2 {-# OPTIONS -fno-implicit-prelude #-}
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 -----------------------------------------------------------------------------
17 module GHC.IOBase where
20 import GHC.Arr -- to derive Ix class
21 import GHC.Enum -- to derive Enum class
24 import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
25 import Data.Maybe ( Maybe(..) )
31 import {-# SOURCE #-} Data.Dynamic
34 -- ---------------------------------------------------------------------------
38 The IO Monad is just an instance of the ST monad, where the state is
39 the real world. We use the exception mechanism (in GHC.Exception) to
40 implement IO exceptions.
42 NOTE: The IO representation is deeply wired in to various parts of the
43 system. The following list may or may not be exhaustive:
45 Compiler - types of various primitives in PrimOp.lhs
47 RTS - forceIO (StgMiscClosures.hc)
48 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
50 - raiseAsync (Schedule.c)
52 Prelude - GHC.IOBase.lhs, and several other places including
55 Libraries - parts of hslibs/lang.
61 A value of type @'IO' a@ is a computation which, when performed,
62 does some I\/O before returning a value of type @a@.
64 There is really only one way to \"perform\" an I\/O action: bind it to
65 @Main.main@ in your program. When your program is run, the I\/O will
66 be performed. It isn't possible to perform I\/O from an arbitrary
67 function, unless that function is itself in the 'IO' monad and called
68 at some point, directly or indirectly, from @Main.main@.
70 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
71 or the '>>' and '>>=' operations from the 'Monad' class.
73 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
75 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
78 instance Functor IO where
79 fmap f x = x >>= (return . f)
81 instance Monad IO where
85 m >> k = m >>= \ _ -> k
91 failIO :: String -> IO a
92 failIO s = ioError (userError s)
94 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
95 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
97 bindIO :: IO a -> (a -> IO b) -> IO b
98 bindIO (IO m) k = IO ( \ s ->
100 (# new_s, a #) -> unIO (k a) new_s
103 thenIO :: IO a -> IO b -> IO b
104 thenIO (IO m) k = IO ( \ s ->
106 (# new_s, a #) -> unIO k new_s
109 returnIO :: a -> IO a
110 returnIO x = IO (\ s -> (# s, x #))
112 -- ---------------------------------------------------------------------------
113 -- Coercions between IO and ST
115 -- | A monad transformer embedding strict state transformers in the 'IO'
116 -- monad. The 'RealWorld' parameter indicates that the internal state
117 -- used by the 'ST' computation is a special one supplied by the 'IO'
118 -- monad, and thus distinct from those used by invocations of 'runST'.
119 stToIO :: ST RealWorld a -> IO a
122 ioToST :: IO a -> ST RealWorld a
123 ioToST (IO m) = (ST m)
125 -- ---------------------------------------------------------------------------
126 -- Unsafe IO operations
129 This is the \"back door\" into the 'IO' monad, allowing
130 'IO' computation to be performed at any time. For
131 this to be safe, the 'IO' computation should be
132 free of side effects and independent of its environment.
134 If the I\/O computation wrapped in 'unsafePerformIO'
135 performs side effects, then the relative order in which those side
136 effects take place (relative to the main I\/O trunk, or other calls to
137 'unsafePerformIO') is indeterminate. You have to be careful when
138 writing and compiling modules that use 'unsafePerformIO':
140 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
141 that calls 'unsafePerformIO'. If the call is inlined,
142 the I\/O may be performed more than once.
144 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
145 elimination being performed on the module, which might combine
146 two side effects that were meant to be separate. A good example
147 is using multiple global variables (like @test@ in the example below).
149 * Make sure that the either you switch off let-floating, or that the
150 call to 'unsafePerformIO' cannot float outside a lambda. For example,
153 f x = unsafePerformIO (newIORef [])
155 you may get only one reference cell shared between all calls to @f@.
158 f x = unsafePerformIO (newIORef [x])
160 because now it can't float outside the lambda.
162 It is less well known that
163 'unsafePerformIO' is not type safe. For example:
166 > test = unsafePerformIO $ newIORef []
169 > writeIORef test [42]
170 > bang <- readIORef test
171 > print (bang :: [Char])
173 This program will core dump. This problem with polymorphic references
174 is well known in the ML community, and does not arise with normal
175 monadic use of references. There is no easy way to make it impossible
176 once you use 'unsafePerformIO'. Indeed, it is
177 possible to write @coerce :: a -> b@ with the
178 help of 'unsafePerformIO'. So be careful!
180 {-# NOINLINE unsafePerformIO #-}
181 unsafePerformIO :: IO a -> a
182 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
184 -- Why do we NOINLINE unsafePerformIO? See the comment with
185 -- GHC.ST.runST. Essentially the issue is that the IO computation
186 -- inside unsafePerformIO must be atomic: it must either all run, or
187 -- not at all. If we let the compiler see the application of the IO
188 -- to realWorld#, it might float out part of the IO.
191 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
192 When passed a value of type @IO a@, the 'IO' will only be performed
193 when the value of the @a@ is demanded. This is used to implement lazy
194 file reading, see 'System.IO.hGetContents'.
196 {-# INLINE unsafeInterleaveIO #-}
197 unsafeInterleaveIO :: IO a -> IO a
198 unsafeInterleaveIO (IO m)
200 r = case m s of (# _, res #) -> res
204 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
205 -- state from this IO thread is passed explicitly to the interleaved
206 -- IO, so it cannot be floated out and shared.
208 -- ---------------------------------------------------------------------------
211 data MVar a = MVar (MVar# RealWorld a)
213 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
214 for communication between concurrent threads. It can be thought of
215 as a a box, which may be empty or full.
218 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
219 instance Eq (MVar a) where
220 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
222 -- A Handle is represented by (a reference to) a record
223 -- containing the state of the I/O port/device. We record
224 -- the following pieces of info:
226 -- * type (read,write,closed etc.)
227 -- * the underlying file descriptor
229 -- * buffer, and spare buffers
230 -- * user-friendly name (usually the
231 -- FilePath used when IO.openFile was called)
233 -- Note: when a Handle is garbage collected, we want to flush its buffer
234 -- and close the OS file handle, so as to free up a (precious) resource.
236 -- | Haskell defines operations to read and write characters from and to files,
237 -- represented by values of type @Handle@. Each value of this type is a
238 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
239 -- with file system objects. A handle has at least the following properties:
241 -- * whether it manages input or output or both;
243 -- * whether it is /open/, /closed/ or /semi-closed/;
245 -- * whether the object is seekable;
247 -- * whether buffering is disabled, or enabled on a line or block basis;
249 -- * a buffer (whose length may be zero).
251 -- Most handles will also have a current I\/O position indicating where the next
252 -- input or output operation will occur. A handle is /readable/ if it
253 -- manages only input or both input and output; likewise, it is /writable/ if
254 -- it manages only output or both input and output. A handle is /open/ when
256 -- Once it is closed it can no longer be used for either input or output,
257 -- though an implementation cannot re-use its storage while references
258 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
259 -- produced by showing a handle is system dependent; it should include
260 -- enough information to identify the handle for debugging. A handle is
261 -- equal according to '==' only to itself; no attempt
262 -- is made to compare the internal state of different handles for equality.
265 = FileHandle -- A normal handle to a file
266 FilePath -- the file (invariant)
269 | DuplexHandle -- A handle to a read/write stream
270 FilePath -- file for a FIFO, otherwise some
271 -- descriptive string.
272 !(MVar Handle__) -- The read side
273 !(MVar Handle__) -- The write side
276 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
279 instance Eq Handle where
280 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
281 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
284 type FD = Int -- XXX ToDo: should be CInt
288 haFD :: !FD, -- file descriptor
289 haType :: HandleType, -- type (read/write/append etc.)
290 haIsBin :: Bool, -- binary mode?
291 haIsStream :: Bool, -- is this a stream handle?
292 haBufferMode :: BufferMode, -- buffer contains read/write data?
293 haBuffer :: !(IORef Buffer), -- the current buffer
294 haBuffers :: !(IORef BufferList), -- spare buffers
295 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
299 -- ---------------------------------------------------------------------------
302 -- The buffer is represented by a mutable variable containing a
303 -- record, where the record contains the raw buffer and the start/end
304 -- points of the filled portion. We use a mutable variable so that
305 -- the common operation of writing (or reading) some data from (to)
306 -- the buffer doesn't need to modify, and hence copy, the handle
307 -- itself, it just updates the buffer.
309 -- There will be some allocation involved in a simple hPutChar in
310 -- order to create the new Buffer structure (below), but this is
311 -- relatively small, and this only has to be done once per write
314 -- The buffer contains its size - we could also get the size by
315 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
316 -- to be rounded up to the nearest Word.
318 type RawBuffer = MutableByteArray# RealWorld
320 -- INVARIANTS on a Buffer:
322 -- * A handle *always* has a buffer, even if it is only 1 character long
323 -- (an unbuffered handle needs a 1 character buffer in order to support
324 -- hLookAhead and hIsEOF).
326 -- * if r == w, then r == 0 && w == 0
327 -- * if state == WriteBuffer, then r == 0
328 -- * a write buffer is never full. If an operation
329 -- fills up the buffer, it will always flush it before
331 -- * a read buffer may be full as a result of hLookAhead. In normal
332 -- operation, a read buffer always has at least one character of space.
340 bufState :: BufferState
343 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
345 -- we keep a few spare buffers around in a handle to avoid allocating
346 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
347 -- same size as the main buffer.
350 | BufferListCons RawBuffer BufferList
353 bufferIsWritable :: Buffer -> Bool
354 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
355 bufferIsWritable _other = False
357 bufferEmpty :: Buffer -> Bool
358 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
360 -- only makes sense for a write buffer
361 bufferFull :: Buffer -> Bool
362 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
364 -- Internally, we classify handles as being one
375 isReadableHandleType ReadHandle = True
376 isReadableHandleType ReadWriteHandle = True
377 isReadableHandleType _ = False
379 isWritableHandleType AppendHandle = True
380 isWritableHandleType WriteHandle = True
381 isWritableHandleType ReadWriteHandle = True
382 isWritableHandleType _ = False
384 -- | File and directory names are values of type 'String', whose precise
385 -- meaning is operating system dependent. Files can be opened, yielding a
386 -- handle which can then be used to operate on the contents of that file.
388 type FilePath = String
390 -- ---------------------------------------------------------------------------
393 -- | Three kinds of buffering are supported: line-buffering,
394 -- block-buffering or no-buffering. These modes have the following
395 -- effects. For output, items are written out, or /flushed/,
396 -- from the internal buffer according to the buffer mode:
398 -- * /line-buffering/: the entire output buffer is flushed
399 -- whenever a newline is output, the buffer overflows,
400 -- a 'System.IO.hFlush' is issued, or the handle is closed.
402 -- * /block-buffering/: the entire buffer is written out whenever it
403 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
405 -- * /no-buffering/: output is written immediately, and never stored
408 -- An implementation is free to flush the buffer more frequently,
409 -- but not less frequently, than specified above.
410 -- The output buffer is emptied as soon as it has been written out.
412 -- Similarly, input occurs according to the buffer mode for the handle:
414 -- * /line-buffering/: when the buffer for the handle is not empty,
415 -- the next item is obtained from the buffer; otherwise, when the
416 -- buffer is empty, characters up to and including the next newline
417 -- character are read into the buffer. No characters are available
418 -- until the newline character is available or the buffer is full.
420 -- * /block-buffering/: when the buffer for the handle becomes empty,
421 -- the next block of data is read into the buffer.
423 -- * /no-buffering/: the next input item is read and returned.
424 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
425 -- handle may require a one-character buffer.
427 -- The default buffering mode when a handle is opened is
428 -- implementation-dependent and may depend on the file system object
429 -- which is attached to that handle.
430 -- For most implementations, physical files will normally be block-buffered
431 -- and terminals will normally be line-buffered.
434 = NoBuffering -- ^ buffering is disabled if possible.
436 -- ^ line-buffering should be enabled if possible.
437 | BlockBuffering (Maybe Int)
438 -- ^ block-buffering should be enabled if possible.
439 -- The size of the buffer is @n@ items if the argument
440 -- is 'Just' @n@ and is otherwise implementation-dependent.
441 deriving (Eq, Ord, Read, Show)
443 -- ---------------------------------------------------------------------------
446 -- |A mutable variable in the 'IO' monad
447 newtype IORef a = IORef (STRef RealWorld a)
449 -- explicit instance because Haddock can't figure out a derived one
450 instance Eq (IORef a) where
451 IORef x == IORef y = x == y
453 -- |Build a new 'IORef'
454 newIORef :: a -> IO (IORef a)
455 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
457 -- |Read the value of an 'IORef'
458 readIORef :: IORef a -> IO a
459 readIORef (IORef var) = stToIO (readSTRef var)
461 -- |Write a new value into an 'IORef'
462 writeIORef :: IORef a -> a -> IO ()
463 writeIORef (IORef var) v = stToIO (writeSTRef var v)
465 -- ---------------------------------------------------------------------------
466 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
467 -- The type arguments are as follows:
469 -- * @i@: the index type of the array (should be an instance of 'Ix')
471 -- * @e@: the element type of the array.
475 newtype IOArray i e = IOArray (STArray RealWorld i e)
477 -- explicit instance because Haddock can't figure out a derived one
478 instance Eq (IOArray i e) where
479 IOArray x == IOArray y = x == y
481 -- |Build a new 'IOArray'
482 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
483 {-# INLINE newIOArray #-}
484 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
486 -- | Read a value from an 'IOArray'
487 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
488 {-# INLINE unsafeReadIOArray #-}
489 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
491 -- | Write a new value into an 'IOArray'
492 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
493 {-# INLINE unsafeWriteIOArray #-}
494 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
496 -- | Read a value from an 'IOArray'
497 readIOArray :: Ix i => IOArray i e -> i -> IO e
498 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
500 -- | Write a new value into an 'IOArray'
501 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
502 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
505 -- ---------------------------------------------------------------------------
506 -- Show instance for Handles
508 -- handle types are 'show'n when printing error msgs, so
509 -- we provide a more user-friendly Show instance for it
510 -- than the derived one.
512 instance Show HandleType where
515 ClosedHandle -> showString "closed"
516 SemiClosedHandle -> showString "semi-closed"
517 ReadHandle -> showString "readable"
518 WriteHandle -> showString "writable"
519 AppendHandle -> showString "writable (append)"
520 ReadWriteHandle -> showString "read-writable"
522 instance Show Handle where
523 showsPrec p (FileHandle file _) = showHandle file
524 showsPrec p (DuplexHandle file _ _) = showHandle file
526 showHandle file = showString "{handle: " . showString file . showString "}"
528 -- ------------------------------------------------------------------------
529 -- Exception datatype and operations
531 -- |The type of exceptions. Every kind of system-generated exception
532 -- has a constructor in the 'Exception' type, and values of other
533 -- types may be injected into 'Exception' by coercing them to
534 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
535 -- "Control.Exception\#DynamicExceptions").
537 = ArithException ArithException
538 -- ^Exceptions raised by arithmetic
539 -- operations. (NOTE: GHC currently does not throw
540 -- 'ArithException's except for 'DivideByZero').
541 | ArrayException ArrayException
542 -- ^Exceptions raised by array-related
543 -- operations. (NOTE: GHC currently does not throw
544 -- 'ArrayException's).
545 | AssertionFailed String
546 -- ^This exception is thrown by the
547 -- 'assert' operation when the condition
548 -- fails. The 'String' argument contains the
549 -- location of the assertion in the source program.
550 | AsyncException AsyncException
551 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
553 -- ^The current thread was executing a call to
554 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
555 -- because there are no other references to this 'MVar'.
557 -- ^There are no runnable threads, so the program is
558 -- deadlocked. The 'Deadlock' exception is
559 -- raised in the main thread only (see also: "Control.Concurrent").
560 | DynException Dynamic
561 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
563 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
564 -- argument of 'ErrorCall' is the string passed to 'error' when it was
566 | ExitException ExitCode
567 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
568 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
569 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
570 -- main thread will cause the program to be terminated with the given
572 | IOException IOException
573 -- ^These are the standard IO exceptions generated by
574 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
575 | NoMethodError String
576 -- ^An attempt was made to invoke a class method which has
577 -- no definition in this instance, and there was no default
578 -- definition given in the class declaration. GHC issues a
579 -- warning when you compile an instance which has missing
582 -- ^The current thread is stuck in an infinite loop. This
583 -- exception may or may not be thrown when the program is
585 | PatternMatchFail String
586 -- ^A pattern matching failure. The 'String' argument should contain a
587 -- descriptive message including the function name, source file
590 -- ^An attempt was made to evaluate a field of a record
591 -- for which no value was given at construction time. The
592 -- 'String' argument gives the location of the
593 -- record construction in the source program.
595 -- ^A field selection was attempted on a constructor that
596 -- doesn\'t have the requested field. This can happen with
597 -- multi-constructor records when one or more fields are
598 -- missing from some of the constructors. The
599 -- 'String' argument gives the location of the
600 -- record selection in the source program.
602 -- ^An attempt was made to update a field in a record,
603 -- where the record doesn\'t have the requested field. This can
604 -- only occur with multi-constructor records, when one or more
605 -- fields are missing from some of the constructors. The
606 -- 'String' argument gives the location of the
607 -- record update in the source program.
609 -- |The type of arithmetic exceptions
619 -- |Asynchronous exceptions
622 -- ^The current thread\'s stack exceeded its limit.
623 -- Since an exception has been raised, the thread\'s stack
624 -- will certainly be below its limit again, but the
625 -- programmer should take remedial action
628 -- ^The program\'s heap is reaching its limit, and
629 -- the program should take action to reduce the amount of
630 -- live data it has. Notes:
632 -- * It is undefined which thread receives this exception.
634 -- * GHC currently does not throw 'HeapOverflow' exceptions.
636 -- ^This exception is raised by another thread
637 -- calling 'Control.Concurrent.killThread', or by the system
638 -- if it needs to terminate the thread for some
642 -- | Exceptions generated by array operations
644 = IndexOutOfBounds String
645 -- ^An attempt was made to index an array outside
646 -- its declared bounds.
647 | UndefinedElement String
648 -- ^An attempt was made to evaluate an element of an
649 -- array that had not been initialized.
652 stackOverflow, heapOverflow :: Exception -- for the RTS
653 stackOverflow = AsyncException StackOverflow
654 heapOverflow = AsyncException HeapOverflow
656 instance Show ArithException where
657 showsPrec _ Overflow = showString "arithmetic overflow"
658 showsPrec _ Underflow = showString "arithmetic underflow"
659 showsPrec _ LossOfPrecision = showString "loss of precision"
660 showsPrec _ DivideByZero = showString "divide by zero"
661 showsPrec _ Denormal = showString "denormal"
663 instance Show AsyncException where
664 showsPrec _ StackOverflow = showString "stack overflow"
665 showsPrec _ HeapOverflow = showString "heap overflow"
666 showsPrec _ ThreadKilled = showString "thread killed"
668 instance Show ArrayException where
669 showsPrec _ (IndexOutOfBounds s)
670 = showString "array index out of range"
671 . (if not (null s) then showString ": " . showString s
673 showsPrec _ (UndefinedElement s)
674 = showString "undefined array element"
675 . (if not (null s) then showString ": " . showString s
678 instance Show Exception where
679 showsPrec _ (IOException err) = shows err
680 showsPrec _ (ArithException err) = shows err
681 showsPrec _ (ArrayException err) = shows err
682 showsPrec _ (ErrorCall err) = showString err
683 showsPrec _ (ExitException err) = showString "exit: " . shows err
684 showsPrec _ (NoMethodError err) = showString err
685 showsPrec _ (PatternMatchFail err) = showString err
686 showsPrec _ (RecSelError err) = showString err
687 showsPrec _ (RecConError err) = showString err
688 showsPrec _ (RecUpdError err) = showString err
689 showsPrec _ (AssertionFailed err) = showString err
690 showsPrec _ (DynException _err) = showString "unknown exception"
691 showsPrec _ (AsyncException e) = shows e
692 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
693 showsPrec _ (NonTermination) = showString "<<loop>>"
694 showsPrec _ (Deadlock) = showString "<<deadlock>>"
696 instance Eq Exception where
697 IOException e1 == IOException e2 = e1 == e2
698 ArithException e1 == ArithException e2 = e1 == e2
699 ArrayException e1 == ArrayException e2 = e1 == e2
700 ErrorCall e1 == ErrorCall e2 = e1 == e2
701 ExitException e1 == ExitException e2 = e1 == e2
702 NoMethodError e1 == NoMethodError e2 = e1 == e2
703 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
704 RecSelError e1 == RecSelError e2 = e1 == e2
705 RecConError e1 == RecConError e2 = e1 == e2
706 RecUpdError e1 == RecUpdError e2 = e1 == e2
707 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
708 DynException _ == DynException _ = False -- incomparable
709 AsyncException e1 == AsyncException e2 = e1 == e2
710 BlockedOnDeadMVar == BlockedOnDeadMVar = True
711 NonTermination == NonTermination = True
712 Deadlock == Deadlock = True
715 -- -----------------------------------------------------------------------------
718 -- We need it here because it is used in ExitException in the
719 -- Exception datatype (above).
722 = ExitSuccess -- ^ indicates successful termination;
724 -- ^ indicates program failure with an exit code.
725 -- The exact interpretation of the code is
726 -- operating-system dependent. In particular, some values
727 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
728 deriving (Eq, Ord, Read, Show)
730 -- --------------------------------------------------------------------------
733 -- | Throw an exception. Exceptions may be thrown from purely
734 -- functional code, but may only be caught within the 'IO' monad.
735 throw :: Exception -> a
736 throw exception = raise# exception
738 -- | A variant of 'throw' that can be used within the 'IO' monad.
740 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
741 -- two functions are subtly different:
743 -- > throw e `seq` return () ===> throw e
744 -- > throwIO e `seq` return () ===> return ()
746 -- The first example will cause the exception @e@ to be raised,
747 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
748 -- an exception to be raised when it is used within the 'IO' monad.
749 -- The 'throwIO' variant should be used in preference to 'throw' to
750 -- raise an exception within the 'IO' monad because it guarantees
751 -- ordering with respect to other 'IO' operations, whereas 'throw'
753 throwIO :: Exception -> IO a
754 throwIO err = IO $ raiseIO# err
756 ioException :: IOException -> IO a
757 ioException err = IO $ raiseIO# (IOException err)
759 -- | Raise an 'IOError' in the 'IO' monad.
760 ioError :: IOError -> IO a
761 ioError = ioException
763 -- ---------------------------------------------------------------------------
766 -- | The Haskell 98 type for exceptions in the 'IO' monad.
767 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
768 -- For a more general type of exception, including also those that arise
769 -- in pure code, see 'Control.Exception.Exception'.
771 -- In Haskell 98, this is an opaque type.
772 type IOError = IOException
774 -- |Exceptions that occur in the @IO@ monad.
775 -- An @IOException@ records a more specific error type, a descriptive
776 -- string and maybe the handle that was used when the error was
780 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
782 ioe_type :: IOErrorType, -- what it was.
783 ioe_location :: String, -- location.
784 ioe_description :: String, -- error type specific information.
785 ioe_filename :: Maybe FilePath -- filename the error is related to.
788 instance Eq IOException where
789 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
790 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
792 -- | An abstract type that contains a value for each variant of 'IOError'.
804 | UnsatisfiedConstraints
811 | UnsupportedOperation
815 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
817 instance Eq IOErrorType where
820 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
821 _ -> getTag x ==# getTag y
823 instance Show IOErrorType where
827 AlreadyExists -> "already exists"
828 NoSuchThing -> "does not exist"
829 ResourceBusy -> "resource busy"
830 ResourceExhausted -> "resource exhausted"
832 IllegalOperation -> "illegal operation"
833 PermissionDenied -> "permission denied"
834 UserError -> "user error"
835 HardwareFault -> "hardware fault"
836 InappropriateType -> "inappropriate type"
837 Interrupted -> "interrupted"
838 InvalidArgument -> "invalid argument"
839 OtherError -> "failed"
840 ProtocolError -> "protocol error"
841 ResourceVanished -> "resource vanished"
842 SystemError -> "system error"
843 TimeExpired -> "timeout"
844 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
845 UnsupportedOperation -> "unsupported operation"
846 DynIOError{} -> "unknown IO error"
848 -- | Construct an 'IOError' value with a string describing the error.
849 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
850 -- 'userError', thus:
852 -- > instance Monad IO where
854 -- > fail s = ioError (userError s)
856 userError :: String -> IOError
857 userError str = IOError Nothing UserError "" str Nothing
859 -- ---------------------------------------------------------------------------
862 instance Show IOException where
863 showsPrec p (IOError hdl iot loc s fn) =
865 Nothing -> case hdl of
867 Just h -> showsPrec p h . showString ": "
868 Just name -> showString name . showString ": ") .
871 _ -> showString loc . showString ": ") .
875 _ -> showString " (" . showString s . showString ")")
877 -- -----------------------------------------------------------------------------
880 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
881 deriving (Eq, Ord, Ix, Enum, Read, Show)