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,
22 -- To and from from ST
23 stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
26 IORef(..), newIORef, readIORef, writeIORef,
27 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
30 -- Handles, file descriptors,
32 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
33 isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
36 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
37 bufferIsWritable, bufferEmpty, bufferFull,
40 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
41 stackOverflow, heapOverflow, throw, throwIO, ioException,
42 IOError, IOException(..), IOErrorType(..), ioError, userError,
47 import GHC.Arr -- to derive Ix class
48 import GHC.Enum -- to derive Enum class
51 -- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
52 import Data.Maybe ( Maybe(..) )
58 import {-# SOURCE #-} GHC.Dynamic
61 -- ---------------------------------------------------------------------------
65 The IO Monad is just an instance of the ST monad, where the state is
66 the real world. We use the exception mechanism (in GHC.Exception) to
67 implement IO exceptions.
69 NOTE: The IO representation is deeply wired in to various parts of the
70 system. The following list may or may not be exhaustive:
72 Compiler - types of various primitives in PrimOp.lhs
74 RTS - forceIO (StgMiscClosures.hc)
75 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
77 - raiseAsync (Schedule.c)
79 Prelude - GHC.IOBase.lhs, and several other places including
82 Libraries - parts of hslibs/lang.
88 A value of type @'IO' a@ is a computation which, when performed,
89 does some I\/O before returning a value of type @a@.
91 There is really only one way to \"perform\" an I\/O action: bind it to
92 @Main.main@ in your program. When your program is run, the I\/O will
93 be performed. It isn't possible to perform I\/O from an arbitrary
94 function, unless that function is itself in the 'IO' monad and called
95 at some point, directly or indirectly, from @Main.main@.
97 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
98 or the '>>' and '>>=' operations from the 'Monad' class.
100 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
102 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
105 instance Functor IO where
106 fmap f x = x >>= (return . f)
108 instance Monad IO where
109 {-# INLINE return #-}
112 m >> k = m >>= \ _ -> k
113 return x = returnIO x
118 failIO :: String -> IO a
119 failIO s = ioError (userError s)
121 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
122 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
124 bindIO :: IO a -> (a -> IO b) -> IO b
125 bindIO (IO m) k = IO ( \ s ->
127 (# new_s, a #) -> unIO (k a) new_s
130 thenIO :: IO a -> IO b -> IO b
131 thenIO (IO m) k = IO ( \ s ->
133 (# new_s, a #) -> unIO k new_s
136 returnIO :: a -> IO a
137 returnIO x = IO (\ s -> (# s, x #))
139 -- ---------------------------------------------------------------------------
140 -- Coercions between IO and ST
142 -- | A monad transformer embedding strict state transformers in the 'IO'
143 -- monad. The 'RealWorld' parameter indicates that the internal state
144 -- used by the 'ST' computation is a special one supplied by the 'IO'
145 -- monad, and thus distinct from those used by invocations of 'runST'.
146 stToIO :: ST RealWorld a -> IO a
149 ioToST :: IO a -> ST RealWorld a
150 ioToST (IO m) = (ST m)
152 -- This relies on IO and ST having the same representation modulo the
153 -- constraint on the type of the state
155 unsafeIOToST :: IO a -> ST s a
156 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
158 unsafeSTToIO :: ST s a -> IO a
159 unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
161 -- ---------------------------------------------------------------------------
162 -- Unsafe IO operations
165 This is the \"back door\" into the 'IO' monad, allowing
166 'IO' computation to be performed at any time. For
167 this to be safe, the 'IO' computation should be
168 free of side effects and independent of its environment.
170 If the I\/O computation wrapped in 'unsafePerformIO'
171 performs side effects, then the relative order in which those side
172 effects take place (relative to the main I\/O trunk, or other calls to
173 'unsafePerformIO') is indeterminate. You have to be careful when
174 writing and compiling modules that use 'unsafePerformIO':
176 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
177 that calls 'unsafePerformIO'. If the call is inlined,
178 the I\/O may be performed more than once.
180 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
181 elimination being performed on the module, which might combine
182 two side effects that were meant to be separate. A good example
183 is using multiple global variables (like @test@ in the example below).
185 * Make sure that the either you switch off let-floating, or that the
186 call to 'unsafePerformIO' cannot float outside a lambda. For example,
189 f x = unsafePerformIO (newIORef [])
191 you may get only one reference cell shared between all calls to @f@.
194 f x = unsafePerformIO (newIORef [x])
196 because now it can't float outside the lambda.
198 It is less well known that
199 'unsafePerformIO' is not type safe. For example:
202 > test = unsafePerformIO $ newIORef []
205 > writeIORef test [42]
206 > bang <- readIORef test
207 > print (bang :: [Char])
209 This program will core dump. This problem with polymorphic references
210 is well known in the ML community, and does not arise with normal
211 monadic use of references. There is no easy way to make it impossible
212 once you use 'unsafePerformIO'. Indeed, it is
213 possible to write @coerce :: a -> b@ with the
214 help of 'unsafePerformIO'. So be careful!
216 {-# NOINLINE unsafePerformIO #-}
217 unsafePerformIO :: IO a -> a
218 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
220 -- Why do we NOINLINE unsafePerformIO? See the comment with
221 -- GHC.ST.runST. Essentially the issue is that the IO computation
222 -- inside unsafePerformIO must be atomic: it must either all run, or
223 -- not at all. If we let the compiler see the application of the IO
224 -- to realWorld#, it might float out part of the IO.
227 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
228 When passed a value of type @IO a@, the 'IO' will only be performed
229 when the value of the @a@ is demanded. This is used to implement lazy
230 file reading, see 'System.IO.hGetContents'.
232 {-# INLINE unsafeInterleaveIO #-}
233 unsafeInterleaveIO :: IO a -> IO a
234 unsafeInterleaveIO (IO m)
236 r = case m s of (# _, res #) -> res
240 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
241 -- state from this IO thread is passed explicitly to the interleaved
242 -- IO, so it cannot be floated out and shared.
244 -- ---------------------------------------------------------------------------
247 data MVar a = MVar (MVar# RealWorld a)
249 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
250 for communication between concurrent threads. It can be thought of
251 as a a box, which may be empty or full.
254 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
255 instance Eq (MVar a) where
256 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
258 -- A Handle is represented by (a reference to) a record
259 -- containing the state of the I/O port/device. We record
260 -- the following pieces of info:
262 -- * type (read,write,closed etc.)
263 -- * the underlying file descriptor
265 -- * buffer, and spare buffers
266 -- * user-friendly name (usually the
267 -- FilePath used when IO.openFile was called)
269 -- Note: when a Handle is garbage collected, we want to flush its buffer
270 -- and close the OS file handle, so as to free up a (precious) resource.
272 -- | Haskell defines operations to read and write characters from and to files,
273 -- represented by values of type @Handle@. Each value of this type is a
274 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
275 -- with file system objects. A handle has at least the following properties:
277 -- * whether it manages input or output or both;
279 -- * whether it is /open/, /closed/ or /semi-closed/;
281 -- * whether the object is seekable;
283 -- * whether buffering is disabled, or enabled on a line or block basis;
285 -- * a buffer (whose length may be zero).
287 -- Most handles will also have a current I\/O position indicating where the next
288 -- input or output operation will occur. A handle is /readable/ if it
289 -- manages only input or both input and output; likewise, it is /writable/ if
290 -- it manages only output or both input and output. A handle is /open/ when
292 -- Once it is closed it can no longer be used for either input or output,
293 -- though an implementation cannot re-use its storage while references
294 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
295 -- produced by showing a handle is system dependent; it should include
296 -- enough information to identify the handle for debugging. A handle is
297 -- equal according to '==' only to itself; no attempt
298 -- is made to compare the internal state of different handles for equality.
300 -- GHC note: a 'Handle' will be automatically closed when the garbage
301 -- collector detects that it has become unreferenced by the program.
302 -- However, relying on this behaviour is not generally recommended:
303 -- the garbage collector is unpredictable. If possible, use explicit
304 -- an explicit 'hClose' to close 'Handle's when they are no longer
305 -- required. GHC does not currently attempt to free up file
306 -- descriptors when they have run out, it is your responsibility to
307 -- ensure that this doesn't happen.
310 = FileHandle -- A normal handle to a file
311 FilePath -- the file (invariant)
314 | DuplexHandle -- A handle to a read/write stream
315 FilePath -- file for a FIFO, otherwise some
316 -- descriptive string.
317 !(MVar Handle__) -- The read side
318 !(MVar Handle__) -- The write side
321 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
324 instance Eq Handle where
325 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
326 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
329 type FD = Int -- XXX ToDo: should be CInt
333 haFD :: !FD, -- file descriptor
334 haType :: HandleType, -- type (read/write/append etc.)
335 haIsBin :: Bool, -- binary mode?
336 haIsStream :: Bool, -- is this a stream handle?
337 haBufferMode :: BufferMode, -- buffer contains read/write data?
338 haBuffer :: !(IORef Buffer), -- the current buffer
339 haBuffers :: !(IORef BufferList), -- spare buffers
340 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
344 -- ---------------------------------------------------------------------------
347 -- The buffer is represented by a mutable variable containing a
348 -- record, where the record contains the raw buffer and the start/end
349 -- points of the filled portion. We use a mutable variable so that
350 -- the common operation of writing (or reading) some data from (to)
351 -- the buffer doesn't need to modify, and hence copy, the handle
352 -- itself, it just updates the buffer.
354 -- There will be some allocation involved in a simple hPutChar in
355 -- order to create the new Buffer structure (below), but this is
356 -- relatively small, and this only has to be done once per write
359 -- The buffer contains its size - we could also get the size by
360 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
361 -- to be rounded up to the nearest Word.
363 type RawBuffer = MutableByteArray# RealWorld
365 -- INVARIANTS on a Buffer:
367 -- * A handle *always* has a buffer, even if it is only 1 character long
368 -- (an unbuffered handle needs a 1 character buffer in order to support
369 -- hLookAhead and hIsEOF).
371 -- * if r == w, then r == 0 && w == 0
372 -- * if state == WriteBuffer, then r == 0
373 -- * a write buffer is never full. If an operation
374 -- fills up the buffer, it will always flush it before
376 -- * a read buffer may be full as a result of hLookAhead. In normal
377 -- operation, a read buffer always has at least one character of space.
385 bufState :: BufferState
388 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
390 -- we keep a few spare buffers around in a handle to avoid allocating
391 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
392 -- same size as the main buffer.
395 | BufferListCons RawBuffer BufferList
398 bufferIsWritable :: Buffer -> Bool
399 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
400 bufferIsWritable _other = False
402 bufferEmpty :: Buffer -> Bool
403 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
405 -- only makes sense for a write buffer
406 bufferFull :: Buffer -> Bool
407 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
409 -- Internally, we classify handles as being one
420 isReadableHandleType ReadHandle = True
421 isReadableHandleType ReadWriteHandle = True
422 isReadableHandleType _ = False
424 isWritableHandleType AppendHandle = True
425 isWritableHandleType WriteHandle = True
426 isWritableHandleType ReadWriteHandle = True
427 isWritableHandleType _ = False
429 isReadWriteHandleType ReadWriteHandle{} = True
430 isReadWriteHandleType _ = False
432 -- | File and directory names are values of type 'String', whose precise
433 -- meaning is operating system dependent. Files can be opened, yielding a
434 -- handle which can then be used to operate on the contents of that file.
436 type FilePath = String
438 -- ---------------------------------------------------------------------------
441 -- | Three kinds of buffering are supported: line-buffering,
442 -- block-buffering or no-buffering. These modes have the following
443 -- effects. For output, items are written out, or /flushed/,
444 -- from the internal buffer according to the buffer mode:
446 -- * /line-buffering/: the entire output buffer is flushed
447 -- whenever a newline is output, the buffer overflows,
448 -- a 'System.IO.hFlush' is issued, or the handle is closed.
450 -- * /block-buffering/: the entire buffer is written out whenever it
451 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
453 -- * /no-buffering/: output is written immediately, and never stored
456 -- An implementation is free to flush the buffer more frequently,
457 -- but not less frequently, than specified above.
458 -- The output buffer is emptied as soon as it has been written out.
460 -- Similarly, input occurs according to the buffer mode for the handle:
462 -- * /line-buffering/: when the buffer for the handle is not empty,
463 -- the next item is obtained from the buffer; otherwise, when the
464 -- buffer is empty, characters up to and including the next newline
465 -- character are read into the buffer. No characters are available
466 -- until the newline character is available or the buffer is full.
468 -- * /block-buffering/: when the buffer for the handle becomes empty,
469 -- the next block of data is read into the buffer.
471 -- * /no-buffering/: the next input item is read and returned.
472 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
473 -- handle may require a one-character buffer.
475 -- The default buffering mode when a handle is opened is
476 -- implementation-dependent and may depend on the file system object
477 -- which is attached to that handle.
478 -- For most implementations, physical files will normally be block-buffered
479 -- and terminals will normally be line-buffered.
482 = NoBuffering -- ^ buffering is disabled if possible.
484 -- ^ line-buffering should be enabled if possible.
485 | BlockBuffering (Maybe Int)
486 -- ^ block-buffering should be enabled if possible.
487 -- The size of the buffer is @n@ items if the argument
488 -- is 'Just' @n@ and is otherwise implementation-dependent.
489 deriving (Eq, Ord, Read, Show)
491 -- ---------------------------------------------------------------------------
494 -- |A mutable variable in the 'IO' monad
495 newtype IORef a = IORef (STRef RealWorld a)
497 -- explicit instance because Haddock can't figure out a derived one
498 instance Eq (IORef a) where
499 IORef x == IORef y = x == y
501 -- |Build a new 'IORef'
502 newIORef :: a -> IO (IORef a)
503 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
505 -- |Read the value of an 'IORef'
506 readIORef :: IORef a -> IO a
507 readIORef (IORef var) = stToIO (readSTRef var)
509 -- |Write a new value into an 'IORef'
510 writeIORef :: IORef a -> a -> IO ()
511 writeIORef (IORef var) v = stToIO (writeSTRef var v)
513 -- ---------------------------------------------------------------------------
514 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
515 -- The type arguments are as follows:
517 -- * @i@: the index type of the array (should be an instance of 'Ix')
519 -- * @e@: the element type of the array.
523 newtype IOArray i e = IOArray (STArray RealWorld i e)
525 -- explicit instance because Haddock can't figure out a derived one
526 instance Eq (IOArray i e) where
527 IOArray x == IOArray y = x == y
529 -- |Build a new 'IOArray'
530 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
531 {-# INLINE newIOArray #-}
532 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
534 -- | Read a value from an 'IOArray'
535 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
536 {-# INLINE unsafeReadIOArray #-}
537 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
539 -- | Write a new value into an 'IOArray'
540 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
541 {-# INLINE unsafeWriteIOArray #-}
542 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
544 -- | Read a value from an 'IOArray'
545 readIOArray :: Ix i => IOArray i e -> i -> IO e
546 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
548 -- | Write a new value into an 'IOArray'
549 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
550 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
553 -- ---------------------------------------------------------------------------
554 -- Show instance for Handles
556 -- handle types are 'show'n when printing error msgs, so
557 -- we provide a more user-friendly Show instance for it
558 -- than the derived one.
560 instance Show HandleType where
563 ClosedHandle -> showString "closed"
564 SemiClosedHandle -> showString "semi-closed"
565 ReadHandle -> showString "readable"
566 WriteHandle -> showString "writable"
567 AppendHandle -> showString "writable (append)"
568 ReadWriteHandle -> showString "read-writable"
570 instance Show Handle where
571 showsPrec p (FileHandle file _) = showHandle file
572 showsPrec p (DuplexHandle file _ _) = showHandle file
574 showHandle file = showString "{handle: " . showString file . showString "}"
576 -- ------------------------------------------------------------------------
577 -- Exception datatype and operations
579 -- |The type of exceptions. Every kind of system-generated exception
580 -- has a constructor in the 'Exception' type, and values of other
581 -- types may be injected into 'Exception' by coercing them to
582 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
583 -- "Control.Exception\#DynamicExceptions").
585 = ArithException ArithException
586 -- ^Exceptions raised by arithmetic
587 -- operations. (NOTE: GHC currently does not throw
588 -- 'ArithException's except for 'DivideByZero').
589 | ArrayException ArrayException
590 -- ^Exceptions raised by array-related
591 -- operations. (NOTE: GHC currently does not throw
592 -- 'ArrayException's).
593 | AssertionFailed String
594 -- ^This exception is thrown by the
595 -- 'assert' operation when the condition
596 -- fails. The 'String' argument contains the
597 -- location of the assertion in the source program.
598 | AsyncException AsyncException
599 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
601 -- ^The current thread was executing a call to
602 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
603 -- because there are no other references to this 'MVar'.
604 | BlockedIndefinitely
605 -- ^The current thread was waiting to retry an atomic memory transaction
606 -- that could never become possible to complete because there are no other
607 -- threads referring to any of teh TVars involved.
609 -- ^The runtime detected an attempt to nest one STM transaction
610 -- inside another one, presumably due to the use of
611 -- 'unsafePeformIO' with 'atomically'.
613 -- ^There are no runnable threads, so the program is
614 -- deadlocked. The 'Deadlock' exception is
615 -- raised in the main thread only (see also: "Control.Concurrent").
616 | DynException Dynamic
617 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
619 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
620 -- argument of 'ErrorCall' is the string passed to 'error' when it was
622 | ExitException ExitCode
623 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
624 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
625 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
626 -- main thread will cause the program to be terminated with the given
628 | IOException IOException
629 -- ^These are the standard IO exceptions generated by
630 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
631 | NoMethodError String
632 -- ^An attempt was made to invoke a class method which has
633 -- no definition in this instance, and there was no default
634 -- definition given in the class declaration. GHC issues a
635 -- warning when you compile an instance which has missing
638 -- ^The current thread is stuck in an infinite loop. This
639 -- exception may or may not be thrown when the program is
641 | PatternMatchFail String
642 -- ^A pattern matching failure. The 'String' argument should contain a
643 -- descriptive message including the function name, source file
646 -- ^An attempt was made to evaluate a field of a record
647 -- for which no value was given at construction time. The
648 -- 'String' argument gives the location of the
649 -- record construction in the source program.
651 -- ^A field selection was attempted on a constructor that
652 -- doesn\'t have the requested field. This can happen with
653 -- multi-constructor records when one or more fields are
654 -- missing from some of the constructors. The
655 -- 'String' argument gives the location of the
656 -- record selection in the source program.
658 -- ^An attempt was made to update a field in a record,
659 -- where the record doesn\'t have the requested field. This can
660 -- only occur with multi-constructor records, when one or more
661 -- fields are missing from some of the constructors. The
662 -- 'String' argument gives the location of the
663 -- record update in the source program.
665 -- |The type of arithmetic exceptions
675 -- |Asynchronous exceptions
678 -- ^The current thread\'s stack exceeded its limit.
679 -- Since an exception has been raised, the thread\'s stack
680 -- will certainly be below its limit again, but the
681 -- programmer should take remedial action
684 -- ^The program\'s heap is reaching its limit, and
685 -- the program should take action to reduce the amount of
686 -- live data it has. Notes:
688 -- * It is undefined which thread receives this exception.
690 -- * GHC currently does not throw 'HeapOverflow' exceptions.
692 -- ^This exception is raised by another thread
693 -- calling 'Control.Concurrent.killThread', or by the system
694 -- if it needs to terminate the thread for some
698 -- | Exceptions generated by array operations
700 = IndexOutOfBounds String
701 -- ^An attempt was made to index an array outside
702 -- its declared bounds.
703 | UndefinedElement String
704 -- ^An attempt was made to evaluate an element of an
705 -- array that had not been initialized.
708 stackOverflow, heapOverflow :: Exception -- for the RTS
709 stackOverflow = AsyncException StackOverflow
710 heapOverflow = AsyncException HeapOverflow
712 instance Show ArithException where
713 showsPrec _ Overflow = showString "arithmetic overflow"
714 showsPrec _ Underflow = showString "arithmetic underflow"
715 showsPrec _ LossOfPrecision = showString "loss of precision"
716 showsPrec _ DivideByZero = showString "divide by zero"
717 showsPrec _ Denormal = showString "denormal"
719 instance Show AsyncException where
720 showsPrec _ StackOverflow = showString "stack overflow"
721 showsPrec _ HeapOverflow = showString "heap overflow"
722 showsPrec _ ThreadKilled = showString "thread killed"
724 instance Show ArrayException where
725 showsPrec _ (IndexOutOfBounds s)
726 = showString "array index out of range"
727 . (if not (null s) then showString ": " . showString s
729 showsPrec _ (UndefinedElement s)
730 = showString "undefined array element"
731 . (if not (null s) then showString ": " . showString s
734 instance Show Exception where
735 showsPrec _ (IOException err) = shows err
736 showsPrec _ (ArithException err) = shows err
737 showsPrec _ (ArrayException err) = shows err
738 showsPrec _ (ErrorCall err) = showString err
739 showsPrec _ (ExitException err) = showString "exit: " . shows err
740 showsPrec _ (NoMethodError err) = showString err
741 showsPrec _ (PatternMatchFail err) = showString err
742 showsPrec _ (RecSelError err) = showString err
743 showsPrec _ (RecConError err) = showString err
744 showsPrec _ (RecUpdError err) = showString err
745 showsPrec _ (AssertionFailed err) = showString err
746 showsPrec _ (DynException err) = showString "exception :: " . showsTypeRep (dynTypeRep err)
747 showsPrec _ (AsyncException e) = shows e
748 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
749 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
750 showsPrec _ (NestedAtomically) = showString "Control.Concurrent.STM.atomically was nested"
751 showsPrec _ (NonTermination) = showString "<<loop>>"
752 showsPrec _ (Deadlock) = showString "<<deadlock>>"
754 instance Eq Exception where
755 IOException e1 == IOException e2 = e1 == e2
756 ArithException e1 == ArithException e2 = e1 == e2
757 ArrayException e1 == ArrayException e2 = e1 == e2
758 ErrorCall e1 == ErrorCall e2 = e1 == e2
759 ExitException e1 == ExitException e2 = e1 == e2
760 NoMethodError e1 == NoMethodError e2 = e1 == e2
761 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
762 RecSelError e1 == RecSelError e2 = e1 == e2
763 RecConError e1 == RecConError e2 = e1 == e2
764 RecUpdError e1 == RecUpdError e2 = e1 == e2
765 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
766 DynException _ == DynException _ = False -- incomparable
767 AsyncException e1 == AsyncException e2 = e1 == e2
768 BlockedOnDeadMVar == BlockedOnDeadMVar = True
769 NonTermination == NonTermination = True
770 NestedAtomically == NestedAtomically = True
771 Deadlock == Deadlock = True
774 -- -----------------------------------------------------------------------------
777 -- We need it here because it is used in ExitException in the
778 -- Exception datatype (above).
781 = ExitSuccess -- ^ indicates successful termination;
783 -- ^ indicates program failure with an exit code.
784 -- The exact interpretation of the code is
785 -- operating-system dependent. In particular, some values
786 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
787 deriving (Eq, Ord, Read, Show)
789 -- --------------------------------------------------------------------------
792 -- | Throw an exception. Exceptions may be thrown from purely
793 -- functional code, but may only be caught within the 'IO' monad.
794 throw :: Exception -> a
795 throw exception = raise# exception
797 -- | A variant of 'throw' that can be used within the 'IO' monad.
799 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
800 -- two functions are subtly different:
802 -- > throw e `seq` return () ===> throw e
803 -- > throwIO e `seq` return () ===> return ()
805 -- The first example will cause the exception @e@ to be raised,
806 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
807 -- an exception to be raised when it is used within the 'IO' monad.
808 -- The 'throwIO' variant should be used in preference to 'throw' to
809 -- raise an exception within the 'IO' monad because it guarantees
810 -- ordering with respect to other 'IO' operations, whereas 'throw'
812 throwIO :: Exception -> IO a
813 throwIO err = IO $ raiseIO# err
815 ioException :: IOException -> IO a
816 ioException err = IO $ raiseIO# (IOException err)
818 -- | Raise an 'IOError' in the 'IO' monad.
819 ioError :: IOError -> IO a
820 ioError = ioException
822 -- ---------------------------------------------------------------------------
825 -- | The Haskell 98 type for exceptions in the 'IO' monad.
826 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
827 -- For a more general type of exception, including also those that arise
828 -- in pure code, see 'Control.Exception.Exception'.
830 -- In Haskell 98, this is an opaque type.
831 type IOError = IOException
833 -- |Exceptions that occur in the @IO@ monad.
834 -- An @IOException@ records a more specific error type, a descriptive
835 -- string and maybe the handle that was used when the error was
839 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
841 ioe_type :: IOErrorType, -- what it was.
842 ioe_location :: String, -- location.
843 ioe_description :: String, -- error type specific information.
844 ioe_filename :: Maybe FilePath -- filename the error is related to.
847 instance Eq IOException where
848 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
849 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
851 -- | An abstract type that contains a value for each variant of 'IOError'.
863 | UnsatisfiedConstraints
870 | UnsupportedOperation
874 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
876 instance Eq IOErrorType where
879 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
880 _ -> getTag x ==# getTag y
882 instance Show IOErrorType where
886 AlreadyExists -> "already exists"
887 NoSuchThing -> "does not exist"
888 ResourceBusy -> "resource busy"
889 ResourceExhausted -> "resource exhausted"
891 IllegalOperation -> "illegal operation"
892 PermissionDenied -> "permission denied"
893 UserError -> "user error"
894 HardwareFault -> "hardware fault"
895 InappropriateType -> "inappropriate type"
896 Interrupted -> "interrupted"
897 InvalidArgument -> "invalid argument"
898 OtherError -> "failed"
899 ProtocolError -> "protocol error"
900 ResourceVanished -> "resource vanished"
901 SystemError -> "system error"
902 TimeExpired -> "timeout"
903 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
904 UnsupportedOperation -> "unsupported operation"
905 DynIOError{} -> "unknown IO error"
907 -- | Construct an 'IOError' value with a string describing the error.
908 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
909 -- 'userError', thus:
911 -- > instance Monad IO where
913 -- > fail s = ioError (userError s)
915 userError :: String -> IOError
916 userError str = IOError Nothing UserError "" str Nothing
918 -- ---------------------------------------------------------------------------
921 instance Show IOException where
922 showsPrec p (IOError hdl iot loc s fn) =
924 Nothing -> case hdl of
926 Just h -> showsPrec p h . showString ": "
927 Just name -> showString name . showString ": ") .
930 _ -> showString loc . showString ": ") .
934 _ -> showString " (" . showString s . showString ")")
936 -- -----------------------------------------------------------------------------
939 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
940 deriving (Eq, Ord, Ix, Enum, Read, Show)