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 is a technical device to keep the
117 -- state used by such computations separate from those inside 'runST'.
118 stToIO :: ST RealWorld a -> IO a
121 ioToST :: IO a -> ST RealWorld a
122 ioToST (IO m) = (ST m)
124 -- ---------------------------------------------------------------------------
125 -- Unsafe IO operations
128 This is the \"back door\" into the 'IO' monad, allowing
129 'IO' computation to be performed at any time. For
130 this to be safe, the 'IO' computation should be
131 free of side effects and independent of its environment.
133 If the I\/O computation wrapped in 'unsafePerformIO'
134 performs side effects, then the relative order in which those side
135 effects take place (relative to the main I\/O trunk, or other calls to
136 'unsafePerformIO') is indeterminate. You have to be careful when
137 writing and compiling modules that use 'unsafePerformIO':
139 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
140 that calls 'unsafePerformIO'. If the call is inlined,
141 the I\/O may be performed more than once.
143 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
144 elimination being performed on the module, which might combine
145 two side effects that were meant to be separate. A good example
146 is using multiple global variables (like @test@ in the example below).
148 * Make sure that the either you switch off let-floating, or that the
149 call to 'unsafePerformIO' cannot float outside a lambda. For example,
152 f x = unsafePerformIO (newIORef [])
154 you may get only one reference cell shared between all calls to @f@.
157 f x = unsafePerformIO (newIORef [x])
159 because now it can't float outside the lambda.
161 It is less well known that
162 'unsafePerformIO' is not type safe. For example:
165 > test = unsafePerformIO $ newIORef []
168 > writeIORef test [42]
169 > bang \<- readIORef test
170 > print (bang :: [Char])
172 This program will core dump. This problem with polymorphic references
173 is well known in the ML community, and does not arise with normal
174 monadic use of references. There is no easy way to make it impossible
175 once you use 'unsafePerformIO'. Indeed, it is
176 possible to write @coerce :: a -> b@ with the
177 help of 'unsafePerformIO'. So be careful!
179 {-# NOINLINE unsafePerformIO #-}
180 unsafePerformIO :: IO a -> a
181 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
184 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
185 When passed a value of type @IO a@, the 'IO' will only be performed
186 when the value of the @a@ is demanded. This is used to implement lazy
187 file reading, see 'System.IO.hGetContents'.
189 {-# NOINLINE unsafeInterleaveIO #-}
190 unsafeInterleaveIO :: IO a -> IO a
191 unsafeInterleaveIO (IO m)
193 r = case m s of (# _, res #) -> res
197 -- ---------------------------------------------------------------------------
200 data MVar a = MVar (MVar# RealWorld a)
202 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
203 for communication between concurrent threads. It can be thought of
204 as a a box, which may be empty or full.
207 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
208 instance Eq (MVar a) where
209 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
211 -- A Handle is represented by (a reference to) a record
212 -- containing the state of the I/O port/device. We record
213 -- the following pieces of info:
215 -- * type (read,write,closed etc.)
216 -- * the underlying file descriptor
218 -- * buffer, and spare buffers
219 -- * user-friendly name (usually the
220 -- FilePath used when IO.openFile was called)
222 -- Note: when a Handle is garbage collected, we want to flush its buffer
223 -- and close the OS file handle, so as to free up a (precious) resource.
225 -- | Haskell defines operations to read and write characters from and to files,
226 -- represented by values of type @Handle@. Each value of this type is a
227 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
228 -- with file system objects. A handle has at least the following properties:
230 -- * whether it manages input or output or both;
232 -- * whether it is /open/, /closed/ or /semi-closed/;
234 -- * whether the object is seekable;
236 -- * whether buffering is disabled, or enabled on a line or block basis;
238 -- * a buffer (whose length may be zero).
240 -- Most handles will also have a current I\/O position indicating where the next
241 -- input or output operation will occur. A handle is /readable/ if it
242 -- manages only input or both input and output; likewise, it is /writable/ if
243 -- it manages only output or both input and output. A handle is /open/ when
245 -- Once it is closed it can no longer be used for either input or output,
246 -- though an implementation cannot re-use its storage while references
247 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
248 -- produced by showing a handle is system dependent; it should include
249 -- enough information to identify the handle for debugging. A handle is
250 -- equal according to '==' only to itself; no attempt
251 -- is made to compare the internal state of different handles for equality.
254 = FileHandle -- A normal handle to a file
255 FilePath -- the file (invariant)
258 | DuplexHandle -- A handle to a read/write stream
259 FilePath -- file for a FIFO, otherwise some
260 -- descriptive string.
261 !(MVar Handle__) -- The read side
262 !(MVar Handle__) -- The write side
265 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
268 instance Eq Handle where
269 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
270 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
273 type FD = Int -- XXX ToDo: should be CInt
277 haFD :: !FD, -- file descriptor
278 haType :: HandleType, -- type (read/write/append etc.)
279 haIsBin :: Bool, -- binary mode?
280 haIsStream :: Bool, -- is this a stream handle?
281 haBufferMode :: BufferMode, -- buffer contains read/write data?
282 haBuffer :: !(IORef Buffer), -- the current buffer
283 haBuffers :: !(IORef BufferList), -- spare buffers
284 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
288 -- ---------------------------------------------------------------------------
291 -- The buffer is represented by a mutable variable containing a
292 -- record, where the record contains the raw buffer and the start/end
293 -- points of the filled portion. We use a mutable variable so that
294 -- the common operation of writing (or reading) some data from (to)
295 -- the buffer doesn't need to modify, and hence copy, the handle
296 -- itself, it just updates the buffer.
298 -- There will be some allocation involved in a simple hPutChar in
299 -- order to create the new Buffer structure (below), but this is
300 -- relatively small, and this only has to be done once per write
303 -- The buffer contains its size - we could also get the size by
304 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
305 -- to be rounded up to the nearest Word.
307 type RawBuffer = MutableByteArray# RealWorld
309 -- INVARIANTS on a Buffer:
311 -- * A handle *always* has a buffer, even if it is only 1 character long
312 -- (an unbuffered handle needs a 1 character buffer in order to support
313 -- hLookAhead and hIsEOF).
315 -- * if r == w, then r == 0 && w == 0
316 -- * if state == WriteBuffer, then r == 0
317 -- * a write buffer is never full. If an operation
318 -- fills up the buffer, it will always flush it before
320 -- * a read buffer may be full as a result of hLookAhead. In normal
321 -- operation, a read buffer always has at least one character of space.
329 bufState :: BufferState
332 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
334 -- we keep a few spare buffers around in a handle to avoid allocating
335 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
336 -- same size as the main buffer.
339 | BufferListCons RawBuffer BufferList
342 bufferIsWritable :: Buffer -> Bool
343 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
344 bufferIsWritable _other = False
346 bufferEmpty :: Buffer -> Bool
347 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
349 -- only makes sense for a write buffer
350 bufferFull :: Buffer -> Bool
351 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
353 -- Internally, we classify handles as being one
364 isReadableHandleType ReadHandle = True
365 isReadableHandleType ReadWriteHandle = True
366 isReadableHandleType _ = False
368 isWritableHandleType AppendHandle = True
369 isWritableHandleType WriteHandle = True
370 isWritableHandleType ReadWriteHandle = True
371 isWritableHandleType _ = False
373 -- | File and directory names are values of type 'String', whose precise
374 -- meaning is operating system dependent. Files can be opened, yielding a
375 -- handle which can then be used to operate on the contents of that file.
377 type FilePath = String
379 -- ---------------------------------------------------------------------------
382 -- | Three kinds of buffering are supported: line-buffering,
383 -- block-buffering or no-buffering. These modes have the following
384 -- effects. For output, items are written out, or /flushed/,
385 -- from the internal buffer according to the buffer mode:
387 -- * /line-buffering/: the entire output buffer is flushed
388 -- whenever a newline is output, the buffer overflows,
389 -- a 'System.IO.hFlush' is issued, or the handle is closed.
391 -- * /block-buffering/: the entire buffer is written out whenever it
392 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
394 -- * /no-buffering/: output is written immediately, and never stored
397 -- An implementation is free to flush the buffer more frequently,
398 -- but not less frequently, than specified above.
399 -- The output buffer is emptied as soon as it has been written out.
401 -- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
403 -- * /line-buffering/: when the buffer for the handle is not empty,
404 -- the next item is obtained from the buffer; otherwise, when the
405 -- buffer is empty, characters up to and including the next newline
406 -- character are read into the buffer. No characters are available
407 -- until the newline character is available or the buffer is full.
409 -- * /block-buffering/: when the buffer for the handle becomes empty,
410 -- the next block of data is read into the buffer.
412 -- * /no-buffering/: the next input item is read and returned.
413 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
414 -- handle may require a one-character buffer.
416 -- The default buffering mode when a handle is opened is
417 -- implementation-dependent and may depend on the file system object
418 -- which is attached to that handle.
419 -- For most implementations, physical files will normally be block-buffered
420 -- and terminals will normally be line-buffered.
423 = NoBuffering -- ^ buffering is disabled if possible.
425 -- ^ line-buffering should be enabled if possible.
426 | BlockBuffering (Maybe Int)
427 -- ^ block-buffering should be enabled if possible.
428 -- The size of the buffer is @n@ items if the argument
429 -- is 'Just' @n@ and is otherwise implementation-dependent.
430 deriving (Eq, Ord, Read, Show)
432 -- ---------------------------------------------------------------------------
435 -- |A mutable variable in the 'IO' monad
436 newtype IORef a = IORef (STRef RealWorld a)
438 -- explicit instance because Haddock can't figure out a derived one
439 instance Eq (IORef a) where
440 IORef x == IORef y = x == y
442 -- |Build a new 'IORef'
443 newIORef :: a -> IO (IORef a)
444 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
446 -- |Read the value of an 'IORef'
447 readIORef :: IORef a -> IO a
448 readIORef (IORef var) = stToIO (readSTRef var)
450 -- |Write a new value into an 'IORef'
451 writeIORef :: IORef a -> a -> IO ()
452 writeIORef (IORef var) v = stToIO (writeSTRef var v)
454 -- ---------------------------------------------------------------------------
455 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
456 -- The type arguments are as follows:
458 -- * @i@: the index type of the array (should be an instance of 'Ix')
460 -- * @e@: the element type of the array.
464 newtype IOArray i e = IOArray (STArray RealWorld i e)
466 -- explicit instance because Haddock can't figure out a derived one
467 instance Eq (IOArray i e) where
468 IOArray x == IOArray y = x == y
470 -- |Build a new 'IOArray'
471 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
472 {-# INLINE newIOArray #-}
473 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
475 -- | Read a value from an 'IOArray'
476 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
477 {-# INLINE unsafeReadIOArray #-}
478 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
480 -- | Write a new value into an 'IOArray'
481 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
482 {-# INLINE unsafeWriteIOArray #-}
483 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
485 -- | Read a value from an 'IOArray'
486 readIOArray :: Ix i => IOArray i e -> i -> IO e
487 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
489 -- | Write a new value into an 'IOArray'
490 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
491 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
494 -- ---------------------------------------------------------------------------
495 -- Show instance for Handles
497 -- handle types are 'show'n when printing error msgs, so
498 -- we provide a more user-friendly Show instance for it
499 -- than the derived one.
501 instance Show HandleType where
504 ClosedHandle -> showString "closed"
505 SemiClosedHandle -> showString "semi-closed"
506 ReadHandle -> showString "readable"
507 WriteHandle -> showString "writable"
508 AppendHandle -> showString "writable (append)"
509 ReadWriteHandle -> showString "read-writable"
511 instance Show Handle where
512 showsPrec p (FileHandle file _) = showHandle file
513 showsPrec p (DuplexHandle file _ _) = showHandle file
515 showHandle file = showString "{handle: " . showString file . showString "}"
517 -- ------------------------------------------------------------------------
518 -- Exception datatype and operations
520 -- |The type of exceptions. Every kind of system-generated exception
521 -- has a constructor in the 'Exception' type, and values of other
522 -- types may be injected into 'Exception' by coercing them to
523 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
524 -- "Control.Exception\#DynamicExceptions").
526 = ArithException ArithException
527 -- ^Exceptions raised by arithmetic
528 -- operations. (NOTE: GHC currently does not throw
529 -- 'ArithException's except for 'DivideByZero').
530 | ArrayException ArrayException
531 -- ^Exceptions raised by array-related
532 -- operations. (NOTE: GHC currently does not throw
533 -- 'ArrayException's).
534 | AssertionFailed String
535 -- ^This exception is thrown by the
536 -- 'assert' operation when the condition
537 -- fails. The 'String' argument contains the
538 -- location of the assertion in the source program.
539 | AsyncException AsyncException
540 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
542 -- ^The current thread was executing a call to
543 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
544 -- because there are no other references to this 'MVar'.
546 -- ^There are no runnable threads, so the program is
547 -- deadlocked. The 'Deadlock' exception is
548 -- raised in the main thread only (see also: "Control.Concurrent").
549 | DynException Dynamic
550 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
552 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
553 -- argument of 'ErrorCall' is the string passed to 'error' when it was
555 | ExitException ExitCode
556 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
557 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
558 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
559 -- main thread will cause the program to be terminated with the given
561 | IOException IOException
562 -- ^These are the standard IO exceptions generated by
563 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
564 | NoMethodError String
565 -- ^An attempt was made to invoke a class method which has
566 -- no definition in this instance, and there was no default
567 -- definition given in the class declaration. GHC issues a
568 -- warning when you compile an instance which has missing
571 -- ^The current thread is stuck in an infinite loop. This
572 -- exception may or may not be thrown when the program is
574 | PatternMatchFail String
575 -- ^A pattern matching failure. The 'String' argument should contain a
576 -- descriptive message including the function name, source file
579 -- ^An attempt was made to evaluate a field of a record
580 -- for which no value was given at construction time. The
581 -- 'String' argument gives the location of the
582 -- record construction in the source program.
584 -- ^A field selection was attempted on a constructor that
585 -- doesn\'t have the requested field. This can happen with
586 -- multi-constructor records when one or more fields are
587 -- missing from some of the constructors. The
588 -- 'String' argument gives the location of the
589 -- record selection in the source program.
591 -- ^An attempt was made to update a field in a record,
592 -- where the record doesn\'t have the requested field. This can
593 -- only occur with multi-constructor records, when one or more
594 -- fields are missing from some of the constructors. The
595 -- 'String' argument gives the location of the
596 -- record update in the source program.
598 -- |The type of arithmetic exceptions
608 -- |Asynchronous exceptions
611 -- ^The current thread\'s stack exceeded its limit.
612 -- Since an exception has been raised, the thread\'s stack
613 -- will certainly be below its limit again, but the
614 -- programmer should take remedial action
617 -- ^The program\'s heap is reaching its limit, and
618 -- the program should take action to reduce the amount of
619 -- live data it has. Notes:
621 -- * It is undefined which thread receives this exception.
623 -- * GHC currently does not throw 'HeapOverflow' exceptions.
625 -- ^This exception is raised by another thread
626 -- calling 'Control.Concurrent.killThread', or by the system
627 -- if it needs to terminate the thread for some
631 -- | Exceptions generated by array operations
633 = IndexOutOfBounds String
634 -- ^An attempt was made to index an array outside
635 -- its declared bounds.
636 | UndefinedElement String
637 -- ^An attempt was made to evaluate an element of an
638 -- array that had not been initialized.
641 stackOverflow, heapOverflow :: Exception -- for the RTS
642 stackOverflow = AsyncException StackOverflow
643 heapOverflow = AsyncException HeapOverflow
645 instance Show ArithException where
646 showsPrec _ Overflow = showString "arithmetic overflow"
647 showsPrec _ Underflow = showString "arithmetic underflow"
648 showsPrec _ LossOfPrecision = showString "loss of precision"
649 showsPrec _ DivideByZero = showString "divide by zero"
650 showsPrec _ Denormal = showString "denormal"
652 instance Show AsyncException where
653 showsPrec _ StackOverflow = showString "stack overflow"
654 showsPrec _ HeapOverflow = showString "heap overflow"
655 showsPrec _ ThreadKilled = showString "thread killed"
657 instance Show ArrayException where
658 showsPrec _ (IndexOutOfBounds s)
659 = showString "array index out of range"
660 . (if not (null s) then showString ": " . showString s
662 showsPrec _ (UndefinedElement s)
663 = showString "undefined array element"
664 . (if not (null s) then showString ": " . showString s
667 instance Show Exception where
668 showsPrec _ (IOException err) = shows err
669 showsPrec _ (ArithException err) = shows err
670 showsPrec _ (ArrayException err) = shows err
671 showsPrec _ (ErrorCall err) = showString err
672 showsPrec _ (ExitException err) = showString "exit: " . shows err
673 showsPrec _ (NoMethodError err) = showString err
674 showsPrec _ (PatternMatchFail err) = showString err
675 showsPrec _ (RecSelError err) = showString err
676 showsPrec _ (RecConError err) = showString err
677 showsPrec _ (RecUpdError err) = showString err
678 showsPrec _ (AssertionFailed err) = showString err
679 showsPrec _ (DynException _err) = showString "unknown exception"
680 showsPrec _ (AsyncException e) = shows e
681 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
682 showsPrec _ (NonTermination) = showString "<<loop>>"
683 showsPrec _ (Deadlock) = showString "<<deadlock>>"
685 instance Eq Exception where
686 IOException e1 == IOException e2 = e1 == e2
687 ArithException e1 == ArithException e2 = e1 == e2
688 ArrayException e1 == ArrayException e2 = e1 == e2
689 ErrorCall e1 == ErrorCall e2 = e1 == e2
690 ExitException e1 == ExitException e2 = e1 == e2
691 NoMethodError e1 == NoMethodError e2 = e1 == e2
692 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
693 RecSelError e1 == RecSelError e2 = e1 == e2
694 RecConError e1 == RecConError e2 = e1 == e2
695 RecUpdError e1 == RecUpdError e2 = e1 == e2
696 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
697 DynException _ == DynException _ = False -- incomparable
698 AsyncException e1 == AsyncException e2 = e1 == e2
699 BlockedOnDeadMVar == BlockedOnDeadMVar = True
700 NonTermination == NonTermination = True
701 Deadlock == Deadlock = True
704 -- -----------------------------------------------------------------------------
707 -- We need it here because it is used in ExitException in the
708 -- Exception datatype (above).
711 = ExitSuccess -- ^ indicates successful termination;
713 -- ^ indicates program failure with an exit code.
714 -- The exact interpretation of the code is
715 -- operating-system dependent. In particular, some values
716 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
717 deriving (Eq, Ord, Read, Show)
719 -- --------------------------------------------------------------------------
722 -- | Throw an exception. Exceptions may be thrown from purely
723 -- functional code, but may only be caught within the 'IO' monad.
724 throw :: Exception -> a
725 throw exception = raise# exception
727 -- | A variant of 'throw' that can be used within the 'IO' monad.
729 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
730 -- two functions are subtly different:
732 -- > throw e `seq` return () ===> throw e
733 -- > throwIO e `seq` return () ===> return ()
735 -- The first example will cause the exception @e@ to be raised,
736 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
737 -- an exception to be raised when it is used within the 'IO' monad.
738 -- The 'throwIO' variant should be used in preference to 'throw' to
739 -- raise an exception within the 'IO' monad because it guarantees
740 -- ordering with respect to other 'IO' operations, whereas 'throw'
742 throwIO :: Exception -> IO a
743 throwIO err = IO $ raiseIO# err
745 ioException :: IOException -> IO a
746 ioException err = IO $ raiseIO# (IOException err)
748 -- | Raise an 'IOError' in the 'IO' monad.
749 ioError :: IOError -> IO a
750 ioError = ioException
752 -- ---------------------------------------------------------------------------
755 -- | The Haskell 98 type for exceptions in the 'IO' monad.
756 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
757 -- For a more general type of exception, including also those that arise
758 -- in pure code, see 'Control.Exception.Exception'.
760 -- In Haskell 98, this is an opaque type.
761 type IOError = IOException
763 -- |Exceptions that occur in the @IO@ monad.
764 -- An @IOException@ records a more specific error type, a descriptive
765 -- string and maybe the handle that was used when the error was
769 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
771 ioe_type :: IOErrorType, -- what it was.
772 ioe_location :: String, -- location.
773 ioe_description :: String, -- error type specific information.
774 ioe_filename :: Maybe FilePath -- filename the error is related to.
777 instance Eq IOException where
778 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
779 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
781 -- | An abstract type that contains a value for each variant of 'IOError'.
793 | UnsatisfiedConstraints
800 | UnsupportedOperation
804 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
806 instance Eq IOErrorType where
809 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
810 _ -> getTag x ==# getTag y
812 instance Show IOErrorType where
816 AlreadyExists -> "already exists"
817 NoSuchThing -> "does not exist"
818 ResourceBusy -> "resource busy"
819 ResourceExhausted -> "resource exhausted"
821 IllegalOperation -> "illegal operation"
822 PermissionDenied -> "permission denied"
823 UserError -> "user error"
824 HardwareFault -> "hardware fault"
825 InappropriateType -> "inappropriate type"
826 Interrupted -> "interrupted"
827 InvalidArgument -> "invalid argument"
828 OtherError -> "failed"
829 ProtocolError -> "protocol error"
830 ResourceVanished -> "resource vanished"
831 SystemError -> "system error"
832 TimeExpired -> "timeout"
833 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
834 UnsupportedOperation -> "unsupported operation"
835 DynIOError{} -> "unknown IO error"
837 -- | Construct an 'IOError' value with a string describing the error.
838 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
839 -- 'userError', thus:
841 -- > instance Monad IO where
843 -- > fail s = ioError (userError s)
845 userError :: String -> IOError
846 userError str = IOError Nothing UserError "" str Nothing
848 -- ---------------------------------------------------------------------------
851 instance Show IOException where
852 showsPrec p (IOError hdl iot loc s fn) =
854 Nothing -> case hdl of
856 Just h -> showsPrec p h . showString ": "
857 Just name -> showString name . showString ": ") .
860 _ -> showString loc . showString ": ") .
864 _ -> showString " (" . showString s . showString ")")
866 -- -----------------------------------------------------------------------------
869 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
870 deriving (Eq, Ord, Ix, Enum, Read, Show)