2 {-# OPTIONS_GHC -fno-implicit-prelude -funbox-strict-fields #-}
3 {-# OPTIONS_HADDOCK hide #-}
4 -----------------------------------------------------------------------------
7 -- Copyright : (c) The University of Glasgow 1994-2002
8 -- License : see libraries/base/LICENSE
10 -- Maintainer : cvs-ghc@haskell.org
11 -- Stability : internal
12 -- Portability : non-portable (GHC Extensions)
14 -- Definitions for the 'IO' monad and its friends.
16 -----------------------------------------------------------------------------
20 IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
21 unsafePerformIO, unsafeInterleaveIO,
22 unsafeDupablePerformIO, unsafeDupableInterleaveIO,
25 -- To and from from ST
26 stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
29 IORef(..), newIORef, readIORef, writeIORef,
30 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
33 -- Handles, file descriptors,
35 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
36 isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
39 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
40 bufferIsWritable, bufferEmpty, bufferFull,
43 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
44 stackOverflow, heapOverflow, throw, throwIO, ioException,
45 IOError, IOException(..), IOErrorType(..), ioError, userError,
50 import GHC.Arr -- to derive Ix class
51 import GHC.Enum -- to derive Enum class
54 -- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
55 import Data.Maybe ( Maybe(..) )
59 import Foreign.C.Types (CInt)
62 import {-# SOURCE #-} Data.Typeable ( showsTypeRep )
63 import {-# SOURCE #-} Data.Dynamic ( Dynamic, dynTypeRep )
66 -- ---------------------------------------------------------------------------
70 The IO Monad is just an instance of the ST monad, where the state is
71 the real world. We use the exception mechanism (in GHC.Exception) to
72 implement IO exceptions.
74 NOTE: The IO representation is deeply wired in to various parts of the
75 system. The following list may or may not be exhaustive:
77 Compiler - types of various primitives in PrimOp.lhs
79 RTS - forceIO (StgMiscClosures.hc)
80 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
82 - raiseAsync (Schedule.c)
84 Prelude - GHC.IOBase.lhs, and several other places including
87 Libraries - parts of hslibs/lang.
93 A value of type @'IO' a@ is a computation which, when performed,
94 does some I\/O before returning a value of type @a@.
96 There is really only one way to \"perform\" an I\/O action: bind it to
97 @Main.main@ in your program. When your program is run, the I\/O will
98 be performed. It isn't possible to perform I\/O from an arbitrary
99 function, unless that function is itself in the 'IO' monad and called
100 at some point, directly or indirectly, from @Main.main@.
102 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
103 or the '>>' and '>>=' operations from the 'Monad' class.
105 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
107 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
110 instance Functor IO where
111 fmap f x = x >>= (return . f)
113 instance Monad IO where
114 {-# INLINE return #-}
117 m >> k = m >>= \ _ -> k
118 return x = returnIO x
123 failIO :: String -> IO a
124 failIO s = ioError (userError s)
126 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
127 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
129 bindIO :: IO a -> (a -> IO b) -> IO b
130 bindIO (IO m) k = IO ( \ s ->
132 (# new_s, a #) -> unIO (k a) new_s
135 thenIO :: IO a -> IO b -> IO b
136 thenIO (IO m) k = IO ( \ s ->
138 (# new_s, a #) -> unIO k new_s
141 returnIO :: a -> IO a
142 returnIO x = IO (\ s -> (# s, x #))
144 -- ---------------------------------------------------------------------------
145 -- Coercions between IO and ST
147 -- | A monad transformer embedding strict state transformers in the 'IO'
148 -- monad. The 'RealWorld' parameter indicates that the internal state
149 -- used by the 'ST' computation is a special one supplied by the 'IO'
150 -- monad, and thus distinct from those used by invocations of 'runST'.
151 stToIO :: ST RealWorld a -> IO a
154 ioToST :: IO a -> ST RealWorld a
155 ioToST (IO m) = (ST m)
157 -- This relies on IO and ST having the same representation modulo the
158 -- constraint on the type of the state
160 unsafeIOToST :: IO a -> ST s a
161 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
163 unsafeSTToIO :: ST s a -> IO a
164 unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
166 -- ---------------------------------------------------------------------------
167 -- Unsafe IO operations
170 This is the \"back door\" into the 'IO' monad, allowing
171 'IO' computation to be performed at any time. For
172 this to be safe, the 'IO' computation should be
173 free of side effects and independent of its environment.
175 If the I\/O computation wrapped in 'unsafePerformIO'
176 performs side effects, then the relative order in which those side
177 effects take place (relative to the main I\/O trunk, or other calls to
178 'unsafePerformIO') is indeterminate. You have to be careful when
179 writing and compiling modules that use 'unsafePerformIO':
181 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
182 that calls 'unsafePerformIO'. If the call is inlined,
183 the I\/O may be performed more than once.
185 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
186 elimination being performed on the module, which might combine
187 two side effects that were meant to be separate. A good example
188 is using multiple global variables (like @test@ in the example below).
190 * Make sure that the either you switch off let-floating, or that the
191 call to 'unsafePerformIO' cannot float outside a lambda. For example,
194 f x = unsafePerformIO (newIORef [])
196 you may get only one reference cell shared between all calls to @f@.
199 f x = unsafePerformIO (newIORef [x])
201 because now it can't float outside the lambda.
203 It is less well known that
204 'unsafePerformIO' is not type safe. For example:
207 > test = unsafePerformIO $ newIORef []
210 > writeIORef test [42]
211 > bang <- readIORef test
212 > print (bang :: [Char])
214 This program will core dump. This problem with polymorphic references
215 is well known in the ML community, and does not arise with normal
216 monadic use of references. There is no easy way to make it impossible
217 once you use 'unsafePerformIO'. Indeed, it is
218 possible to write @coerce :: a -> b@ with the
219 help of 'unsafePerformIO'. So be careful!
221 unsafePerformIO :: IO a -> a
222 unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)
225 This version of 'unsafePerformIO' is slightly more efficient,
226 because it omits the check that the IO is only being performed by a
227 single thread. Hence, when you write 'unsafeDupablePerformIO',
228 there is a possibility that the IO action may be performed multiple
229 times (on a multiprocessor), and you should therefore ensure that
230 it gives the same results each time.
232 {-# NOINLINE unsafeDupablePerformIO #-}
233 unsafeDupablePerformIO :: IO a -> a
234 unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
236 -- Why do we NOINLINE unsafeDupablePerformIO? See the comment with
237 -- GHC.ST.runST. Essentially the issue is that the IO computation
238 -- inside unsafePerformIO must be atomic: it must either all run, or
239 -- not at all. If we let the compiler see the application of the IO
240 -- to realWorld#, it might float out part of the IO.
242 -- Why is there a call to 'lazy' in unsafeDupablePerformIO?
243 -- If we don't have it, the demand analyser discovers the following strictness
244 -- for unsafeDupablePerformIO: C(U(AV))
246 -- unsafeDupablePerformIO (\s -> let r = f x in
247 -- case writeIORef v r s of (# s1, _ #) ->
249 -- The strictness analyser will find that the binding for r is strict,
250 -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
251 -- doing the writeIORef. This actually makes tests/lib/should_run/memo002
254 -- Solution: don't expose the strictness of unsafeDupablePerformIO,
255 -- by hiding it with 'lazy'
258 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
259 When passed a value of type @IO a@, the 'IO' will only be performed
260 when the value of the @a@ is demanded. This is used to implement lazy
261 file reading, see 'System.IO.hGetContents'.
263 {-# INLINE unsafeInterleaveIO #-}
264 unsafeInterleaveIO :: IO a -> IO a
265 unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
267 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
268 -- state from this IO thread is passed explicitly to the interleaved
269 -- IO, so it cannot be floated out and shared.
271 {-# INLINE unsafeDupableInterleaveIO #-}
272 unsafeDupableInterleaveIO :: IO a -> IO a
273 unsafeDupableInterleaveIO (IO m)
275 r = case m s of (# _, res #) -> res
280 Ensures that the suspensions under evaluation by the current thread
281 are unique; that is, the current thread is not evaluating anything
282 that is also under evaluation by another thread that has also executed
285 This operation is used in the definition of 'unsafePerformIO' to
286 prevent the IO action from being executed multiple times, which is usually
290 noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)
292 -- ---------------------------------------------------------------------------
295 data MVar a = MVar (MVar# RealWorld a)
297 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
298 for communication between concurrent threads. It can be thought of
299 as a a box, which may be empty or full.
302 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
303 instance Eq (MVar a) where
304 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
306 -- A Handle is represented by (a reference to) a record
307 -- containing the state of the I/O port/device. We record
308 -- the following pieces of info:
310 -- * type (read,write,closed etc.)
311 -- * the underlying file descriptor
313 -- * buffer, and spare buffers
314 -- * user-friendly name (usually the
315 -- FilePath used when IO.openFile was called)
317 -- Note: when a Handle is garbage collected, we want to flush its buffer
318 -- and close the OS file handle, so as to free up a (precious) resource.
320 -- | Haskell defines operations to read and write characters from and to files,
321 -- represented by values of type @Handle@. Each value of this type is a
322 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
323 -- with file system objects. A handle has at least the following properties:
325 -- * whether it manages input or output or both;
327 -- * whether it is /open/, /closed/ or /semi-closed/;
329 -- * whether the object is seekable;
331 -- * whether buffering is disabled, or enabled on a line or block basis;
333 -- * a buffer (whose length may be zero).
335 -- Most handles will also have a current I\/O position indicating where the next
336 -- input or output operation will occur. A handle is /readable/ if it
337 -- manages only input or both input and output; likewise, it is /writable/ if
338 -- it manages only output or both input and output. A handle is /open/ when
340 -- Once it is closed it can no longer be used for either input or output,
341 -- though an implementation cannot re-use its storage while references
342 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
343 -- produced by showing a handle is system dependent; it should include
344 -- enough information to identify the handle for debugging. A handle is
345 -- equal according to '==' only to itself; no attempt
346 -- is made to compare the internal state of different handles for equality.
348 -- GHC note: a 'Handle' will be automatically closed when the garbage
349 -- collector detects that it has become unreferenced by the program.
350 -- However, relying on this behaviour is not generally recommended:
351 -- the garbage collector is unpredictable. If possible, use explicit
352 -- an explicit 'hClose' to close 'Handle's when they are no longer
353 -- required. GHC does not currently attempt to free up file
354 -- descriptors when they have run out, it is your responsibility to
355 -- ensure that this doesn't happen.
358 = FileHandle -- A normal handle to a file
359 FilePath -- the file (invariant)
362 | DuplexHandle -- A handle to a read/write stream
363 FilePath -- file for a FIFO, otherwise some
364 -- descriptive string.
365 !(MVar Handle__) -- The read side
366 !(MVar Handle__) -- The write side
369 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
372 instance Eq Handle where
373 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
374 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
381 haFD :: !FD, -- file descriptor
382 haType :: HandleType, -- type (read/write/append etc.)
383 haIsBin :: Bool, -- binary mode?
384 haIsStream :: Bool, -- Windows : is this a socket?
385 -- Unix : is O_NONBLOCK set?
386 haBufferMode :: BufferMode, -- buffer contains read/write data?
387 haBuffer :: !(IORef Buffer), -- the current buffer
388 haBuffers :: !(IORef BufferList), -- spare buffers
389 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
393 -- ---------------------------------------------------------------------------
396 -- The buffer is represented by a mutable variable containing a
397 -- record, where the record contains the raw buffer and the start/end
398 -- points of the filled portion. We use a mutable variable so that
399 -- the common operation of writing (or reading) some data from (to)
400 -- the buffer doesn't need to modify, and hence copy, the handle
401 -- itself, it just updates the buffer.
403 -- There will be some allocation involved in a simple hPutChar in
404 -- order to create the new Buffer structure (below), but this is
405 -- relatively small, and this only has to be done once per write
408 -- The buffer contains its size - we could also get the size by
409 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
410 -- to be rounded up to the nearest Word.
412 type RawBuffer = MutableByteArray# RealWorld
414 -- INVARIANTS on a Buffer:
416 -- * A handle *always* has a buffer, even if it is only 1 character long
417 -- (an unbuffered handle needs a 1 character buffer in order to support
418 -- hLookAhead and hIsEOF).
420 -- * if r == w, then r == 0 && w == 0
421 -- * if state == WriteBuffer, then r == 0
422 -- * a write buffer is never full. If an operation
423 -- fills up the buffer, it will always flush it before
425 -- * a read buffer may be full as a result of hLookAhead. In normal
426 -- operation, a read buffer always has at least one character of space.
434 bufState :: BufferState
437 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
439 -- we keep a few spare buffers around in a handle to avoid allocating
440 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
441 -- same size as the main buffer.
444 | BufferListCons RawBuffer BufferList
447 bufferIsWritable :: Buffer -> Bool
448 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
449 bufferIsWritable _other = False
451 bufferEmpty :: Buffer -> Bool
452 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
454 -- only makes sense for a write buffer
455 bufferFull :: Buffer -> Bool
456 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
458 -- Internally, we classify handles as being one
469 isReadableHandleType ReadHandle = True
470 isReadableHandleType ReadWriteHandle = True
471 isReadableHandleType _ = False
473 isWritableHandleType AppendHandle = True
474 isWritableHandleType WriteHandle = True
475 isWritableHandleType ReadWriteHandle = True
476 isWritableHandleType _ = False
478 isReadWriteHandleType ReadWriteHandle{} = True
479 isReadWriteHandleType _ = False
481 -- | File and directory names are values of type 'String', whose precise
482 -- meaning is operating system dependent. Files can be opened, yielding a
483 -- handle which can then be used to operate on the contents of that file.
485 type FilePath = String
487 -- ---------------------------------------------------------------------------
490 -- | Three kinds of buffering are supported: line-buffering,
491 -- block-buffering or no-buffering. These modes have the following
492 -- effects. For output, items are written out, or /flushed/,
493 -- from the internal buffer according to the buffer mode:
495 -- * /line-buffering/: the entire output buffer is flushed
496 -- whenever a newline is output, the buffer overflows,
497 -- a 'System.IO.hFlush' is issued, or the handle is closed.
499 -- * /block-buffering/: the entire buffer is written out whenever it
500 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
502 -- * /no-buffering/: output is written immediately, and never stored
505 -- An implementation is free to flush the buffer more frequently,
506 -- but not less frequently, than specified above.
507 -- The output buffer is emptied as soon as it has been written out.
509 -- Similarly, input occurs according to the buffer mode for the handle:
511 -- * /line-buffering/: when the buffer for the handle is not empty,
512 -- the next item is obtained from the buffer; otherwise, when the
513 -- buffer is empty, characters up to and including the next newline
514 -- character are read into the buffer. No characters are available
515 -- until the newline character is available or the buffer is full.
517 -- * /block-buffering/: when the buffer for the handle becomes empty,
518 -- the next block of data is read into the buffer.
520 -- * /no-buffering/: the next input item is read and returned.
521 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
522 -- handle may require a one-character buffer.
524 -- The default buffering mode when a handle is opened is
525 -- implementation-dependent and may depend on the file system object
526 -- which is attached to that handle.
527 -- For most implementations, physical files will normally be block-buffered
528 -- and terminals will normally be line-buffered.
531 = NoBuffering -- ^ buffering is disabled if possible.
533 -- ^ line-buffering should be enabled if possible.
534 | BlockBuffering (Maybe Int)
535 -- ^ block-buffering should be enabled if possible.
536 -- The size of the buffer is @n@ items if the argument
537 -- is 'Just' @n@ and is otherwise implementation-dependent.
538 deriving (Eq, Ord, Read, Show)
540 -- ---------------------------------------------------------------------------
543 -- |A mutable variable in the 'IO' monad
544 newtype IORef a = IORef (STRef RealWorld a)
546 -- explicit instance because Haddock can't figure out a derived one
547 instance Eq (IORef a) where
548 IORef x == IORef y = x == y
550 -- |Build a new 'IORef'
551 newIORef :: a -> IO (IORef a)
552 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
554 -- |Read the value of an 'IORef'
555 readIORef :: IORef a -> IO a
556 readIORef (IORef var) = stToIO (readSTRef var)
558 -- |Write a new value into an 'IORef'
559 writeIORef :: IORef a -> a -> IO ()
560 writeIORef (IORef var) v = stToIO (writeSTRef var v)
562 -- ---------------------------------------------------------------------------
563 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
564 -- The type arguments are as follows:
566 -- * @i@: the index type of the array (should be an instance of 'Ix')
568 -- * @e@: the element type of the array.
572 newtype IOArray i e = IOArray (STArray RealWorld i e)
574 -- explicit instance because Haddock can't figure out a derived one
575 instance Eq (IOArray i e) where
576 IOArray x == IOArray y = x == y
578 -- |Build a new 'IOArray'
579 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
580 {-# INLINE newIOArray #-}
581 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
583 -- | Read a value from an 'IOArray'
584 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
585 {-# INLINE unsafeReadIOArray #-}
586 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
588 -- | Write a new value into an 'IOArray'
589 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
590 {-# INLINE unsafeWriteIOArray #-}
591 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
593 -- | Read a value from an 'IOArray'
594 readIOArray :: Ix i => IOArray i e -> i -> IO e
595 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
597 -- | Write a new value into an 'IOArray'
598 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
599 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
602 -- ---------------------------------------------------------------------------
603 -- Show instance for Handles
605 -- handle types are 'show'n when printing error msgs, so
606 -- we provide a more user-friendly Show instance for it
607 -- than the derived one.
609 instance Show HandleType where
612 ClosedHandle -> showString "closed"
613 SemiClosedHandle -> showString "semi-closed"
614 ReadHandle -> showString "readable"
615 WriteHandle -> showString "writable"
616 AppendHandle -> showString "writable (append)"
617 ReadWriteHandle -> showString "read-writable"
619 instance Show Handle where
620 showsPrec p (FileHandle file _) = showHandle file
621 showsPrec p (DuplexHandle file _ _) = showHandle file
623 showHandle file = showString "{handle: " . showString file . showString "}"
625 -- ------------------------------------------------------------------------
626 -- Exception datatype and operations
628 -- |The type of exceptions. Every kind of system-generated exception
629 -- has a constructor in the 'Exception' type, and values of other
630 -- types may be injected into 'Exception' by coercing them to
631 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
632 -- "Control.Exception\#DynamicExceptions").
634 = ArithException ArithException
635 -- ^Exceptions raised by arithmetic
636 -- operations. (NOTE: GHC currently does not throw
637 -- 'ArithException's except for 'DivideByZero').
638 | ArrayException ArrayException
639 -- ^Exceptions raised by array-related
640 -- operations. (NOTE: GHC currently does not throw
641 -- 'ArrayException's).
642 | AssertionFailed String
643 -- ^This exception is thrown by the
644 -- 'assert' operation when the condition
645 -- fails. The 'String' argument contains the
646 -- location of the assertion in the source program.
647 | AsyncException AsyncException
648 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
650 -- ^The current thread was executing a call to
651 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
652 -- because there are no other references to this 'MVar'.
653 | BlockedIndefinitely
654 -- ^The current thread was waiting to retry an atomic memory transaction
655 -- that could never become possible to complete because there are no other
656 -- threads referring to any of the TVars involved.
658 -- ^The runtime detected an attempt to nest one STM transaction
659 -- inside another one, presumably due to the use of
660 -- 'unsafePeformIO' with 'atomically'.
662 -- ^There are no runnable threads, so the program is
663 -- deadlocked. The 'Deadlock' exception is
664 -- raised in the main thread only (see also: "Control.Concurrent").
665 | DynException Dynamic
666 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
668 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
669 -- argument of 'ErrorCall' is the string passed to 'error' when it was
671 | ExitException ExitCode
672 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
673 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
674 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
675 -- main thread will cause the program to be terminated with the given
677 | IOException IOException
678 -- ^These are the standard IO exceptions generated by
679 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
680 | NoMethodError String
681 -- ^An attempt was made to invoke a class method which has
682 -- no definition in this instance, and there was no default
683 -- definition given in the class declaration. GHC issues a
684 -- warning when you compile an instance which has missing
687 -- ^The current thread is stuck in an infinite loop. This
688 -- exception may or may not be thrown when the program is
690 | PatternMatchFail String
691 -- ^A pattern matching failure. The 'String' argument should contain a
692 -- descriptive message including the function name, source file
695 -- ^An attempt was made to evaluate a field of a record
696 -- for which no value was given at construction time. The
697 -- 'String' argument gives the location of the
698 -- record construction in the source program.
700 -- ^A field selection was attempted on a constructor that
701 -- doesn\'t have the requested field. This can happen with
702 -- multi-constructor records when one or more fields are
703 -- missing from some of the constructors. The
704 -- 'String' argument gives the location of the
705 -- record selection in the source program.
707 -- ^An attempt was made to update a field in a record,
708 -- where the record doesn\'t have the requested field. This can
709 -- only occur with multi-constructor records, when one or more
710 -- fields are missing from some of the constructors. The
711 -- 'String' argument gives the location of the
712 -- record update in the source program.
714 -- |The type of arithmetic exceptions
724 -- |Asynchronous exceptions
727 -- ^The current thread\'s stack exceeded its limit.
728 -- Since an exception has been raised, the thread\'s stack
729 -- will certainly be below its limit again, but the
730 -- programmer should take remedial action
733 -- ^The program\'s heap is reaching its limit, and
734 -- the program should take action to reduce the amount of
735 -- live data it has. Notes:
737 -- * It is undefined which thread receives this exception.
739 -- * GHC currently does not throw 'HeapOverflow' exceptions.
741 -- ^This exception is raised by another thread
742 -- calling 'Control.Concurrent.killThread', or by the system
743 -- if it needs to terminate the thread for some
747 -- | Exceptions generated by array operations
749 = IndexOutOfBounds String
750 -- ^An attempt was made to index an array outside
751 -- its declared bounds.
752 | UndefinedElement String
753 -- ^An attempt was made to evaluate an element of an
754 -- array that had not been initialized.
757 stackOverflow, heapOverflow :: Exception -- for the RTS
758 stackOverflow = AsyncException StackOverflow
759 heapOverflow = AsyncException HeapOverflow
761 instance Show ArithException where
762 showsPrec _ Overflow = showString "arithmetic overflow"
763 showsPrec _ Underflow = showString "arithmetic underflow"
764 showsPrec _ LossOfPrecision = showString "loss of precision"
765 showsPrec _ DivideByZero = showString "divide by zero"
766 showsPrec _ Denormal = showString "denormal"
768 instance Show AsyncException where
769 showsPrec _ StackOverflow = showString "stack overflow"
770 showsPrec _ HeapOverflow = showString "heap overflow"
771 showsPrec _ ThreadKilled = showString "thread killed"
773 instance Show ArrayException where
774 showsPrec _ (IndexOutOfBounds s)
775 = showString "array index out of range"
776 . (if not (null s) then showString ": " . showString s
778 showsPrec _ (UndefinedElement s)
779 = showString "undefined array element"
780 . (if not (null s) then showString ": " . showString s
783 instance Show Exception where
784 showsPrec _ (IOException err) = shows err
785 showsPrec _ (ArithException err) = shows err
786 showsPrec _ (ArrayException err) = shows err
787 showsPrec _ (ErrorCall err) = showString err
788 showsPrec _ (ExitException err) = showString "exit: " . shows err
789 showsPrec _ (NoMethodError err) = showString err
790 showsPrec _ (PatternMatchFail err) = showString err
791 showsPrec _ (RecSelError err) = showString err
792 showsPrec _ (RecConError err) = showString err
793 showsPrec _ (RecUpdError err) = showString err
794 showsPrec _ (AssertionFailed err) = showString err
795 showsPrec _ (DynException err) = showString "exception :: " . showsTypeRep (dynTypeRep err)
796 showsPrec _ (AsyncException e) = shows e
797 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
798 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
799 showsPrec _ (NestedAtomically) = showString "Control.Concurrent.STM.atomically was nested"
800 showsPrec _ (NonTermination) = showString "<<loop>>"
801 showsPrec _ (Deadlock) = showString "<<deadlock>>"
803 instance Eq Exception where
804 IOException e1 == IOException e2 = e1 == e2
805 ArithException e1 == ArithException e2 = e1 == e2
806 ArrayException e1 == ArrayException e2 = e1 == e2
807 ErrorCall e1 == ErrorCall e2 = e1 == e2
808 ExitException e1 == ExitException e2 = e1 == e2
809 NoMethodError e1 == NoMethodError e2 = e1 == e2
810 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
811 RecSelError e1 == RecSelError e2 = e1 == e2
812 RecConError e1 == RecConError e2 = e1 == e2
813 RecUpdError e1 == RecUpdError e2 = e1 == e2
814 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
815 DynException _ == DynException _ = False -- incomparable
816 AsyncException e1 == AsyncException e2 = e1 == e2
817 BlockedOnDeadMVar == BlockedOnDeadMVar = True
818 NonTermination == NonTermination = True
819 NestedAtomically == NestedAtomically = True
820 Deadlock == Deadlock = True
823 -- -----------------------------------------------------------------------------
826 -- We need it here because it is used in ExitException in the
827 -- Exception datatype (above).
830 = ExitSuccess -- ^ indicates successful termination;
832 -- ^ indicates program failure with an exit code.
833 -- The exact interpretation of the code is
834 -- operating-system dependent. In particular, some values
835 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
836 deriving (Eq, Ord, Read, Show)
838 -- --------------------------------------------------------------------------
841 -- | Throw an exception. Exceptions may be thrown from purely
842 -- functional code, but may only be caught within the 'IO' monad.
843 throw :: Exception -> a
844 throw exception = raise# exception
846 -- | A variant of 'throw' that can be used within the 'IO' monad.
848 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
849 -- two functions are subtly different:
851 -- > throw e `seq` x ===> throw e
852 -- > throwIO e `seq` x ===> x
854 -- The first example will cause the exception @e@ to be raised,
855 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
856 -- an exception to be raised when it is used within the 'IO' monad.
857 -- The 'throwIO' variant should be used in preference to 'throw' to
858 -- raise an exception within the 'IO' monad because it guarantees
859 -- ordering with respect to other 'IO' operations, whereas 'throw'
861 throwIO :: Exception -> IO a
862 throwIO err = IO $ raiseIO# err
864 ioException :: IOException -> IO a
865 ioException err = IO $ raiseIO# (IOException err)
867 -- | Raise an 'IOError' in the 'IO' monad.
868 ioError :: IOError -> IO a
869 ioError = ioException
871 -- ---------------------------------------------------------------------------
874 -- | The Haskell 98 type for exceptions in the 'IO' monad.
875 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
876 -- For a more general type of exception, including also those that arise
877 -- in pure code, see 'Control.Exception.Exception'.
879 -- In Haskell 98, this is an opaque type.
880 type IOError = IOException
882 -- |Exceptions that occur in the @IO@ monad.
883 -- An @IOException@ records a more specific error type, a descriptive
884 -- string and maybe the handle that was used when the error was
888 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
890 ioe_type :: IOErrorType, -- what it was.
891 ioe_location :: String, -- location.
892 ioe_description :: String, -- error type specific information.
893 ioe_filename :: Maybe FilePath -- filename the error is related to.
896 instance Eq IOException where
897 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
898 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
900 -- | An abstract type that contains a value for each variant of 'IOError'.
912 | UnsatisfiedConstraints
919 | UnsupportedOperation
923 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
925 instance Eq IOErrorType where
928 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
929 _ -> getTag x ==# getTag y
931 instance Show IOErrorType where
935 AlreadyExists -> "already exists"
936 NoSuchThing -> "does not exist"
937 ResourceBusy -> "resource busy"
938 ResourceExhausted -> "resource exhausted"
940 IllegalOperation -> "illegal operation"
941 PermissionDenied -> "permission denied"
942 UserError -> "user error"
943 HardwareFault -> "hardware fault"
944 InappropriateType -> "inappropriate type"
945 Interrupted -> "interrupted"
946 InvalidArgument -> "invalid argument"
947 OtherError -> "failed"
948 ProtocolError -> "protocol error"
949 ResourceVanished -> "resource vanished"
950 SystemError -> "system error"
951 TimeExpired -> "timeout"
952 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
953 UnsupportedOperation -> "unsupported operation"
954 DynIOError{} -> "unknown IO error"
956 -- | Construct an 'IOError' value with a string describing the error.
957 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
958 -- 'userError', thus:
960 -- > instance Monad IO where
962 -- > fail s = ioError (userError s)
964 userError :: String -> IOError
965 userError str = IOError Nothing UserError "" str Nothing
967 -- ---------------------------------------------------------------------------
970 instance Show IOException where
971 showsPrec p (IOError hdl iot loc s fn) =
973 Nothing -> case hdl of
975 Just h -> showsPrec p h . showString ": "
976 Just name -> showString name . showString ": ") .
979 _ -> showString loc . showString ": ") .
983 _ -> showString " (" . showString s . showString ")")
985 -- -----------------------------------------------------------------------------
988 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
989 deriving (Eq, Ord, Ix, Enum, Read, Show)