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, catchAny, catchException,
49 ErrorCall(..), ArithException(..), AsyncException(..),
50 BlockedOnDeadMVar(..), BlockedIndefinitely(..),
54 import GHC.Arr -- to derive Ix class
55 import GHC.Enum -- to derive Enum class
58 -- import GHC.Num -- To get fromInteger etc, needed because of -XNoImplicitPrelude
59 import Data.Maybe ( Maybe(..) )
63 import Foreign.C.Types (CInt)
67 import {-# SOURCE #-} Data.Typeable ( Typeable, showsTypeRep )
68 import {-# SOURCE #-} Data.Dynamic ( Dynamic, dynTypeRep )
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, a #) -> 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 ReadHandle = True
475 isReadableHandleType ReadWriteHandle = True
476 isReadableHandleType _ = False
478 isWritableHandleType AppendHandle = True
479 isWritableHandleType WriteHandle = True
480 isWritableHandleType ReadWriteHandle = True
481 isWritableHandleType _ = False
483 isReadWriteHandleType ReadWriteHandle{} = True
484 isReadWriteHandleType _ = False
486 -- | File and directory names are values of type 'String', whose precise
487 -- meaning is operating system dependent. Files can be opened, yielding a
488 -- handle which can then be used to operate on the contents of that file.
490 type FilePath = String
492 -- ---------------------------------------------------------------------------
495 -- | Three kinds of buffering are supported: line-buffering,
496 -- block-buffering or no-buffering. These modes have the following
497 -- effects. For output, items are written out, or /flushed/,
498 -- from the internal buffer according to the buffer mode:
500 -- * /line-buffering/: the entire output buffer is flushed
501 -- whenever a newline is output, the buffer overflows,
502 -- a 'System.IO.hFlush' is issued, or the handle is closed.
504 -- * /block-buffering/: the entire buffer is written out whenever it
505 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
507 -- * /no-buffering/: output is written immediately, and never stored
510 -- An implementation is free to flush the buffer more frequently,
511 -- but not less frequently, than specified above.
512 -- The output buffer is emptied as soon as it has been written out.
514 -- Similarly, input occurs according to the buffer mode for the handle:
516 -- * /line-buffering/: when the buffer for the handle is not empty,
517 -- the next item is obtained from the buffer; otherwise, when the
518 -- buffer is empty, characters up to and including the next newline
519 -- character are read into the buffer. No characters are available
520 -- until the newline character is available or the buffer is full.
522 -- * /block-buffering/: when the buffer for the handle becomes empty,
523 -- the next block of data is read into the buffer.
525 -- * /no-buffering/: the next input item is read and returned.
526 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
527 -- handle may require a one-character buffer.
529 -- The default buffering mode when a handle is opened is
530 -- implementation-dependent and may depend on the file system object
531 -- which is attached to that handle.
532 -- For most implementations, physical files will normally be block-buffered
533 -- and terminals will normally be line-buffered.
536 = NoBuffering -- ^ buffering is disabled if possible.
538 -- ^ line-buffering should be enabled if possible.
539 | BlockBuffering (Maybe Int)
540 -- ^ block-buffering should be enabled if possible.
541 -- The size of the buffer is @n@ items if the argument
542 -- is 'Just' @n@ and is otherwise implementation-dependent.
543 deriving (Eq, Ord, Read, Show)
545 -- ---------------------------------------------------------------------------
548 -- |A mutable variable in the 'IO' monad
549 newtype IORef a = IORef (STRef RealWorld a)
551 -- explicit instance because Haddock can't figure out a derived one
552 instance Eq (IORef a) where
553 IORef x == IORef y = x == y
555 -- |Build a new 'IORef'
556 newIORef :: a -> IO (IORef a)
557 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
559 -- |Read the value of an 'IORef'
560 readIORef :: IORef a -> IO a
561 readIORef (IORef var) = stToIO (readSTRef var)
563 -- |Write a new value into an 'IORef'
564 writeIORef :: IORef a -> a -> IO ()
565 writeIORef (IORef var) v = stToIO (writeSTRef var v)
567 -- ---------------------------------------------------------------------------
568 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
569 -- The type arguments are as follows:
571 -- * @i@: the index type of the array (should be an instance of 'Ix')
573 -- * @e@: the element type of the array.
577 newtype IOArray i e = IOArray (STArray RealWorld i e)
579 -- explicit instance because Haddock can't figure out a derived one
580 instance Eq (IOArray i e) where
581 IOArray x == IOArray y = x == y
583 -- |Build a new 'IOArray'
584 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
585 {-# INLINE newIOArray #-}
586 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
588 -- | Read a value from an 'IOArray'
589 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
590 {-# INLINE unsafeReadIOArray #-}
591 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
593 -- | Write a new value into an 'IOArray'
594 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
595 {-# INLINE unsafeWriteIOArray #-}
596 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
598 -- | Read a value from an 'IOArray'
599 readIOArray :: Ix i => IOArray i e -> i -> IO e
600 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
602 -- | Write a new value into an 'IOArray'
603 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
604 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
607 -- ---------------------------------------------------------------------------
608 -- Show instance for Handles
610 -- handle types are 'show'n when printing error msgs, so
611 -- we provide a more user-friendly Show instance for it
612 -- than the derived one.
614 instance Show HandleType where
617 ClosedHandle -> showString "closed"
618 SemiClosedHandle -> showString "semi-closed"
619 ReadHandle -> showString "readable"
620 WriteHandle -> showString "writable"
621 AppendHandle -> showString "writable (append)"
622 ReadWriteHandle -> showString "read-writable"
624 instance Show Handle where
625 showsPrec p (FileHandle file _) = showHandle file
626 showsPrec p (DuplexHandle file _ _) = showHandle file
628 showHandle file = showString "{handle: " . showString file . showString "}"
630 -- ------------------------------------------------------------------------
631 -- Exception datatypes and operations
633 data ErrorCall = ErrorCall String
636 instance Exception ErrorCall
638 instance Show ErrorCall where
639 showsPrec _ (ErrorCall err) = showString err
643 data BlockedOnDeadMVar = BlockedOnDeadMVar
646 instance Exception BlockedOnDeadMVar
648 instance Show BlockedOnDeadMVar where
649 showsPrec _ BlockedOnDeadMVar = showString "thread blocked indefinitely"
653 data BlockedIndefinitely = BlockedIndefinitely
656 instance Exception BlockedIndefinitely
658 instance Show BlockedIndefinitely where
659 showsPrec _ BlockedIndefinitely = showString "thread blocked indefinitely"
663 -- |The type of arithmetic exceptions
670 deriving (Eq, Ord, Typeable)
672 instance Exception ArithException
674 -- |Asynchronous exceptions
677 -- ^The current thread\'s stack exceeded its limit.
678 -- Since an exception has been raised, the thread\'s stack
679 -- will certainly be below its limit again, but the
680 -- programmer should take remedial action
683 -- ^The program\'s heap is reaching its limit, and
684 -- the program should take action to reduce the amount of
685 -- live data it has. Notes:
687 -- * It is undefined which thread receives this exception.
689 -- * GHC currently does not throw 'HeapOverflow' exceptions.
691 -- ^This exception is raised by another thread
692 -- calling 'Control.Concurrent.killThread', or by the system
693 -- if it needs to terminate the thread for some
696 -- ^This exception is raised by default in the main thread of
697 -- the program when the user requests to terminate the program
698 -- via the usual mechanism(s) (e.g. Control-C in the console).
699 deriving (Eq, Ord, Typeable)
701 instance Exception AsyncException
703 -- | Exceptions generated by array operations
705 = IndexOutOfBounds String
706 -- ^An attempt was made to index an array outside
707 -- its declared bounds.
708 | UndefinedElement String
709 -- ^An attempt was made to evaluate an element of an
710 -- array that had not been initialized.
711 deriving (Eq, Ord, Typeable)
713 instance Exception ArrayException
715 stackOverflow, heapOverflow :: SomeException -- for the RTS
716 stackOverflow = toException StackOverflow
717 heapOverflow = toException HeapOverflow
719 instance Show ArithException where
720 showsPrec _ Overflow = showString "arithmetic overflow"
721 showsPrec _ Underflow = showString "arithmetic underflow"
722 showsPrec _ LossOfPrecision = showString "loss of precision"
723 showsPrec _ DivideByZero = showString "divide by zero"
724 showsPrec _ Denormal = showString "denormal"
726 instance Show AsyncException where
727 showsPrec _ StackOverflow = showString "stack overflow"
728 showsPrec _ HeapOverflow = showString "heap overflow"
729 showsPrec _ ThreadKilled = showString "thread killed"
731 instance Show ArrayException where
732 showsPrec _ (IndexOutOfBounds s)
733 = showString "array index out of range"
734 . (if not (null s) then showString ": " . showString s
736 showsPrec _ (UndefinedElement s)
737 = showString "undefined array element"
738 . (if not (null s) then showString ": " . showString s
741 -- -----------------------------------------------------------------------------
744 -- We need it here because it is used in ExitException in the
745 -- Exception datatype (above).
748 = ExitSuccess -- ^ indicates successful termination;
750 -- ^ indicates program failure with an exit code.
751 -- The exact interpretation of the code is
752 -- operating-system dependent. In particular, some values
753 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
754 deriving (Eq, Ord, Read, Show, Typeable)
756 instance Exception ExitCode
758 ioException :: IOException -> IO a
759 ioException err = throwIO err
761 -- | Raise an 'IOError' in the 'IO' monad.
762 ioError :: IOError -> IO a
763 ioError = ioException
765 -- ---------------------------------------------------------------------------
768 -- | The Haskell 98 type for exceptions in the 'IO' monad.
769 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
770 -- For a more general type of exception, including also those that arise
771 -- in pure code, see 'Control.Exception.Exception'.
773 -- In Haskell 98, this is an opaque type.
774 type IOError = IOException
776 -- |Exceptions that occur in the @IO@ monad.
777 -- An @IOException@ records a more specific error type, a descriptive
778 -- string and maybe the handle that was used when the error was
782 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
784 ioe_type :: IOErrorType, -- what it was.
785 ioe_location :: String, -- location.
786 ioe_description :: String, -- error type specific information.
787 ioe_filename :: Maybe FilePath -- filename the error is related to.
791 instance Exception IOException
793 instance Eq IOException where
794 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
795 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
797 -- | An abstract type that contains a value for each variant of 'IOError'.
809 | UnsatisfiedConstraints
816 | UnsupportedOperation
820 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
822 instance Eq IOErrorType where
825 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
826 _ -> getTag x ==# getTag y
828 instance Show IOErrorType where
832 AlreadyExists -> "already exists"
833 NoSuchThing -> "does not exist"
834 ResourceBusy -> "resource busy"
835 ResourceExhausted -> "resource exhausted"
837 IllegalOperation -> "illegal operation"
838 PermissionDenied -> "permission denied"
839 UserError -> "user error"
840 HardwareFault -> "hardware fault"
841 InappropriateType -> "inappropriate type"
842 Interrupted -> "interrupted"
843 InvalidArgument -> "invalid argument"
844 OtherError -> "failed"
845 ProtocolError -> "protocol error"
846 ResourceVanished -> "resource vanished"
847 SystemError -> "system error"
848 TimeExpired -> "timeout"
849 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
850 UnsupportedOperation -> "unsupported operation"
851 DynIOError{} -> "unknown IO error"
853 -- | Construct an 'IOError' value with a string describing the error.
854 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
855 -- 'userError', thus:
857 -- > instance Monad IO where
859 -- > fail s = ioError (userError s)
861 userError :: String -> IOError
862 userError str = IOError Nothing UserError "" str Nothing
864 -- ---------------------------------------------------------------------------
867 instance Show IOException where
868 showsPrec p (IOError hdl iot loc s fn) =
870 Nothing -> case hdl of
872 Just h -> showsPrec p h . showString ": "
873 Just name -> showString name . showString ": ") .
876 _ -> showString loc . showString ": ") .
880 _ -> showString " (" . showString s . showString ")")
882 -- -----------------------------------------------------------------------------
885 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
886 deriving (Eq, Ord, Ix, Enum, Read, Show)
889 %*********************************************************
891 \subsection{Primitive catch and throwIO}
893 %*********************************************************
895 catchException used to handle the passing around of the state to the
896 action and the handler. This turned out to be a bad idea - it meant
897 that we had to wrap both arguments in thunks so they could be entered
898 as normal (remember IO returns an unboxed pair...).
902 catch# :: IO a -> (b -> IO a) -> IO a
904 (well almost; the compiler doesn't know about the IO newtype so we
905 have to work around that in the definition of catchException below).
908 catchException :: Exception e => IO a -> (e -> IO a) -> IO a
909 catchException (IO io) handler = IO $ catch# io handler'
910 where handler' e = case fromException e of
911 Just e' -> unIO (handler e')
914 catchAny :: IO a -> (forall e . Exception e => e -> IO a) -> IO a
915 catchAny (IO io) handler = IO $ catch# io handler'
916 where handler' (SomeException e) = unIO (handler e)
918 -- | A variant of 'throw' that can be used within the 'IO' monad.
920 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
921 -- two functions are subtly different:
923 -- > throw e `seq` x ===> throw e
924 -- > throwIO e `seq` x ===> x
926 -- The first example will cause the exception @e@ to be raised,
927 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
928 -- an exception to be raised when it is used within the 'IO' monad.
929 -- The 'throwIO' variant should be used in preference to 'throw' to
930 -- raise an exception within the 'IO' monad because it guarantees
931 -- ordering with respect to other 'IO' operations, whereas 'throw'
933 throwIO :: Exception e => e -> IO a
934 throwIO e = IO (raiseIO# (toException e))
938 %*********************************************************
940 \subsection{Controlling asynchronous exception delivery}
942 %*********************************************************
945 -- | Applying 'block' to a computation will
946 -- execute that computation with asynchronous exceptions
947 -- /blocked/. That is, any thread which
948 -- attempts to raise an exception in the current thread with 'Control.Exception.throwTo' will be
949 -- blocked until asynchronous exceptions are enabled again. There\'s
950 -- no need to worry about re-enabling asynchronous exceptions; that is
951 -- done automatically on exiting the scope of
954 -- Threads created by 'Control.Concurrent.forkIO' inherit the blocked
955 -- state from the parent; that is, to start a thread in blocked mode,
956 -- use @block $ forkIO ...@. This is particularly useful if you need to
957 -- establish an exception handler in the forked thread before any
958 -- asynchronous exceptions are received.
959 block :: IO a -> IO a
961 -- | To re-enable asynchronous exceptions inside the scope of
962 -- 'block', 'unblock' can be
963 -- used. It scopes in exactly the same way, so on exit from
964 -- 'unblock' asynchronous exception delivery will
965 -- be disabled again.
966 unblock :: IO a -> IO a
968 block (IO io) = IO $ blockAsyncExceptions# io
969 unblock (IO io) = IO $ unblockAsyncExceptions# io
973 -- | Forces its argument to be evaluated when the resultant 'IO' action
974 -- is executed. It can be used to order evaluation with respect to
975 -- other 'IO' operations; its semantics are given by
977 -- > evaluate x `seq` y ==> y
978 -- > evaluate x `catch` f ==> (return $! x) `catch` f
979 -- > evaluate x >>= f ==> (return $! x) >>= f
981 -- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
982 -- same as @(return $! x)@. A correct definition is
984 -- > evaluate x = (return $! x) >>= return
986 evaluate :: a -> IO a
987 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
989 -- a `seq` (# s, a #)
990 -- because we can't have an unboxed tuple as a function argument