2 {-# OPTIONS_GHC -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 -----------------------------------------------------------------------------
19 IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
20 unsafePerformIO, unsafeInterleaveIO,
21 unsafeDupablePerformIO, unsafeDupableInterleaveIO,
24 -- To and from from ST
25 stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
28 IORef(..), newIORef, readIORef, writeIORef,
29 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
32 -- Handles, file descriptors,
34 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
35 isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
38 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
39 bufferIsWritable, bufferEmpty, bufferFull,
42 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
43 stackOverflow, heapOverflow, throw, throwIO, ioException,
44 IOError, IOException(..), IOErrorType(..), ioError, userError,
49 import GHC.Arr -- to derive Ix class
50 import GHC.Enum -- to derive Enum class
53 -- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
54 import Data.Maybe ( Maybe(..) )
58 import Foreign.C.Types (CInt)
61 import {-# SOURCE #-} Data.Typeable ( showsTypeRep )
62 import {-# SOURCE #-} Data.Dynamic ( Dynamic, dynTypeRep )
65 -- ---------------------------------------------------------------------------
69 The IO Monad is just an instance of the ST monad, where the state is
70 the real world. We use the exception mechanism (in GHC.Exception) to
71 implement IO exceptions.
73 NOTE: The IO representation is deeply wired in to various parts of the
74 system. The following list may or may not be exhaustive:
76 Compiler - types of various primitives in PrimOp.lhs
78 RTS - forceIO (StgMiscClosures.hc)
79 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
81 - raiseAsync (Schedule.c)
83 Prelude - GHC.IOBase.lhs, and several other places including
86 Libraries - parts of hslibs/lang.
92 A value of type @'IO' a@ is a computation which, when performed,
93 does some I\/O before returning a value of type @a@.
95 There is really only one way to \"perform\" an I\/O action: bind it to
96 @Main.main@ in your program. When your program is run, the I\/O will
97 be performed. It isn't possible to perform I\/O from an arbitrary
98 function, unless that function is itself in the 'IO' monad and called
99 at some point, directly or indirectly, from @Main.main@.
101 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
102 or the '>>' and '>>=' operations from the 'Monad' class.
104 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
106 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
109 instance Functor IO where
110 fmap f x = x >>= (return . f)
112 instance Monad IO where
113 {-# INLINE return #-}
116 m >> k = m >>= \ _ -> k
117 return x = returnIO x
122 failIO :: String -> IO a
123 failIO s = ioError (userError s)
125 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
126 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
128 bindIO :: IO a -> (a -> IO b) -> IO b
129 bindIO (IO m) k = IO ( \ s ->
131 (# new_s, a #) -> unIO (k a) new_s
134 thenIO :: IO a -> IO b -> IO b
135 thenIO (IO m) k = IO ( \ s ->
137 (# new_s, a #) -> unIO k new_s
140 returnIO :: a -> IO a
141 returnIO x = IO (\ s -> (# s, x #))
143 -- ---------------------------------------------------------------------------
144 -- Coercions between IO and ST
146 -- | A monad transformer embedding strict state transformers in the 'IO'
147 -- monad. The 'RealWorld' parameter indicates that the internal state
148 -- used by the 'ST' computation is a special one supplied by the 'IO'
149 -- monad, and thus distinct from those used by invocations of 'runST'.
150 stToIO :: ST RealWorld a -> IO a
153 ioToST :: IO a -> ST RealWorld a
154 ioToST (IO m) = (ST m)
156 -- This relies on IO and ST having the same representation modulo the
157 -- constraint on the type of the state
159 unsafeIOToST :: IO a -> ST s a
160 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
162 unsafeSTToIO :: ST s a -> IO a
163 unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
165 -- ---------------------------------------------------------------------------
166 -- Unsafe IO operations
169 This is the \"back door\" into the 'IO' monad, allowing
170 'IO' computation to be performed at any time. For
171 this to be safe, the 'IO' computation should be
172 free of side effects and independent of its environment.
174 If the I\/O computation wrapped in 'unsafePerformIO'
175 performs side effects, then the relative order in which those side
176 effects take place (relative to the main I\/O trunk, or other calls to
177 'unsafePerformIO') is indeterminate. You have to be careful when
178 writing and compiling modules that use 'unsafePerformIO':
180 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
181 that calls 'unsafePerformIO'. If the call is inlined,
182 the I\/O may be performed more than once.
184 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
185 elimination being performed on the module, which might combine
186 two side effects that were meant to be separate. A good example
187 is using multiple global variables (like @test@ in the example below).
189 * Make sure that the either you switch off let-floating, or that the
190 call to 'unsafePerformIO' cannot float outside a lambda. For example,
193 f x = unsafePerformIO (newIORef [])
195 you may get only one reference cell shared between all calls to @f@.
198 f x = unsafePerformIO (newIORef [x])
200 because now it can't float outside the lambda.
202 It is less well known that
203 'unsafePerformIO' is not type safe. For example:
206 > test = unsafePerformIO $ newIORef []
209 > writeIORef test [42]
210 > bang <- readIORef test
211 > print (bang :: [Char])
213 This program will core dump. This problem with polymorphic references
214 is well known in the ML community, and does not arise with normal
215 monadic use of references. There is no easy way to make it impossible
216 once you use 'unsafePerformIO'. Indeed, it is
217 possible to write @coerce :: a -> b@ with the
218 help of 'unsafePerformIO'. So be careful!
220 unsafePerformIO :: IO a -> a
221 unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)
224 This version of 'unsafePerformIO' is slightly more efficient,
225 because it omits the check that the IO is only being performed by a
226 single thread. Hence, when you write 'unsafeDupablePerformIO',
227 there is a possibility that the IO action may be performed multiple
228 times (on a multiprocessor), and you should therefore ensure that
229 it gives the same results each time.
231 {-# NOINLINE unsafeDupablePerformIO #-}
232 unsafeDupablePerformIO :: IO a -> a
233 unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
235 -- Why do we NOINLINE unsafeDupablePerformIO? See the comment with
236 -- GHC.ST.runST. Essentially the issue is that the IO computation
237 -- inside unsafePerformIO must be atomic: it must either all run, or
238 -- not at all. If we let the compiler see the application of the IO
239 -- to realWorld#, it might float out part of the IO.
241 -- Why is there a call to 'lazy' in unsafeDupablePerformIO?
242 -- If we don't have it, the demand analyser discovers the following strictness
243 -- for unsafeDupablePerformIO: C(U(AV))
245 -- unsafeDupablePerformIO (\s -> let r = f x in
246 -- case writeIORef v r s of (# s1, _ #) ->
248 -- The strictness analyser will find that the binding for r is strict,
249 -- (becuase of uPIO's strictness sig), and so it'll evaluate it before
250 -- doing the writeIORef. This actually makes tests/lib/should_run/memo002
253 -- Solution: don't expose the strictness of unsafeDupablePerformIO,
254 -- by hiding it with 'lazy'
257 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
258 When passed a value of type @IO a@, the 'IO' will only be performed
259 when the value of the @a@ is demanded. This is used to implement lazy
260 file reading, see 'System.IO.hGetContents'.
262 {-# INLINE unsafeInterleaveIO #-}
263 unsafeInterleaveIO :: IO a -> IO a
264 unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
266 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
267 -- state from this IO thread is passed explicitly to the interleaved
268 -- IO, so it cannot be floated out and shared.
270 {-# INLINE unsafeDupableInterleaveIO #-}
271 unsafeDupableInterleaveIO :: IO a -> IO a
272 unsafeDupableInterleaveIO (IO m)
274 r = case m s of (# _, res #) -> res
279 Ensures that the suspensions under evaluation by the current thread
280 are unique; that is, the current thread is not evaluating anything
281 that is also under evaluation by another thread that has also executed
284 This operation is used in the definition of 'unsafePerformIO' to
285 prevent the IO action from being executed multiple times, which is usually
289 noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)
291 -- ---------------------------------------------------------------------------
294 data MVar a = MVar (MVar# RealWorld a)
296 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
297 for communication between concurrent threads. It can be thought of
298 as a a box, which may be empty or full.
301 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
302 instance Eq (MVar a) where
303 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
305 -- A Handle is represented by (a reference to) a record
306 -- containing the state of the I/O port/device. We record
307 -- the following pieces of info:
309 -- * type (read,write,closed etc.)
310 -- * the underlying file descriptor
312 -- * buffer, and spare buffers
313 -- * user-friendly name (usually the
314 -- FilePath used when IO.openFile was called)
316 -- Note: when a Handle is garbage collected, we want to flush its buffer
317 -- and close the OS file handle, so as to free up a (precious) resource.
319 -- | Haskell defines operations to read and write characters from and to files,
320 -- represented by values of type @Handle@. Each value of this type is a
321 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
322 -- with file system objects. A handle has at least the following properties:
324 -- * whether it manages input or output or both;
326 -- * whether it is /open/, /closed/ or /semi-closed/;
328 -- * whether the object is seekable;
330 -- * whether buffering is disabled, or enabled on a line or block basis;
332 -- * a buffer (whose length may be zero).
334 -- Most handles will also have a current I\/O position indicating where the next
335 -- input or output operation will occur. A handle is /readable/ if it
336 -- manages only input or both input and output; likewise, it is /writable/ if
337 -- it manages only output or both input and output. A handle is /open/ when
339 -- Once it is closed it can no longer be used for either input or output,
340 -- though an implementation cannot re-use its storage while references
341 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
342 -- produced by showing a handle is system dependent; it should include
343 -- enough information to identify the handle for debugging. A handle is
344 -- equal according to '==' only to itself; no attempt
345 -- is made to compare the internal state of different handles for equality.
347 -- GHC note: a 'Handle' will be automatically closed when the garbage
348 -- collector detects that it has become unreferenced by the program.
349 -- However, relying on this behaviour is not generally recommended:
350 -- the garbage collector is unpredictable. If possible, use explicit
351 -- an explicit 'hClose' to close 'Handle's when they are no longer
352 -- required. GHC does not currently attempt to free up file
353 -- descriptors when they have run out, it is your responsibility to
354 -- ensure that this doesn't happen.
357 = FileHandle -- A normal handle to a file
358 FilePath -- the file (invariant)
361 | DuplexHandle -- A handle to a read/write stream
362 FilePath -- file for a FIFO, otherwise some
363 -- descriptive string.
364 !(MVar Handle__) -- The read side
365 !(MVar Handle__) -- The write side
368 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
371 instance Eq Handle where
372 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
373 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
380 haFD :: !FD, -- file descriptor
381 haType :: HandleType, -- type (read/write/append etc.)
382 haIsBin :: Bool, -- binary mode?
383 haIsStream :: Bool, -- is this a stream handle?
384 haBufferMode :: BufferMode, -- buffer contains read/write data?
385 haBuffer :: !(IORef Buffer), -- the current buffer
386 haBuffers :: !(IORef BufferList), -- spare buffers
387 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
391 -- ---------------------------------------------------------------------------
394 -- The buffer is represented by a mutable variable containing a
395 -- record, where the record contains the raw buffer and the start/end
396 -- points of the filled portion. We use a mutable variable so that
397 -- the common operation of writing (or reading) some data from (to)
398 -- the buffer doesn't need to modify, and hence copy, the handle
399 -- itself, it just updates the buffer.
401 -- There will be some allocation involved in a simple hPutChar in
402 -- order to create the new Buffer structure (below), but this is
403 -- relatively small, and this only has to be done once per write
406 -- The buffer contains its size - we could also get the size by
407 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
408 -- to be rounded up to the nearest Word.
410 type RawBuffer = MutableByteArray# RealWorld
412 -- INVARIANTS on a Buffer:
414 -- * A handle *always* has a buffer, even if it is only 1 character long
415 -- (an unbuffered handle needs a 1 character buffer in order to support
416 -- hLookAhead and hIsEOF).
418 -- * if r == w, then r == 0 && w == 0
419 -- * if state == WriteBuffer, then r == 0
420 -- * a write buffer is never full. If an operation
421 -- fills up the buffer, it will always flush it before
423 -- * a read buffer may be full as a result of hLookAhead. In normal
424 -- operation, a read buffer always has at least one character of space.
432 bufState :: BufferState
435 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
437 -- we keep a few spare buffers around in a handle to avoid allocating
438 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
439 -- same size as the main buffer.
442 | BufferListCons RawBuffer BufferList
445 bufferIsWritable :: Buffer -> Bool
446 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
447 bufferIsWritable _other = False
449 bufferEmpty :: Buffer -> Bool
450 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
452 -- only makes sense for a write buffer
453 bufferFull :: Buffer -> Bool
454 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
456 -- Internally, we classify handles as being one
467 isReadableHandleType ReadHandle = True
468 isReadableHandleType ReadWriteHandle = True
469 isReadableHandleType _ = False
471 isWritableHandleType AppendHandle = True
472 isWritableHandleType WriteHandle = True
473 isWritableHandleType ReadWriteHandle = True
474 isWritableHandleType _ = False
476 isReadWriteHandleType ReadWriteHandle{} = True
477 isReadWriteHandleType _ = False
479 -- | File and directory names are values of type 'String', whose precise
480 -- meaning is operating system dependent. Files can be opened, yielding a
481 -- handle which can then be used to operate on the contents of that file.
483 type FilePath = String
485 -- ---------------------------------------------------------------------------
488 -- | Three kinds of buffering are supported: line-buffering,
489 -- block-buffering or no-buffering. These modes have the following
490 -- effects. For output, items are written out, or /flushed/,
491 -- from the internal buffer according to the buffer mode:
493 -- * /line-buffering/: the entire output buffer is flushed
494 -- whenever a newline is output, the buffer overflows,
495 -- a 'System.IO.hFlush' is issued, or the handle is closed.
497 -- * /block-buffering/: the entire buffer is written out whenever it
498 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
500 -- * /no-buffering/: output is written immediately, and never stored
503 -- An implementation is free to flush the buffer more frequently,
504 -- but not less frequently, than specified above.
505 -- The output buffer is emptied as soon as it has been written out.
507 -- Similarly, input occurs according to the buffer mode for the handle:
509 -- * /line-buffering/: when the buffer for the handle is not empty,
510 -- the next item is obtained from the buffer; otherwise, when the
511 -- buffer is empty, characters up to and including the next newline
512 -- character are read into the buffer. No characters are available
513 -- until the newline character is available or the buffer is full.
515 -- * /block-buffering/: when the buffer for the handle becomes empty,
516 -- the next block of data is read into the buffer.
518 -- * /no-buffering/: the next input item is read and returned.
519 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
520 -- handle may require a one-character buffer.
522 -- The default buffering mode when a handle is opened is
523 -- implementation-dependent and may depend on the file system object
524 -- which is attached to that handle.
525 -- For most implementations, physical files will normally be block-buffered
526 -- and terminals will normally be line-buffered.
529 = NoBuffering -- ^ buffering is disabled if possible.
531 -- ^ line-buffering should be enabled if possible.
532 | BlockBuffering (Maybe Int)
533 -- ^ block-buffering should be enabled if possible.
534 -- The size of the buffer is @n@ items if the argument
535 -- is 'Just' @n@ and is otherwise implementation-dependent.
536 deriving (Eq, Ord, Read, Show)
538 -- ---------------------------------------------------------------------------
541 -- |A mutable variable in the 'IO' monad
542 newtype IORef a = IORef (STRef RealWorld a)
544 -- explicit instance because Haddock can't figure out a derived one
545 instance Eq (IORef a) where
546 IORef x == IORef y = x == y
548 -- |Build a new 'IORef'
549 newIORef :: a -> IO (IORef a)
550 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
552 -- |Read the value of an 'IORef'
553 readIORef :: IORef a -> IO a
554 readIORef (IORef var) = stToIO (readSTRef var)
556 -- |Write a new value into an 'IORef'
557 writeIORef :: IORef a -> a -> IO ()
558 writeIORef (IORef var) v = stToIO (writeSTRef var v)
560 -- ---------------------------------------------------------------------------
561 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
562 -- The type arguments are as follows:
564 -- * @i@: the index type of the array (should be an instance of 'Ix')
566 -- * @e@: the element type of the array.
570 newtype IOArray i e = IOArray (STArray RealWorld i e)
572 -- explicit instance because Haddock can't figure out a derived one
573 instance Eq (IOArray i e) where
574 IOArray x == IOArray y = x == y
576 -- |Build a new 'IOArray'
577 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
578 {-# INLINE newIOArray #-}
579 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
581 -- | Read a value from an 'IOArray'
582 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
583 {-# INLINE unsafeReadIOArray #-}
584 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
586 -- | Write a new value into an 'IOArray'
587 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
588 {-# INLINE unsafeWriteIOArray #-}
589 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
591 -- | Read a value from an 'IOArray'
592 readIOArray :: Ix i => IOArray i e -> i -> IO e
593 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
595 -- | Write a new value into an 'IOArray'
596 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
597 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
600 -- ---------------------------------------------------------------------------
601 -- Show instance for Handles
603 -- handle types are 'show'n when printing error msgs, so
604 -- we provide a more user-friendly Show instance for it
605 -- than the derived one.
607 instance Show HandleType where
610 ClosedHandle -> showString "closed"
611 SemiClosedHandle -> showString "semi-closed"
612 ReadHandle -> showString "readable"
613 WriteHandle -> showString "writable"
614 AppendHandle -> showString "writable (append)"
615 ReadWriteHandle -> showString "read-writable"
617 instance Show Handle where
618 showsPrec p (FileHandle file _) = showHandle file
619 showsPrec p (DuplexHandle file _ _) = showHandle file
621 showHandle file = showString "{handle: " . showString file . showString "}"
623 -- ------------------------------------------------------------------------
624 -- Exception datatype and operations
626 -- |The type of exceptions. Every kind of system-generated exception
627 -- has a constructor in the 'Exception' type, and values of other
628 -- types may be injected into 'Exception' by coercing them to
629 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
630 -- "Control.Exception\#DynamicExceptions").
632 = ArithException ArithException
633 -- ^Exceptions raised by arithmetic
634 -- operations. (NOTE: GHC currently does not throw
635 -- 'ArithException's except for 'DivideByZero').
636 | ArrayException ArrayException
637 -- ^Exceptions raised by array-related
638 -- operations. (NOTE: GHC currently does not throw
639 -- 'ArrayException's).
640 | AssertionFailed String
641 -- ^This exception is thrown by the
642 -- 'assert' operation when the condition
643 -- fails. The 'String' argument contains the
644 -- location of the assertion in the source program.
645 | AsyncException AsyncException
646 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
648 -- ^The current thread was executing a call to
649 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
650 -- because there are no other references to this 'MVar'.
651 | BlockedIndefinitely
652 -- ^The current thread was waiting to retry an atomic memory transaction
653 -- that could never become possible to complete because there are no other
654 -- threads referring to any of teh TVars involved.
656 -- ^The runtime detected an attempt to nest one STM transaction
657 -- inside another one, presumably due to the use of
658 -- 'unsafePeformIO' with 'atomically'.
660 -- ^There are no runnable threads, so the program is
661 -- deadlocked. The 'Deadlock' exception is
662 -- raised in the main thread only (see also: "Control.Concurrent").
663 | DynException Dynamic
664 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
666 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
667 -- argument of 'ErrorCall' is the string passed to 'error' when it was
669 | ExitException ExitCode
670 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
671 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
672 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
673 -- main thread will cause the program to be terminated with the given
675 | IOException IOException
676 -- ^These are the standard IO exceptions generated by
677 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
678 | NoMethodError String
679 -- ^An attempt was made to invoke a class method which has
680 -- no definition in this instance, and there was no default
681 -- definition given in the class declaration. GHC issues a
682 -- warning when you compile an instance which has missing
685 -- ^The current thread is stuck in an infinite loop. This
686 -- exception may or may not be thrown when the program is
688 | PatternMatchFail String
689 -- ^A pattern matching failure. The 'String' argument should contain a
690 -- descriptive message including the function name, source file
693 -- ^An attempt was made to evaluate a field of a record
694 -- for which no value was given at construction time. The
695 -- 'String' argument gives the location of the
696 -- record construction in the source program.
698 -- ^A field selection was attempted on a constructor that
699 -- doesn\'t have the requested field. This can happen with
700 -- multi-constructor records when one or more fields are
701 -- missing from some of the constructors. The
702 -- 'String' argument gives the location of the
703 -- record selection in the source program.
705 -- ^An attempt was made to update a field in a record,
706 -- where the record doesn\'t have the requested field. This can
707 -- only occur with multi-constructor records, when one or more
708 -- fields are missing from some of the constructors. The
709 -- 'String' argument gives the location of the
710 -- record update in the source program.
712 -- |The type of arithmetic exceptions
722 -- |Asynchronous exceptions
725 -- ^The current thread\'s stack exceeded its limit.
726 -- Since an exception has been raised, the thread\'s stack
727 -- will certainly be below its limit again, but the
728 -- programmer should take remedial action
731 -- ^The program\'s heap is reaching its limit, and
732 -- the program should take action to reduce the amount of
733 -- live data it has. Notes:
735 -- * It is undefined which thread receives this exception.
737 -- * GHC currently does not throw 'HeapOverflow' exceptions.
739 -- ^This exception is raised by another thread
740 -- calling 'Control.Concurrent.killThread', or by the system
741 -- if it needs to terminate the thread for some
745 -- | Exceptions generated by array operations
747 = IndexOutOfBounds String
748 -- ^An attempt was made to index an array outside
749 -- its declared bounds.
750 | UndefinedElement String
751 -- ^An attempt was made to evaluate an element of an
752 -- array that had not been initialized.
755 stackOverflow, heapOverflow :: Exception -- for the RTS
756 stackOverflow = AsyncException StackOverflow
757 heapOverflow = AsyncException HeapOverflow
759 instance Show ArithException where
760 showsPrec _ Overflow = showString "arithmetic overflow"
761 showsPrec _ Underflow = showString "arithmetic underflow"
762 showsPrec _ LossOfPrecision = showString "loss of precision"
763 showsPrec _ DivideByZero = showString "divide by zero"
764 showsPrec _ Denormal = showString "denormal"
766 instance Show AsyncException where
767 showsPrec _ StackOverflow = showString "stack overflow"
768 showsPrec _ HeapOverflow = showString "heap overflow"
769 showsPrec _ ThreadKilled = showString "thread killed"
771 instance Show ArrayException where
772 showsPrec _ (IndexOutOfBounds s)
773 = showString "array index out of range"
774 . (if not (null s) then showString ": " . showString s
776 showsPrec _ (UndefinedElement s)
777 = showString "undefined array element"
778 . (if not (null s) then showString ": " . showString s
781 instance Show Exception where
782 showsPrec _ (IOException err) = shows err
783 showsPrec _ (ArithException err) = shows err
784 showsPrec _ (ArrayException err) = shows err
785 showsPrec _ (ErrorCall err) = showString err
786 showsPrec _ (ExitException err) = showString "exit: " . shows err
787 showsPrec _ (NoMethodError err) = showString err
788 showsPrec _ (PatternMatchFail err) = showString err
789 showsPrec _ (RecSelError err) = showString err
790 showsPrec _ (RecConError err) = showString err
791 showsPrec _ (RecUpdError err) = showString err
792 showsPrec _ (AssertionFailed err) = showString err
793 showsPrec _ (DynException err) = showString "exception :: " . showsTypeRep (dynTypeRep err)
794 showsPrec _ (AsyncException e) = shows e
795 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
796 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
797 showsPrec _ (NestedAtomically) = showString "Control.Concurrent.STM.atomically was nested"
798 showsPrec _ (NonTermination) = showString "<<loop>>"
799 showsPrec _ (Deadlock) = showString "<<deadlock>>"
801 instance Eq Exception where
802 IOException e1 == IOException e2 = e1 == e2
803 ArithException e1 == ArithException e2 = e1 == e2
804 ArrayException e1 == ArrayException e2 = e1 == e2
805 ErrorCall e1 == ErrorCall e2 = e1 == e2
806 ExitException e1 == ExitException e2 = e1 == e2
807 NoMethodError e1 == NoMethodError e2 = e1 == e2
808 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
809 RecSelError e1 == RecSelError e2 = e1 == e2
810 RecConError e1 == RecConError e2 = e1 == e2
811 RecUpdError e1 == RecUpdError e2 = e1 == e2
812 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
813 DynException _ == DynException _ = False -- incomparable
814 AsyncException e1 == AsyncException e2 = e1 == e2
815 BlockedOnDeadMVar == BlockedOnDeadMVar = True
816 NonTermination == NonTermination = True
817 NestedAtomically == NestedAtomically = True
818 Deadlock == Deadlock = True
821 -- -----------------------------------------------------------------------------
824 -- We need it here because it is used in ExitException in the
825 -- Exception datatype (above).
828 = ExitSuccess -- ^ indicates successful termination;
830 -- ^ indicates program failure with an exit code.
831 -- The exact interpretation of the code is
832 -- operating-system dependent. In particular, some values
833 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
834 deriving (Eq, Ord, Read, Show)
836 -- --------------------------------------------------------------------------
839 -- | Throw an exception. Exceptions may be thrown from purely
840 -- functional code, but may only be caught within the 'IO' monad.
841 throw :: Exception -> a
842 throw exception = raise# exception
844 -- | A variant of 'throw' that can be used within the 'IO' monad.
846 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
847 -- two functions are subtly different:
849 -- > throw e `seq` x ===> throw e
850 -- > throwIO e `seq` x ===> x
852 -- The first example will cause the exception @e@ to be raised,
853 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
854 -- an exception to be raised when it is used within the 'IO' monad.
855 -- The 'throwIO' variant should be used in preference to 'throw' to
856 -- raise an exception within the 'IO' monad because it guarantees
857 -- ordering with respect to other 'IO' operations, whereas 'throw'
859 throwIO :: Exception -> IO a
860 throwIO err = IO $ raiseIO# err
862 ioException :: IOException -> IO a
863 ioException err = IO $ raiseIO# (IOException err)
865 -- | Raise an 'IOError' in the 'IO' monad.
866 ioError :: IOError -> IO a
867 ioError = ioException
869 -- ---------------------------------------------------------------------------
872 -- | The Haskell 98 type for exceptions in the 'IO' monad.
873 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
874 -- For a more general type of exception, including also those that arise
875 -- in pure code, see 'Control.Exception.Exception'.
877 -- In Haskell 98, this is an opaque type.
878 type IOError = IOException
880 -- |Exceptions that occur in the @IO@ monad.
881 -- An @IOException@ records a more specific error type, a descriptive
882 -- string and maybe the handle that was used when the error was
886 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
888 ioe_type :: IOErrorType, -- what it was.
889 ioe_location :: String, -- location.
890 ioe_description :: String, -- error type specific information.
891 ioe_filename :: Maybe FilePath -- filename the error is related to.
894 instance Eq IOException where
895 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
896 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
898 -- | An abstract type that contains a value for each variant of 'IOError'.
910 | UnsatisfiedConstraints
917 | UnsupportedOperation
921 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
923 instance Eq IOErrorType where
926 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
927 _ -> getTag x ==# getTag y
929 instance Show IOErrorType where
933 AlreadyExists -> "already exists"
934 NoSuchThing -> "does not exist"
935 ResourceBusy -> "resource busy"
936 ResourceExhausted -> "resource exhausted"
938 IllegalOperation -> "illegal operation"
939 PermissionDenied -> "permission denied"
940 UserError -> "user error"
941 HardwareFault -> "hardware fault"
942 InappropriateType -> "inappropriate type"
943 Interrupted -> "interrupted"
944 InvalidArgument -> "invalid argument"
945 OtherError -> "failed"
946 ProtocolError -> "protocol error"
947 ResourceVanished -> "resource vanished"
948 SystemError -> "system error"
949 TimeExpired -> "timeout"
950 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
951 UnsupportedOperation -> "unsupported operation"
952 DynIOError{} -> "unknown IO error"
954 -- | Construct an 'IOError' value with a string describing the error.
955 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
956 -- 'userError', thus:
958 -- > instance Monad IO where
960 -- > fail s = ioError (userError s)
962 userError :: String -> IOError
963 userError str = IOError Nothing UserError "" str Nothing
965 -- ---------------------------------------------------------------------------
968 instance Show IOException where
969 showsPrec p (IOError hdl iot loc s fn) =
971 Nothing -> case hdl of
973 Just h -> showsPrec p h . showString ": "
974 Just name -> showString name . showString ": ") .
977 _ -> showString loc . showString ": ") .
981 _ -> showString " (" . showString s . showString ")")
983 -- -----------------------------------------------------------------------------
986 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
987 deriving (Eq, Ord, Ix, Enum, Read, Show)