descriptor
387 haType :: HandleType, -- type (read/write/append etc.)
388 haIsBin :: Bool, -- binary mode?
389 haIsStream :: Bool, -- Windows : is this a socket?
390 -- Unix : is O_NONBLOCK set?
391 haBufferMode :: BufferMode, -- buffer contains read/write data?
392 haBuffer :: !(IORef Buffer), -- the current buffer
393 haBuffers :: !(IORef BufferList), -- spare buffers
394 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
398 -- ---------------------------------------------------------------------------
401 -- The buffer is represented by a mutable variable containing a
402 -- record, where the record contains the raw buffer and the start/end
403 -- points of the filled portion. We use a mutable variable so that
404 -- the common operation of writing (or reading) some data from (to)
405 -- the buffer doesn't need to modify, and hence copy, the handle
406 -- itself, it just updates the buffer.
408 -- There will be some allocation involved in a simple hPutChar in
409 -- order to create the new Buffer structure (below), but this is
410 -- relatively small, and this only has to be done once per write
413 -- The buffer contains its size - we could also get the size by
414 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
415 -- to be rounded up to the nearest Word.
417 type RawBuffer = MutableByteArray# RealWorld
419 -- INVARIANTS on a Buffer:
421 -- * A handle *always* has a buffer, even if it is only 1 character long
422 -- (an unbuffered handle needs a 1 character buffer in order to support
423 -- hLookAhead and hIsEOF).
425 -- * if r == w, then r == 0 && w == 0
426 -- * if state == WriteBuffer, then r == 0
427 -- * a write buffer is never full. If an operation
428 -- fills up the buffer, it will always flush it before
430 -- * a read buffer may be full as a result of hLookAhead. In normal
431 -- operation, a read buffer always has at least one character of space.
439 bufState :: BufferState
442 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
444 -- we keep a few spare buffers around in a handle to avoid allocating
445 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
446 -- same size as the main buffer.
449 | BufferListCons RawBuffer BufferList
452 bufferIsWritable :: Buffer -> Bool
453 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
454 bufferIsWritable _other = False
456 bufferEmpty :: Buffer -> Bool
457 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
459 -- only makes sense for a write buffer
460 bufferFull :: Buffer -> Bool
461 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
463 -- Internally, we classify handles as being one
474 isReadableHandleType :: HandleType -> Bool
475 isReadableHandleType ReadHandle = True
476 isReadableHandleType ReadWriteHandle = True
477 isReadableHandleType _ = False
479 isWritableHandleType :: HandleType -> Bool
480 isWritableHandleType AppendHandle = True
481 isWritableHandleType WriteHandle = True
482 isWritableHandleType ReadWriteHandle = True
483 isWritableHandleType _ = False
485 isReadWriteHandleType :: HandleType -> Bool
486 isReadWriteHandleType ReadWriteHandle{} = True
487 isReadWriteHandleType _ = False
489 -- | File and directory names are values of type 'String', whose precise
490 -- meaning is operating system dependent. Files can be opened, yielding a
491 -- handle which can then be used to operate on the contents of that file.
493 type FilePath = String
495 -- ---------------------------------------------------------------------------
498 -- | Three kinds of buffering are supported: line-buffering,
499 -- block-buffering or no-buffering. These modes have the following
500 -- effects. For output, items are written out, or /flushed/,
501 -- from the internal buffer according to the buffer mode:
503 -- * /line-buffering/: the entire output buffer is flushed
504 -- whenever a newline is output, the buffer overflows,
505 -- a 'System.IO.hFlush' is issued, or the handle is closed.
507 -- * /block-buffering/: the entire buffer is written out whenever it
508 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
510 -- * /no-buffering/: output is written immediately, and never stored
513 -- An implementation is free to flush the buffer more frequently,
514 -- but not less frequently, than specified above.
515 -- The output buffer is emptied as soon as it has been written out.
517 -- Similarly, input occurs according to the buffer mode for the handle:
519 -- * /line-buffering/: when the buffer for the handle is not empty,
520 -- the next item is obtained from the buffer; otherwise, when the
521 -- buffer is empty, characters up to and including the next newline
522 -- character are read into the buffer. No characters are available
523 -- until the newline character is available or the buffer is full.
525 -- * /block-buffering/: when the buffer for the handle becomes empty,
526 -- the next block of data is read into the buffer.
528 -- * /no-buffering/: the next input item is read and returned.
529 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
530 -- handle may require a one-character buffer.
532 -- The default buffering mode when a handle is opened is
533 -- implementation-dependent and may depend on the file system object
534 -- which is attached to that handle.
535 -- For most implementations, physical files will normally be block-buffered
536 -- and terminals will normally be line-buffered.
539 = NoBuffering -- ^ buffering is disabled if possible.
541 -- ^ line-buffering should be enabled if possible.
542 | BlockBuffering (Maybe Int)
543 -- ^ block-buffering should be enabled if possible.
544 -- The size of the buffer is @n@ items if the argument
545 -- is 'Just' @n@ and is otherwise implementation-dependent.
546 deriving (Eq, Ord, Read, Show)
548 -- ---------------------------------------------------------------------------
551 -- |A mutable variable in the 'IO' monad
552 newtype IORef a = IORef (STRef RealWorld a)
554 -- explicit instance because Haddock can't figure out a derived one
555 instance Eq (IORef a) where
556 IORef x == IORef y = x == y
558 -- |Build a new 'IORef'
559 newIORef :: a -> IO (IORef a)
560 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
562 -- |Read the value of an 'IORef'
563 readIORef :: IORef a -> IO a
564 readIORef (IORef var) = stToIO (readSTRef var)
566 -- |Write a new value into an 'IORef'
567 writeIORef :: IORef a -> a -> IO ()
568 writeIORef (IORef var) v = stToIO (writeSTRef var v)
570 -- ---------------------------------------------------------------------------
571 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
572 -- The type arguments are as follows:
574 -- * @i@: the index type of the array (should be an instance of 'Ix')
576 -- * @e@: the element type of the array.
580 newtype IOArray i e = IOArray (STArray RealWorld i e)
582 -- explicit instance because Haddock can't figure out a derived one
583 instance Eq (IOArray i e) where
584 IOArray x == IOArray y = x == y
586 -- |Build a new 'IOArray'
587 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
588 {-# INLINE newIOArray #-}
589 newIOArray lu initial = stToIO $ do {marr <- newSTArray lu initial; return (IOArray marr)}
591 -- | Read a value from an 'IOArray'
592 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
593 {-# INLINE unsafeReadIOArray #-}
594 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
596 -- | Write a new value into an 'IOArray'
597 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
598 {-# INLINE unsafeWriteIOArray #-}
599 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
601 -- | Read a value from an 'IOArray'
602 readIOArray :: Ix i => IOArray i e -> i -> IO e
603 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
605 -- | Write a new value into an 'IOArray'
606 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
607 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
610 -- ---------------------------------------------------------------------------
611 -- Show instance for Handles
613 -- handle types are 'show'n when printing error msgs, so
614 -- we provide a more user-friendly Show instance for it
615 -- than the derived one.
617 instance Show HandleType where
620 ClosedHandle -> showString "closed"
621 SemiClosedHandle -> showString "semi-closed"
622 ReadHandle -> showString "readable"
623 WriteHandle -> showString "writable"
624 AppendHandle -> showString "writable (append)"
625 ReadWriteHandle -> showString "read-writable"
627 instance Show Handle where
628 showsPrec _ (FileHandle file _) = showHandle file
629 showsPrec _ (DuplexHandle file _ _) = showHandle file
631 showHandle :: FilePath -> String -> String
632 showHandle file = showString "{handle: " . showString file . showString "}"
634 -- ------------------------------------------------------------------------
635 -- Exception datatypes and operations
637 -- |The thread is blocked on an @MVar@, but there are no other references
638 -- to the @MVar@ so it can't ever continue.
639 data BlockedOnDeadMVar = BlockedOnDeadMVar
642 instance Exception BlockedOnDeadMVar
644 instance Show BlockedOnDeadMVar where
645 showsPrec _ BlockedOnDeadMVar = showString "thread blocked indefinitely"
647 blockedOnDeadMVar :: SomeException -- for the RTS
648 blockedOnDeadMVar = toException BlockedOnDeadMVar
652 -- |The thread is awiting to retry an STM transaction, but there are no
653 -- other references to any @TVar@s involved, so it can't ever continue.
654 data BlockedIndefinitely = BlockedIndefinitely
657 instance Exception BlockedIndefinitely
659 instance Show BlockedIndefinitely where
660 showsPrec _ BlockedIndefinitely = showString "thread blocked indefinitely"
662 blockedIndefinitely :: SomeException -- for the RTS
663 blockedIndefinitely = toException BlockedIndefinitely
667 -- |There are no runnable threads, so the program is deadlocked.
668 -- The @Deadlock@ exception is raised in the main thread only.
669 data Deadlock = Deadlock
672 instance Exception Deadlock
674 instance Show Deadlock where
675 showsPrec _ Deadlock = showString "<<deadlock>>"
679 -- |Exceptions generated by 'assert'. The @String@ gives information
680 -- about the source location of the assertion.
681 data AssertionFailed = AssertionFailed String
684 instance Exception AssertionFailed
686 instance Show AssertionFailed where
687 showsPrec _ (AssertionFailed err) = showString err
691 -- |Asynchronous exceptions.
694 -- ^The current thread\'s stack exceeded its limit.
695 -- Since an exception has been raised, the thread\'s stack
696 -- will certainly be below its limit again, but the
697 -- programmer should take remedial action
700 -- ^The program\'s heap is reaching its limit, and
701 -- the program should take action to reduce the amount of
702 -- live data it has. Notes:
704 -- * It is undefined which thread receives this exception.
706 -- * GHC currently does not throw 'HeapOverflow' exceptions.
708 -- ^This exception is raised by another thread
709 -- calling 'Control.Concurrent.killThread', or by the system
710 -- if it needs to terminate the thread for some
713 -- ^This exception is raised by default in the main thread of
714 -- the program when the user requests to terminate the program
715 -- via the usual mechanism(s) (e.g. Control-C in the console).
716 deriving (Eq, Ord, Typeable)
718 instance Exception AsyncException
720 -- | Exceptions generated by array operations
722 = IndexOutOfBounds String
723 -- ^An attempt was made to index an array outside
724 -- its declared bounds.
725 | UndefinedElement String
726 -- ^An attempt was made to evaluate an element of an
727 -- array that had not been initialized.
728 deriving (Eq, Ord, Typeable)
730 instance Exception ArrayException
732 stackOverflow, heapOverflow :: SomeException -- for the RTS
733 stackOverflow = toException StackOverflow
734 heapOverflow = toException HeapOverflow
736 instance Show AsyncException where
737 showsPrec _ StackOverflow = showString "stack overflow"
738 showsPrec _ HeapOverflow = showString "heap overflow"
739 showsPrec _ ThreadKilled = showString "thread killed"
740 showsPrec _ UserInterrupt = showString "user interrupt"
742 instance Show ArrayException where
743 showsPrec _ (IndexOutOfBounds s)
744 = showString "array index out of range"
745 . (if not (null s) then showString ": " . showString s
747 showsPrec _ (UndefinedElement s)
748 = showString "undefined array element"
749 . (if not (null s) then showString ": " . showString s
752 -- -----------------------------------------------------------------------------
755 -- We need it here because it is used in ExitException in the
756 -- Exception datatype (above).
759 = ExitSuccess -- ^ indicates successful termination;
761 -- ^ indicates program failure with an exit code.
762 -- The exact interpretation of the code is
763 -- operating-system dependent. In particular, some values
764 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
765 deriving (Eq, Ord, Read, Show, Typeable)
767 instance Exception ExitCode
769 ioException :: IOException -> IO a
770 ioException err = throwIO err
772 -- | Raise an 'IOError' in the 'IO' monad.
773 ioError :: IOError -> IO a
774 ioError = ioException
776 -- ---------------------------------------------------------------------------
779 -- | The Haskell 98 type for exceptions in the 'IO' monad.
780 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
781 -- For a more general type of exception, including also those that arise
782 -- in pure code, see 'Control.Exception.Exception'.
784 -- In Haskell 98, this is an opaque type.
785 type IOError = IOException
787 -- |Exceptions that occur in the @IO@ monad.
788 -- An @IOException@ records a more specific error type, a descriptive
789 -- string and maybe the handle that was used when the error was
793 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
795 ioe_type :: IOErrorType, -- what it was.
796 ioe_location :: String, -- location.
797 ioe_description :: String, -- error type specific information.
798 ioe_errno :: Maybe CInt, -- errno leading to this error, if any.
799 ioe_filename :: Maybe FilePath -- filename the error is related to.
803 instance Exception IOException
805 instance Eq IOException where
806 (IOError h1 e1 loc1 str1 en1 fn1) == (IOError h2 e2 loc2 str2 en2 fn2) =
807 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && en1==en2 && fn1==fn2
809 -- | An abstract type that contains a value for each variant of 'IOError'.
821 | UnsatisfiedConstraints
828 | UnsupportedOperation
833 instance Eq IOErrorType where
834 x == y = getTag x ==# getTag y
836 instance Show IOErrorType where
840 AlreadyExists -> "already exists"
841 NoSuchThing -> "does not exist"
842 ResourceBusy -> "resource busy"
843 ResourceExhausted -> "resource exhausted"
845 IllegalOperation -> "illegal operation"
846 PermissionDenied -> "permission denied"
847 UserError -> "user error"
848 HardwareFault -> "hardware fault"
849 InappropriateType -> "inappropriate type"
850 Interrupted -> "interrupted"
851 InvalidArgument -> "invalid argument"
852 OtherError -> "failed"
853 ProtocolError -> "protocol error"
854 ResourceVanished -> "resource vanished"
855 SystemError -> "system error"
856 TimeExpired -> "timeout"
857 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
858 UnsupportedOperation -> "unsupported operation"
860 -- | Construct an 'IOError' value with a string describing the error.
861 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
862 -- 'userError', thus:
864 -- > instance Monad IO where
866 -- > fail s = ioError (userError s)
868 userError :: String -> IOError
869 userError str = IOError Nothing UserError "" str Nothing Nothing
871 -- ---------------------------------------------------------------------------
874 instance Show IOException where
875 showsPrec p (IOError hdl iot loc s _ fn) =
877 Nothing -> case hdl of
879 Just h -> showsPrec p h . showString ": "
880 Just name -> showString name . showString ": ") .
883 _ -> showString loc . showString ": ") .
887 _ -> showString " (" . showString s . showString ")")
889 -- -----------------------------------------------------------------------------
892 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
893 deriving (Eq, Ord, Ix, Enum, Read, Show)
896 %*********************************************************
898 \subsection{Primitive catch and throwIO}
900 %*********************************************************
902 catchException used to handle the passing around of the state to the
903 action and the handler. This turned out to be a bad idea - it meant
904 that we had to wrap both arguments in thunks so they could be entered
905 as normal (remember IO returns an unboxed pair...).
909 catch# :: IO a -> (b -> IO a) -> IO a
911 (well almost; the compiler doesn't know about the IO newtype so we
912 have to work around that in the definition of catchException below).
915 catchException :: Exception e => IO a -> (e -> IO a) -> IO a
916 catchException (IO io) handler = IO $ catch# io handler'
917 where handler' e = case fromException e of
918 Just e' -> unIO (handler e')
921 catchAny :: IO a -> (forall e . Exception e => e -> IO a) -> IO a
922 catchAny (IO io) handler = IO $ catch# io handler'
923 where handler' (SomeException e) = unIO (handler e)
925 -- | A variant of 'throw' that can only be used within the 'IO' monad.
927 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
928 -- two functions are subtly different:
930 -- > throw e `seq` x ===> throw e
931 -- > throwIO e `seq` x ===> x
933 -- The first example will cause the exception @e@ to be raised,
934 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
935 -- an exception to be raised when it is used within the 'IO' monad.
936 -- The 'throwIO' variant should be used in preference to 'throw' to
937 -- raise an exception within the 'IO' monad because it guarantees
938 -- ordering with respect to other 'IO' operations, whereas 'throw'
940 throwIO :: Exception e => e -> IO a
941 throwIO e = IO (raiseIO# (toException e))
945 %*********************************************************
947 \subsection{Controlling asynchronous exception delivery}
949 %*********************************************************
952 -- | Applying 'block' to a computation will
953 -- execute that computation with asynchronous exceptions
954 -- /blocked/. That is, any thread which
955 -- attempts to raise an exception in the current thread with 'Control.Exception.throwTo' will be
956 -- blocked until asynchronous exceptions are enabled again. There\'s
957 -- no need to worry about re-enabling asynchronous exceptions; that is
958 -- done automatically on exiting the scope of
961 -- Threads created by 'Control.Concurrent.forkIO' inherit the blocked
962 -- state from the parent; that is, to start a thread in blocked mode,
963 -- use @block $ forkIO ...@. This is particularly useful if you need to
964 -- establish an exception handler in the forked thread before any
965 -- asynchronous exceptions are received.
966 block :: IO a -> IO a
968 -- | To re-enable asynchronous exceptions inside the scope of
969 -- 'block', 'unblock' can be
970 -- used. It scopes in exactly the same way, so on exit from
971 -- 'unblock' asynchronous exception delivery will
972 -- be disabled again.
973 unblock :: IO a -> IO a
975 block (IO io) = IO $ blockAsyncExceptions# io
976 unblock (IO io) = IO $ unblockAsyncExceptions# io
978 -- | returns True if asynchronous exceptions are blocked in the
981 blocked = IO $ \s -> case asyncExceptionsBlocked# s of
982 (# s', i #) -> (# s', i /=# 0# #)
986 -- | Forces its argument to be evaluated to weak head normal form when
987 -- the resultant 'IO' action is executed. It can be used to order
988 -- evaluation with respect to other 'IO' operations; its semantics are
991 -- > evaluate x `seq` y ==> y
992 -- > evaluate x `catch` f ==> (return $! x) `catch` f
993 -- > evaluate x >>= f ==> (return $! x) >>= f
995 -- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
996 -- same as @(return $! x)@. A correct definition is
998 -- > evaluate x = (return $! x) >>= return
1000 evaluate :: a -> IO a
1001 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
1003 -- a `seq` (# s, a #)
1004 -- because we can't have an unboxed tuple as a function argument
1008 assertError :: Addr# -> Bool -> a -> a
1009 assertError str predicate v
1011 | otherwise = throw (AssertionFailed (untangle str "Assertion failed"))
1014 (untangle coded message) expects "coded" to be of the form
1017 location message details
1019 untangle :: Addr# -> String -> String
1020 untangle coded message
1027 coded_str = unpackCStringUtf8# coded
1030 = case (span not_bar coded_str) of { (loc, rest) ->
1032 ('|':det) -> (loc, ' ' : det)
1035 not_bar c = c /= '|'