2 {-# OPTIONS_GHC -fno-implicit-prelude -funbox-strict-fields #-}
3 -----------------------------------------------------------------------------
6 -- Copyright : (c) The University of Glasgow 1994-2002
7 -- License : see libraries/base/LICENSE
9 -- Maintainer : cvs-ghc@haskell.org
10 -- Stability : internal
11 -- Portability : non-portable (GHC Extensions)
13 -- Definitions for the 'IO' monad and its friends.
15 -----------------------------------------------------------------------------
19 IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
20 unsafePerformIO, unsafeInterleaveIO,
21 unsafeDupablePerformIO, unsafeDupableInterleaveIO,
24 -- To and from from ST
25 stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
28 IORef(..), newIORef, readIORef, writeIORef,
29 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
32 -- Handles, file descriptors,
34 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
35 isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
38 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
39 bufferIsWritable, bufferEmpty, bufferFull,
42 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
43 stackOverflow, heapOverflow, throw, throwIO, ioException,
44 IOError, IOException(..), IOErrorType(..), ioError, userError,
49 import GHC.Arr -- to derive Ix class
50 import GHC.Enum -- to derive Enum class
53 -- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
54 import Data.Maybe ( Maybe(..) )
58 import Foreign.C.Types (CInt)
61 import {-# SOURCE #-} Data.Typeable ( showsTypeRep )
62 import {-# SOURCE #-} Data.Dynamic ( Dynamic, dynTypeRep )
65 -- ---------------------------------------------------------------------------
69 The IO Monad is just an instance of the ST monad, where the state is
70 the real world. We use the exception mechanism (in GHC.Exception) to
71 implement IO exceptions.
73 NOTE: The IO representation is deeply wired in to various parts of the
74 system. The following list may or may not be exhaustive:
76 Compiler - types of various primitives in PrimOp.lhs
78 RTS - forceIO (StgMiscClosures.hc)
79 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
81 - raiseAsync (Schedule.c)
83 Prelude - GHC.IOBase.lhs, and several other places including
86 Libraries - parts of hslibs/lang.
92 A value of type @'IO' a@ is a computation which, when performed,
93 does some I\/O before returning a value of type @a@.
95 There is really only one way to \"perform\" an I\/O action: bind it to
96 @Main.main@ in your program. When your program is run, the I\/O will
97 be performed. It isn't possible to perform I\/O from an arbitrary
98 function, unless that function is itself in the 'IO' monad and called
99 at some point, directly or indirectly, from @Main.main@.
101 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
102 or the '>>' and '>>=' operations from the 'Monad' class.
104 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
106 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
109 instance Functor IO where
110 fmap f x = x >>= (return . f)
112 instance Monad IO where
113 {-# INLINE return #-}
116 m >> k = m >>= \ _ -> k
117 return x = returnIO x
122 failIO :: String -> IO a
123 failIO s = ioError (userError s)
125 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
126 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
128 bindIO :: IO a -> (a -> IO b) -> IO b
129 bindIO (IO m) k = IO ( \ s ->
131 (# new_s, a #) -> unIO (k a) new_s
134 thenIO :: IO a -> IO b -> IO b
135 thenIO (IO m) k = IO ( \ s ->
137 (# new_s, a #) -> unIO k new_s
140 returnIO :: a -> IO a
141 returnIO x = IO (\ s -> (# s, x #))
143 -- ---------------------------------------------------------------------------
144 -- Coercions between IO and ST
146 -- | A monad transformer embedding strict state transformers in the 'IO'
147 -- monad. The 'RealWorld' parameter indicates that the internal state
148 -- used by the 'ST' computation is a special one supplied by the 'IO'
149 -- monad, and thus distinct from those used by invocations of 'runST'.
150 stToIO :: ST RealWorld a -> IO a
153 ioToST :: IO a -> ST RealWorld a
154 ioToST (IO m) = (ST m)
156 -- This relies on IO and ST having the same representation modulo the
157 -- constraint on the type of the state
159 unsafeIOToST :: IO a -> ST s a
160 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
162 unsafeSTToIO :: ST s a -> IO a
163 unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
165 -- ---------------------------------------------------------------------------
166 -- Unsafe IO operations
169 This is the \"back door\" into the 'IO' monad, allowing
170 'IO' computation to be performed at any time. For
171 this to be safe, the 'IO' computation should be
172 free of side effects and independent of its environment.
174 If the I\/O computation wrapped in 'unsafePerformIO'
175 performs side effects, then the relative order in which those side
176 effects take place (relative to the main I\/O trunk, or other calls to
177 'unsafePerformIO') is indeterminate. You have to be careful when
178 writing and compiling modules that use 'unsafePerformIO':
180 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
181 that calls 'unsafePerformIO'. If the call is inlined,
182 the I\/O may be performed more than once.
184 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
185 elimination being performed on the module, which might combine
186 two side effects that were meant to be separate. A good example
187 is using multiple global variables (like @test@ in the example below).
189 * Make sure that the either you switch off let-floating, or that the
190 call to 'unsafePerformIO' cannot float outside a lambda. For example,
193 f x = unsafePerformIO (newIORef [])
195 you may get only one reference cell shared between all calls to @f@.
198 f x = unsafePerformIO (newIORef [x])
200 because now it can't float outside the lambda.
202 It is less well known that
203 'unsafePerformIO' is not type safe. For example:
206 > test = unsafePerformIO $ newIORef []
209 > writeIORef test [42]
210 > bang <- readIORef test
211 > print (bang :: [Char])
213 This program will core dump. This problem with polymorphic references
214 is well known in the ML community, and does not arise with normal
215 monadic use of references. There is no easy way to make it impossible
216 once you use 'unsafePerformIO'. Indeed, it is
217 possible to write @coerce :: a -> b@ with the
218 help of 'unsafePerformIO'. So be careful!
220 unsafePerformIO :: IO a -> a
221 unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)
224 This version of 'unsafePerformIO' is slightly more efficient,
225 because it omits the check that the IO is only being performed by a
226 single thread. Hence, when you write 'unsafeDupablePerformIO',
227 there is a possibility that the IO action may be performed multiple
228 times (on a multiprocessor), and you should therefore ensure that
229 it gives the same results each time.
231 {-# NOINLINE unsafeDupablePerformIO #-}
232 unsafeDupablePerformIO :: IO a -> a
233 unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
235 -- Why do we NOINLINE unsafeDupablePerformIO? See the comment with
236 -- GHC.ST.runST. Essentially the issue is that the IO computation
237 -- inside unsafePerformIO must be atomic: it must either all run, or
238 -- not at all. If we let the compiler see the application of the IO
239 -- to realWorld#, it might float out part of the IO.
241 -- Why is there a call to 'lazy' in unsafeDupablePerformIO?
242 -- If we don't have it, the demand analyser discovers the following strictness
243 -- for unsafeDupablePerformIO: C(U(AV))
245 -- unsafeDupablePerformIO (\s -> let r = f x in
246 -- case writeIORef v r s of (# s1, _ #) ->
248 -- The strictness analyser will find that the binding for r is strict,
249 -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
250 -- doing the writeIORef. This actually makes tests/lib/should_run/memo002
253 -- Solution: don't expose the strictness of unsafeDupablePerformIO,
254 -- by hiding it with 'lazy'
257 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
258 When passed a value of type @IO a@, the 'IO' will only be performed
259 when the value of the @a@ is demanded. This is used to implement lazy
260 file reading, see 'System.IO.hGetContents'.
262 {-# INLINE unsafeInterleaveIO #-}
263 unsafeInterleaveIO :: IO a -> IO a
264 unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
266 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
267 -- state from this IO thread is passed explicitly to the interleaved
268 -- IO, so it cannot be floated out and shared.
270 {-# INLINE unsafeDupableInterleaveIO #-}
271 unsafeDupableInterleaveIO :: IO a -> IO a
272 unsafeDupableInterleaveIO (IO m)
274 r = case m s of (# _, res #) -> res
279 Ensures that the suspensions under evaluation by the current thread
280 are unique; that is, the current thread is not evaluating anything
281 that is also under evaluation by another thread that has also executed
284 This operation is used in the definition of 'unsafePerformIO' to
285 prevent the IO action from being executed multiple times, which is usually
289 noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)
291 -- ---------------------------------------------------------------------------
294 data MVar a = MVar (MVar# RealWorld a)
296 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
297 for communication between concurrent threads. It can be thought of
298 as a a box, which may be empty or full.
301 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
302 instance Eq (MVar a) where
303 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
305 -- A Handle is represented by (a reference to) a record
306 -- containing the state of the I/O port/device. We record
307 -- the following pieces of info:
309 -- * type (read,write,closed etc.)
310 -- * the underlying file descriptor
312 -- * buffer, and spare buffers
313 -- * user-friendly name (usually the
314 -- FilePath used when IO.openFile was called)
316 -- Note: when a Handle is garbage collected, we want to flush its buffer
317 -- and close the OS file handle, so as to free up a (precious) resource.
319 -- | Haskell defines operations to read and write characters from and to files,
320 -- represented by values of type @Handle@. Each value of this type is a
321 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
322 -- with file system objects. A handle has at least the following properties:
324 -- * whether it manages input or output or both;
326 -- * whether it is /open/, /closed/ or /semi-closed/;
328 -- * whether the object is seekable;
330 -- * whether buffering is disabled, or enabled on a line or block basis;
332 -- * a buffer (whose length may be zero).
334 -- Most handles will also have a current I\/O position indicating where the next
335 -- input or output operation will occur. A handle is /readable/ if it
336 -- manages only input or both input and output; likewise, it is /writable/ if
337 -- it manages only output or both input and output. A handle is /open/ when
339 -- Once it is closed it can no longer be used for either input or output,
340 -- though an implementation cannot re-use its storage while references
341 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
342 -- produced by showing a handle is system dependent; it should include
343 -- enough information to identify the handle for debugging. A handle is
344 -- equal according to '==' only to itself; no attempt
345 -- is made to compare the internal state of different handles for equality.
347 -- GHC note: a 'Handle' will be automatically closed when the garbage
348 -- collector detects that it has become unreferenced by the program.
349 -- However, relying on this behaviour is not generally recommended:
350 -- the garbage collector is unpredictable. If possible, use explicit
351 -- an explicit 'hClose' to close 'Handle's when they are no longer
352 -- required. GHC does not currently attempt to free up file
353 -- descriptors when they have run out, it is your responsibility to
354 -- ensure that this doesn't happen.
357 = FileHandle -- A normal handle to a file
358 FilePath -- the file (invariant)
361 | DuplexHandle -- A handle to a read/write stream
362 FilePath -- file for a FIFO, otherwise some
363 -- descriptive string.
364 !(MVar Handle__) -- The read side
365 !(MVar Handle__) -- The write side
368 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
371 instance Eq Handle where
372 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
373 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
380 haFD :: !FD, -- file descriptor
381 haType :: HandleType, -- type (read/write/append etc.)
382 haIsBin :: Bool, -- binary mode?
383 haIsStream :: Bool, -- Windows : is this a socket?
384 -- Unix : is O_NONBLOCK set?
385 haBufferMode :: BufferMode, -- buffer contains read/write data?
386 haBuffer :: !(IORef Buffer), -- the current buffer
387 haBuffers :: !(IORef BufferList), -- spare buffers
388 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
392 -- ---------------------------------------------------------------------------
395 -- The buffer is represented by a mutable variable containing a
396 -- record, where the record contains the raw buffer and the start/end
397 -- points of the filled portion. We use a mutable variable so that
398 -- the common operation of writing (or reading) some data from (to)
399 -- the buffer doesn't need to modify, and hence copy, the handle
400 -- itself, it just updates the buffer.
402 -- There will be some allocation involved in a simple hPutChar in
403 -- order to create the new Buffer structure (below), but this is
404 -- relatively small, and this only has to be done once per write
407 -- The buffer contains its size - we could also get the size by
408 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
409 -- to be rounded up to the nearest Word.
411 type RawBuffer = MutableByteArray# RealWorld
413 -- INVARIANTS on a Buffer:
415 -- * A handle *always* has a buffer, even if it is only 1 character long
416 -- (an unbuffered handle needs a 1 character buffer in order to support
417 -- hLookAhead and hIsEOF).
419 -- * if r == w, then r == 0 && w == 0
420 -- * if state == WriteBuffer, then r == 0
421 -- * a write buffer is never full. If an operation
422 -- fills up the buffer, it will always flush it before
424 -- * a read buffer may be full as a result of hLookAhead. In normal
425 -- operation, a read buffer always has at least one character of space.
433 bufState :: BufferState
436 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
438 -- we keep a few spare buffers around in a handle to avoid allocating
439 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
440 -- same size as the main buffer.
443 | BufferListCons RawBuffer BufferList
446 bufferIsWritable :: Buffer -> Bool
447 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
448 bufferIsWritable _other = False
450 bufferEmpty :: Buffer -> Bool
451 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
453 -- only makes sense for a write buffer
454 bufferFull :: Buffer -> Bool
455 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
457 -- Internally, we classify handles as being one
468 isReadableHandleType ReadHandle = True
469 isReadableHandleType ReadWriteHandle = True
470 isReadableHandleType _ = False
472 isWritableHandleType AppendHandle = True
473 isWritableHandleType WriteHandle = True
474 isWritableHandleType ReadWriteHandle = True
475 isWritableHandleType _ = False
477 isReadWriteHandleType ReadWriteHandle{} = True
478 isReadWriteHandleType _ = False
480 -- | File and directory names are values of type 'String', whose precise
481 -- meaning is operating system dependent. Files can be opened, yielding a
482 -- handle which can then be used to operate on the contents of that file.
484 type FilePath = String
486 -- ---------------------------------------------------------------------------
489 -- | Three kinds of buffering are supported: line-buffering,
490 -- block-buffering or no-buffering. These modes have the following
491 -- effects. For output, items are written out, or /flushed/,
492 -- from the internal buffer according to the buffer mode:
494 -- * /line-buffering/: the entire output buffer is flushed
495 -- whenever a newline is output, the buffer overflows,
496 -- a 'System.IO.hFlush' is issued, or the handle is closed.
498 -- * /block-buffering/: the entire buffer is written out whenever it
499 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
501 -- * /no-buffering/: output is written immediately, and never stored
504 -- An implementation is free to flush the buffer more frequently,
505 -- but not less frequently, than specified above.
506 -- The output buffer is emptied as soon as it has been written out.
508 -- Similarly, input occurs according to the buffer mode for the handle:
510 -- * /line-buffering/: when the buffer for the handle is not empty,
511 -- the next item is obtained from the buffer; otherwise, when the
512 -- buffer is empty, characters up to and including the next newline
513 -- character are read into the buffer. No characters are available
514 -- until the newline character is available or the buffer is full.
516 -- * /block-buffering/: when the buffer for the handle becomes empty,
517 -- the next block of data is read into the buffer.
519 -- * /no-buffering/: the next input item is read and returned.
520 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
521 -- handle may require a one-character buffer.
523 -- The default buffering mode when a handle is opened is
524 -- implementation-dependent and may depend on the file system object
525 -- which is attached to that handle.
526 -- For most implementations, physical files will normally be block-buffered
527 -- and terminals will normally be line-buffered.
530 = NoBuffering -- ^ buffering is disabled if possible.
532 -- ^ line-buffering should be enabled if possible.
533 | BlockBuffering (Maybe Int)
534 -- ^ block-buffering should be enabled if possible.
535 -- The size of the buffer is @n@ items if the argument
536 -- is 'Just' @n@ and is otherwise implementation-dependent.
537 deriving (Eq, Ord, Read, Show)
539 -- ---------------------------------------------------------------------------
542 -- |A mutable variable in the 'IO' monad
543 newtype IORef a = IORef (STRef RealWorld a)
545 -- explicit instance because Haddock can't figure out a derived one
546 instance Eq (IORef a) where
547 IORef x == IORef y = x == y
549 -- |Build a new 'IORef'
550 newIORef :: a -> IO (IORef a)
551 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
553 -- |Read the value of an 'IORef'
554 readIORef :: IORef a -> IO a
555 readIORef (IORef var) = stToIO (readSTRef var)
557 -- |Write a new value into an 'IORef'
558 writeIORef :: IORef a -> a -> IO ()
559 writeIORef (IORef var) v = stToIO (writeSTRef var v)
561 -- ---------------------------------------------------------------------------
562 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
563 -- The type arguments are as follows:
565 -- * @i@: the index type of the array (should be an instance of 'Ix')
567 -- * @e@: the element type of the array.
571 newtype IOArray i e = IOArray (STArray RealWorld i e)
573 -- explicit instance because Haddock can't figure out a derived one
574 instance Eq (IOArray i e) where
575 IOArray x == IOArray y = x == y
577 -- |Build a new 'IOArray'
578 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
579 {-# INLINE newIOArray #-}
580 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
582 -- | Read a value from an 'IOArray'
583 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
584 {-# INLINE unsafeReadIOArray #-}
585 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
587 -- | Write a new value into an 'IOArray'
588 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
589 {-# INLINE unsafeWriteIOArray #-}
590 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
592 -- | Read a value from an 'IOArray'
593 readIOArray :: Ix i => IOArray i e -> i -> IO e
594 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
596 -- | Write a new value into an 'IOArray'
597 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
598 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
601 -- ---------------------------------------------------------------------------
602 -- Show instance for Handles
604 -- handle types are 'show'n when printing error msgs, so
605 -- we provide a more user-friendly Show instance for it
606 -- than the derived one.
608 instance Show HandleType where
611 ClosedHandle -> showString "closed"
612 SemiClosedHandle -> showString "semi-closed"
613 ReadHandle -> showString "readable"
614 WriteHandle -> showString "writable"
615 AppendHandle -> showString "writable (append)"
616 ReadWriteHandle -> showString "read-writable"
618 instance Show Handle where
619 showsPrec p (FileHandle file _) = showHandle file
620 showsPrec p (DuplexHandle file _ _) = showHandle file
622 showHandle file = showString "{handle: " . showString file . showString "}"
624 -- ------------------------------------------------------------------------
625 -- Exception datatype and operations
627 -- |The type of exceptions. Every kind of system-generated exception
628 -- has a constructor in the 'Exception' type, and values of other
629 -- types may be injected into 'Exception' by coercing them to
630 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
631 -- "Control.Exception\#DynamicExceptions").
633 = ArithException ArithException
634 -- ^Exceptions raised by arithmetic
635 -- operations. (NOTE: GHC currently does not throw
636 -- 'ArithException's except for 'DivideByZero').
637 | ArrayException ArrayException
638 -- ^Exceptions raised by array-related
639 -- operations. (NOTE: GHC currently does not throw
640 -- 'ArrayException's).
641 | AssertionFailed String
642 -- ^This exception is thrown by the
643 -- 'assert' operation when the condition
644 -- fails. The 'String' argument contains the
645 -- location of the assertion in the source program.
646 | AsyncException AsyncException
647 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
649 -- ^The current thread was executing a call to
650 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
651 -- because there are no other references to this 'MVar'.
652 | BlockedIndefinitely
653 -- ^The current thread was waiting to retry an atomic memory transaction
654 -- that could never become possible to complete because there are no other
655 -- threads referring to any of the TVars involved.
657 -- ^The runtime detected an attempt to nest one STM transaction
658 -- inside another one, presumably due to the use of
659 -- 'unsafePeformIO' with 'atomically'.
661 -- ^There are no runnable threads, so the program is
662 -- deadlocked. The 'Deadlock' exception is
663 -- raised in the main thread only (see also: "Control.Concurrent").
664 | DynException Dynamic
665 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
667 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
668 -- argument of 'ErrorCall' is the string passed to 'error' when it was
670 | ExitException ExitCode
671 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
672 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
673 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
674 -- main thread will cause the program to be terminated with the given
676 | IOException IOException
677 -- ^These are the standard IO exceptions generated by
678 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
679 | NoMethodError String
680 -- ^An attempt was made to invoke a class method which has
681 -- no definition in this instance, and there was no default
682 -- definition given in the class declaration. GHC issues a
683 -- warning when you compile an instance which has missing
686 -- ^The current thread is stuck in an infinite loop. This
687 -- exception may or may not be thrown when the program is
689 | PatternMatchFail String
690 -- ^A pattern matching failure. The 'String' argument should contain a
691 -- descriptive message including the function name, source file
694 -- ^An attempt was made to evaluate a field of a record
695 -- for which no value was given at construction time. The
696 -- 'String' argument gives the location of the
697 -- record construction in the source program.
699 -- ^A field selection was attempted on a constructor that
700 -- doesn\'t have the requested field. This can happen with
701 -- multi-constructor records when one or more fields are
702 -- missing from some of the constructors. The
703 -- 'String' argument gives the location of the
704 -- record selection in the source program.
706 -- ^An attempt was made to update a field in a record,
707 -- where the record doesn\'t have the requested field. This can
708 -- only occur with multi-constructor records, when one or more
709 -- fields are missing from some of the constructors. The
710 -- 'String' argument gives the location of the
711 -- record update in the source program.
713 -- |The type of arithmetic exceptions
723 -- |Asynchronous exceptions
726 -- ^The current thread\'s stack exceeded its limit.
727 -- Since an exception has been raised, the thread\'s stack
728 -- will certainly be below its limit again, but the
729 -- programmer should take remedial action
732 -- ^The program\'s heap is reaching its limit, and
733 -- the program should take action to reduce the amount of
734 -- live data it has. Notes:
736 -- * It is undefined which thread receives this exception.
738 -- * GHC currently does not throw 'HeapOverflow' exceptions.
740 -- ^This exception is raised by another thread
741 -- calling 'Control.Concurrent.killThread', or by the system
742 -- if it needs to terminate the thread for some
746 -- | Exceptions generated by array operations
748 = IndexOutOfBounds String
749 -- ^An attempt was made to index an array outside
750 -- its declared bounds.
751 | UndefinedElement String
752 -- ^An attempt was made to evaluate an element of an
753 -- array that had not been initialized.
756 stackOverflow, heapOverflow :: Exception -- for the RTS
757 stackOverflow = AsyncException StackOverflow
758 heapOverflow = AsyncException HeapOverflow
760 instance Show ArithException where
761 showsPrec _ Overflow = showString "arithmetic overflow"
762 showsPrec _ Underflow = showString "arithmetic underflow"
763 showsPrec _ LossOfPrecision = showString "loss of precision"
764 showsPrec _ DivideByZero = showString "divide by zero"
765 showsPrec _ Denormal = showString "denormal"
767 instance Show AsyncException where
768 showsPrec _ StackOverflow = showString "stack overflow"
769 showsPrec _ HeapOverflow = showString "heap overflow"
770 showsPrec _ ThreadKilled = showString "thread killed"
772 instance Show ArrayException where
773 showsPrec _ (IndexOutOfBounds s)
774 = showString "array index out of range"
775 . (if not (null s) then showString ": " . showString s
777 showsPrec _ (UndefinedElement s)
778 = showString "undefined array element"
779 . (if not (null s) then showString ": " . showString s
782 instance Show Exception where
783 showsPrec _ (IOException err) = shows err
784 showsPrec _ (ArithException err) = shows err
785 showsPrec _ (ArrayException err) = shows err
786 showsPrec _ (ErrorCall err) = showString err
787 showsPrec _ (ExitException err) = showString "exit: " . shows err
788 showsPrec _ (NoMethodError err) = showString err
789 showsPrec _ (PatternMatchFail err) = showString err
790 showsPrec _ (RecSelError err) = showString err
791 showsPrec _ (RecConError err) = showString err
792 showsPrec _ (RecUpdError err) = showString err
793 showsPrec _ (AssertionFailed err) = showString err
794 showsPrec _ (DynException err) = showString "exception :: " . showsTypeRep (dynTypeRep err)
795 showsPrec _ (AsyncException e) = shows e
796 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
797 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
798 showsPrec _ (NestedAtomically) = showString "Control.Concurrent.STM.atomically was nested"
799 showsPrec _ (NonTermination) = showString "<<loop>>"
800 showsPrec _ (Deadlock) = showString "<<deadlock>>"
802 instance Eq Exception where
803 IOException e1 == IOException e2 = e1 == e2
804 ArithException e1 == ArithException e2 = e1 == e2
805 ArrayException e1 == ArrayException e2 = e1 == e2
806 ErrorCall e1 == ErrorCall e2 = e1 == e2
807 ExitException e1 == ExitException e2 = e1 == e2
808 NoMethodError e1 == NoMethodError e2 = e1 == e2
809 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
810 RecSelError e1 == RecSelError e2 = e1 == e2
811 RecConError e1 == RecConError e2 = e1 == e2
812 RecUpdError e1 == RecUpdError e2 = e1 == e2
813 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
814 DynException _ == DynException _ = False -- incomparable
815 AsyncException e1 == AsyncException e2 = e1 == e2
816 BlockedOnDeadMVar == BlockedOnDeadMVar = True
817 NonTermination == NonTermination = True
818 NestedAtomically == NestedAtomically = True
819 Deadlock == Deadlock = True
822 -- -----------------------------------------------------------------------------
825 -- We need it here because it is used in ExitException in the
826 -- Exception datatype (above).
829 = ExitSuccess -- ^ indicates successful termination;
831 -- ^ indicates program failure with an exit code.
832 -- The exact interpretation of the code is
833 -- operating-system dependent. In particular, some values
834 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
835 deriving (Eq, Ord, Read, Show)
837 -- --------------------------------------------------------------------------
840 -- | Throw an exception. Exceptions may be thrown from purely
841 -- functional code, but may only be caught within the 'IO' monad.
842 throw :: Exception -> a
843 throw exception = raise# exception
845 -- | A variant of 'throw' that can be used within the 'IO' monad.
847 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
848 -- two functions are subtly different:
850 -- > throw e `seq` x ===> throw e
851 -- > throwIO e `seq` x ===> x
853 -- The first example will cause the exception @e@ to be raised,
854 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
855 -- an exception to be raised when it is used within the 'IO' monad.
856 -- The 'throwIO' variant should be used in preference to 'throw' to
857 -- raise an exception within the 'IO' monad because it guarantees
858 -- ordering with respect to other 'IO' operations, whereas 'throw'
860 throwIO :: Exception -> IO a
861 throwIO err = IO $ raiseIO# err
863 ioException :: IOException -> IO a
864 ioException err = IO $ raiseIO# (IOException err)
866 -- | Raise an 'IOError' in the 'IO' monad.
867 ioError :: IOError -> IO a
868 ioError = ioException
870 -- ---------------------------------------------------------------------------
873 -- | The Haskell 98 type for exceptions in the 'IO' monad.
874 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
875 -- For a more general type of exception, including also those that arise
876 -- in pure code, see 'Control.Exception.Exception'.
878 -- In Haskell 98, this is an opaque type.
879 type IOError = IOException
881 -- |Exceptions that occur in the @IO@ monad.
882 -- An @IOException@ records a more specific error type, a descriptive
883 -- string and maybe the handle that was used when the error was
887 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
889 ioe_type :: IOErrorType, -- what it was.
890 ioe_location :: String, -- location.
891 ioe_description :: String, -- error type specific information.
892 ioe_filename :: Maybe FilePath -- filename the error is related to.
895 instance Eq IOException where
896 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
897 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
899 -- | An abstract type that contains a value for each variant of 'IOError'.
911 | UnsatisfiedConstraints
918 | UnsupportedOperation
922 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
924 instance Eq IOErrorType where
927 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
928 _ -> getTag x ==# getTag y
930 instance Show IOErrorType where
934 AlreadyExists -> "already exists"
935 NoSuchThing -> "does not exist"
936 ResourceBusy -> "resource busy"
937 ResourceExhausted -> "resource exhausted"
939 IllegalOperation -> "illegal operation"
940 PermissionDenied -> "permission denied"
941 UserError -> "user error"
942 HardwareFault -> "hardware fault"
943 InappropriateType -> "inappropriate type"
944 Interrupted -> "interrupted"
945 InvalidArgument -> "invalid argument"
946 OtherError -> "failed"
947 ProtocolError -> "protocol error"
948 ResourceVanished -> "resource vanished"
949 SystemError -> "system error"
950 TimeExpired -> "timeout"
951 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
952 UnsupportedOperation -> "unsupported operation"
953 DynIOError{} -> "unknown IO error"
955 -- | Construct an 'IOError' value with a string describing the error.
956 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
957 -- 'userError', thus:
959 -- > instance Monad IO where
961 -- > fail s = ioError (userError s)
963 userError :: String -> IOError
964 userError str = IOError Nothing UserError "" str Nothing
966 -- ---------------------------------------------------------------------------
969 instance Show IOException where
970 showsPrec p (IOError hdl iot loc s fn) =
972 Nothing -> case hdl of
974 Just h -> showsPrec p h . showString ": "
975 Just name -> showString name . showString ": ") .
978 _ -> showString loc . showString ": ") .
982 _ -> showString " (" . showString s . showString ")")
984 -- -----------------------------------------------------------------------------
987 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
988 deriving (Eq, Ord, Ix, Enum, Read, Show)