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,
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, 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 #-} Data.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 -- ---------------------------------------------------------------------------
159 -- Unsafe IO operations
162 This is the \"back door\" into the 'IO' monad, allowing
163 'IO' computation to be performed at any time. For
164 this to be safe, the 'IO' computation should be
165 free of side effects and independent of its environment.
167 If the I\/O computation wrapped in 'unsafePerformIO'
168 performs side effects, then the relative order in which those side
169 effects take place (relative to the main I\/O trunk, or other calls to
170 'unsafePerformIO') is indeterminate. You have to be careful when
171 writing and compiling modules that use 'unsafePerformIO':
173 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
174 that calls 'unsafePerformIO'. If the call is inlined,
175 the I\/O may be performed more than once.
177 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
178 elimination being performed on the module, which might combine
179 two side effects that were meant to be separate. A good example
180 is using multiple global variables (like @test@ in the example below).
182 * Make sure that the either you switch off let-floating, or that the
183 call to 'unsafePerformIO' cannot float outside a lambda. For example,
186 f x = unsafePerformIO (newIORef [])
188 you may get only one reference cell shared between all calls to @f@.
191 f x = unsafePerformIO (newIORef [x])
193 because now it can't float outside the lambda.
195 It is less well known that
196 'unsafePerformIO' is not type safe. For example:
199 > test = unsafePerformIO $ newIORef []
202 > writeIORef test [42]
203 > bang <- readIORef test
204 > print (bang :: [Char])
206 This program will core dump. This problem with polymorphic references
207 is well known in the ML community, and does not arise with normal
208 monadic use of references. There is no easy way to make it impossible
209 once you use 'unsafePerformIO'. Indeed, it is
210 possible to write @coerce :: a -> b@ with the
211 help of 'unsafePerformIO'. So be careful!
213 {-# NOINLINE unsafePerformIO #-}
214 unsafePerformIO :: IO a -> a
215 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
217 -- Why do we NOINLINE unsafePerformIO? See the comment with
218 -- GHC.ST.runST. Essentially the issue is that the IO computation
219 -- inside unsafePerformIO must be atomic: it must either all run, or
220 -- not at all. If we let the compiler see the application of the IO
221 -- to realWorld#, it might float out part of the IO.
224 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
225 When passed a value of type @IO a@, the 'IO' will only be performed
226 when the value of the @a@ is demanded. This is used to implement lazy
227 file reading, see 'System.IO.hGetContents'.
229 {-# INLINE unsafeInterleaveIO #-}
230 unsafeInterleaveIO :: IO a -> IO a
231 unsafeInterleaveIO (IO m)
233 r = case m s of (# _, res #) -> res
237 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
238 -- state from this IO thread is passed explicitly to the interleaved
239 -- IO, so it cannot be floated out and shared.
241 -- ---------------------------------------------------------------------------
244 data MVar a = MVar (MVar# RealWorld a)
246 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
247 for communication between concurrent threads. It can be thought of
248 as a a box, which may be empty or full.
251 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
252 instance Eq (MVar a) where
253 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
255 -- A Handle is represented by (a reference to) a record
256 -- containing the state of the I/O port/device. We record
257 -- the following pieces of info:
259 -- * type (read,write,closed etc.)
260 -- * the underlying file descriptor
262 -- * buffer, and spare buffers
263 -- * user-friendly name (usually the
264 -- FilePath used when IO.openFile was called)
266 -- Note: when a Handle is garbage collected, we want to flush its buffer
267 -- and close the OS file handle, so as to free up a (precious) resource.
269 -- | Haskell defines operations to read and write characters from and to files,
270 -- represented by values of type @Handle@. Each value of this type is a
271 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
272 -- with file system objects. A handle has at least the following properties:
274 -- * whether it manages input or output or both;
276 -- * whether it is /open/, /closed/ or /semi-closed/;
278 -- * whether the object is seekable;
280 -- * whether buffering is disabled, or enabled on a line or block basis;
282 -- * a buffer (whose length may be zero).
284 -- Most handles will also have a current I\/O position indicating where the next
285 -- input or output operation will occur. A handle is /readable/ if it
286 -- manages only input or both input and output; likewise, it is /writable/ if
287 -- it manages only output or both input and output. A handle is /open/ when
289 -- Once it is closed it can no longer be used for either input or output,
290 -- though an implementation cannot re-use its storage while references
291 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
292 -- produced by showing a handle is system dependent; it should include
293 -- enough information to identify the handle for debugging. A handle is
294 -- equal according to '==' only to itself; no attempt
295 -- is made to compare the internal state of different handles for equality.
297 -- GHC note: a 'Handle' will be automatically closed when the garbage
298 -- collector detects that it has become unreferenced by the program.
299 -- However, relying on this behaviour is not generally recommended:
300 -- the garbage collector is unpredictable. If possible, use explicit
301 -- an explicit 'hClose' to close 'Handle's when they are no longer
302 -- required. GHC does not currently attempt to free up file
303 -- descriptors when they have run out, it is your responsibility to
304 -- ensure that this doesn't happen.
307 = FileHandle -- A normal handle to a file
308 FilePath -- the file (invariant)
311 | DuplexHandle -- A handle to a read/write stream
312 FilePath -- file for a FIFO, otherwise some
313 -- descriptive string.
314 !(MVar Handle__) -- The read side
315 !(MVar Handle__) -- The write side
318 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
321 instance Eq Handle where
322 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
323 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
326 type FD = Int -- XXX ToDo: should be CInt
330 haFD :: !FD, -- file descriptor
331 haType :: HandleType, -- type (read/write/append etc.)
332 haIsBin :: Bool, -- binary mode?
333 haIsStream :: Bool, -- is this a stream handle?
334 haBufferMode :: BufferMode, -- buffer contains read/write data?
335 haBuffer :: !(IORef Buffer), -- the current buffer
336 haBuffers :: !(IORef BufferList), -- spare buffers
337 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
341 -- ---------------------------------------------------------------------------
344 -- The buffer is represented by a mutable variable containing a
345 -- record, where the record contains the raw buffer and the start/end
346 -- points of the filled portion. We use a mutable variable so that
347 -- the common operation of writing (or reading) some data from (to)
348 -- the buffer doesn't need to modify, and hence copy, the handle
349 -- itself, it just updates the buffer.
351 -- There will be some allocation involved in a simple hPutChar in
352 -- order to create the new Buffer structure (below), but this is
353 -- relatively small, and this only has to be done once per write
356 -- The buffer contains its size - we could also get the size by
357 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
358 -- to be rounded up to the nearest Word.
360 type RawBuffer = MutableByteArray# RealWorld
362 -- INVARIANTS on a Buffer:
364 -- * A handle *always* has a buffer, even if it is only 1 character long
365 -- (an unbuffered handle needs a 1 character buffer in order to support
366 -- hLookAhead and hIsEOF).
368 -- * if r == w, then r == 0 && w == 0
369 -- * if state == WriteBuffer, then r == 0
370 -- * a write buffer is never full. If an operation
371 -- fills up the buffer, it will always flush it before
373 -- * a read buffer may be full as a result of hLookAhead. In normal
374 -- operation, a read buffer always has at least one character of space.
382 bufState :: BufferState
385 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
387 -- we keep a few spare buffers around in a handle to avoid allocating
388 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
389 -- same size as the main buffer.
392 | BufferListCons RawBuffer BufferList
395 bufferIsWritable :: Buffer -> Bool
396 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
397 bufferIsWritable _other = False
399 bufferEmpty :: Buffer -> Bool
400 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
402 -- only makes sense for a write buffer
403 bufferFull :: Buffer -> Bool
404 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
406 -- Internally, we classify handles as being one
417 isReadableHandleType ReadHandle = True
418 isReadableHandleType ReadWriteHandle = True
419 isReadableHandleType _ = False
421 isWritableHandleType AppendHandle = True
422 isWritableHandleType WriteHandle = True
423 isWritableHandleType ReadWriteHandle = True
424 isWritableHandleType _ = False
426 -- | File and directory names are values of type 'String', whose precise
427 -- meaning is operating system dependent. Files can be opened, yielding a
428 -- handle which can then be used to operate on the contents of that file.
430 type FilePath = String
432 -- ---------------------------------------------------------------------------
435 -- | Three kinds of buffering are supported: line-buffering,
436 -- block-buffering or no-buffering. These modes have the following
437 -- effects. For output, items are written out, or /flushed/,
438 -- from the internal buffer according to the buffer mode:
440 -- * /line-buffering/: the entire output buffer is flushed
441 -- whenever a newline is output, the buffer overflows,
442 -- a 'System.IO.hFlush' is issued, or the handle is closed.
444 -- * /block-buffering/: the entire buffer is written out whenever it
445 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
447 -- * /no-buffering/: output is written immediately, and never stored
450 -- An implementation is free to flush the buffer more frequently,
451 -- but not less frequently, than specified above.
452 -- The output buffer is emptied as soon as it has been written out.
454 -- Similarly, input occurs according to the buffer mode for the handle:
456 -- * /line-buffering/: when the buffer for the handle is not empty,
457 -- the next item is obtained from the buffer; otherwise, when the
458 -- buffer is empty, characters up to and including the next newline
459 -- character are read into the buffer. No characters are available
460 -- until the newline character is available or the buffer is full.
462 -- * /block-buffering/: when the buffer for the handle becomes empty,
463 -- the next block of data is read into the buffer.
465 -- * /no-buffering/: the next input item is read and returned.
466 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
467 -- handle may require a one-character buffer.
469 -- The default buffering mode when a handle is opened is
470 -- implementation-dependent and may depend on the file system object
471 -- which is attached to that handle.
472 -- For most implementations, physical files will normally be block-buffered
473 -- and terminals will normally be line-buffered.
476 = NoBuffering -- ^ buffering is disabled if possible.
478 -- ^ line-buffering should be enabled if possible.
479 | BlockBuffering (Maybe Int)
480 -- ^ block-buffering should be enabled if possible.
481 -- The size of the buffer is @n@ items if the argument
482 -- is 'Just' @n@ and is otherwise implementation-dependent.
483 deriving (Eq, Ord, Read, Show)
485 -- ---------------------------------------------------------------------------
488 -- |A mutable variable in the 'IO' monad
489 newtype IORef a = IORef (STRef RealWorld a)
491 -- explicit instance because Haddock can't figure out a derived one
492 instance Eq (IORef a) where
493 IORef x == IORef y = x == y
495 -- |Build a new 'IORef'
496 newIORef :: a -> IO (IORef a)
497 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
499 -- |Read the value of an 'IORef'
500 readIORef :: IORef a -> IO a
501 readIORef (IORef var) = stToIO (readSTRef var)
503 -- |Write a new value into an 'IORef'
504 writeIORef :: IORef a -> a -> IO ()
505 writeIORef (IORef var) v = stToIO (writeSTRef var v)
507 -- ---------------------------------------------------------------------------
508 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
509 -- The type arguments are as follows:
511 -- * @i@: the index type of the array (should be an instance of 'Ix')
513 -- * @e@: the element type of the array.
517 newtype IOArray i e = IOArray (STArray RealWorld i e)
519 -- explicit instance because Haddock can't figure out a derived one
520 instance Eq (IOArray i e) where
521 IOArray x == IOArray y = x == y
523 -- |Build a new 'IOArray'
524 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
525 {-# INLINE newIOArray #-}
526 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
528 -- | Read a value from an 'IOArray'
529 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
530 {-# INLINE unsafeReadIOArray #-}
531 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
533 -- | Write a new value into an 'IOArray'
534 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
535 {-# INLINE unsafeWriteIOArray #-}
536 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
538 -- | Read a value from an 'IOArray'
539 readIOArray :: Ix i => IOArray i e -> i -> IO e
540 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
542 -- | Write a new value into an 'IOArray'
543 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
544 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
547 -- ---------------------------------------------------------------------------
548 -- Show instance for Handles
550 -- handle types are 'show'n when printing error msgs, so
551 -- we provide a more user-friendly Show instance for it
552 -- than the derived one.
554 instance Show HandleType where
557 ClosedHandle -> showString "closed"
558 SemiClosedHandle -> showString "semi-closed"
559 ReadHandle -> showString "readable"
560 WriteHandle -> showString "writable"
561 AppendHandle -> showString "writable (append)"
562 ReadWriteHandle -> showString "read-writable"
564 instance Show Handle where
565 showsPrec p (FileHandle file _) = showHandle file
566 showsPrec p (DuplexHandle file _ _) = showHandle file
568 showHandle file = showString "{handle: " . showString file . showString "}"
570 -- ------------------------------------------------------------------------
571 -- Exception datatype and operations
573 -- |The type of exceptions. Every kind of system-generated exception
574 -- has a constructor in the 'Exception' type, and values of other
575 -- types may be injected into 'Exception' by coercing them to
576 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
577 -- "Control.Exception\#DynamicExceptions").
579 = ArithException ArithException
580 -- ^Exceptions raised by arithmetic
581 -- operations. (NOTE: GHC currently does not throw
582 -- 'ArithException's except for 'DivideByZero').
583 | ArrayException ArrayException
584 -- ^Exceptions raised by array-related
585 -- operations. (NOTE: GHC currently does not throw
586 -- 'ArrayException's).
587 | AssertionFailed String
588 -- ^This exception is thrown by the
589 -- 'assert' operation when the condition
590 -- fails. The 'String' argument contains the
591 -- location of the assertion in the source program.
592 | AsyncException AsyncException
593 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
595 -- ^The current thread was executing a call to
596 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
597 -- because there are no other references to this 'MVar'.
598 | BlockedIndefinitely
599 -- ^The current thread was waiting to retry an atomic memory transaction
600 -- that could never become possible to complete because there are no other
601 -- threads referring to any of teh TVars involved.
603 -- ^There are no runnable threads, so the program is
604 -- deadlocked. The 'Deadlock' exception is
605 -- raised in the main thread only (see also: "Control.Concurrent").
606 | DynException Dynamic
607 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
609 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
610 -- argument of 'ErrorCall' is the string passed to 'error' when it was
612 | ExitException ExitCode
613 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
614 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
615 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
616 -- main thread will cause the program to be terminated with the given
618 | IOException IOException
619 -- ^These are the standard IO exceptions generated by
620 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
621 | NoMethodError String
622 -- ^An attempt was made to invoke a class method which has
623 -- no definition in this instance, and there was no default
624 -- definition given in the class declaration. GHC issues a
625 -- warning when you compile an instance which has missing
628 -- ^The current thread is stuck in an infinite loop. This
629 -- exception may or may not be thrown when the program is
631 | PatternMatchFail String
632 -- ^A pattern matching failure. The 'String' argument should contain a
633 -- descriptive message including the function name, source file
636 -- ^An attempt was made to evaluate a field of a record
637 -- for which no value was given at construction time. The
638 -- 'String' argument gives the location of the
639 -- record construction in the source program.
641 -- ^A field selection was attempted on a constructor that
642 -- doesn\'t have the requested field. This can happen with
643 -- multi-constructor records when one or more fields are
644 -- missing from some of the constructors. The
645 -- 'String' argument gives the location of the
646 -- record selection in the source program.
648 -- ^An attempt was made to update a field in a record,
649 -- where the record doesn\'t have the requested field. This can
650 -- only occur with multi-constructor records, when one or more
651 -- fields are missing from some of the constructors. The
652 -- 'String' argument gives the location of the
653 -- record update in the source program.
655 -- |The type of arithmetic exceptions
665 -- |Asynchronous exceptions
668 -- ^The current thread\'s stack exceeded its limit.
669 -- Since an exception has been raised, the thread\'s stack
670 -- will certainly be below its limit again, but the
671 -- programmer should take remedial action
674 -- ^The program\'s heap is reaching its limit, and
675 -- the program should take action to reduce the amount of
676 -- live data it has. Notes:
678 -- * It is undefined which thread receives this exception.
680 -- * GHC currently does not throw 'HeapOverflow' exceptions.
682 -- ^This exception is raised by another thread
683 -- calling 'Control.Concurrent.killThread', or by the system
684 -- if it needs to terminate the thread for some
688 -- | Exceptions generated by array operations
690 = IndexOutOfBounds String
691 -- ^An attempt was made to index an array outside
692 -- its declared bounds.
693 | UndefinedElement String
694 -- ^An attempt was made to evaluate an element of an
695 -- array that had not been initialized.
698 stackOverflow, heapOverflow :: Exception -- for the RTS
699 stackOverflow = AsyncException StackOverflow
700 heapOverflow = AsyncException HeapOverflow
702 instance Show ArithException where
703 showsPrec _ Overflow = showString "arithmetic overflow"
704 showsPrec _ Underflow = showString "arithmetic underflow"
705 showsPrec _ LossOfPrecision = showString "loss of precision"
706 showsPrec _ DivideByZero = showString "divide by zero"
707 showsPrec _ Denormal = showString "denormal"
709 instance Show AsyncException where
710 showsPrec _ StackOverflow = showString "stack overflow"
711 showsPrec _ HeapOverflow = showString "heap overflow"
712 showsPrec _ ThreadKilled = showString "thread killed"
714 instance Show ArrayException where
715 showsPrec _ (IndexOutOfBounds s)
716 = showString "array index out of range"
717 . (if not (null s) then showString ": " . showString s
719 showsPrec _ (UndefinedElement s)
720 = showString "undefined array element"
721 . (if not (null s) then showString ": " . showString s
724 instance Show Exception where
725 showsPrec _ (IOException err) = shows err
726 showsPrec _ (ArithException err) = shows err
727 showsPrec _ (ArrayException err) = shows err
728 showsPrec _ (ErrorCall err) = showString err
729 showsPrec _ (ExitException err) = showString "exit: " . shows err
730 showsPrec _ (NoMethodError err) = showString err
731 showsPrec _ (PatternMatchFail err) = showString err
732 showsPrec _ (RecSelError err) = showString err
733 showsPrec _ (RecConError err) = showString err
734 showsPrec _ (RecUpdError err) = showString err
735 showsPrec _ (AssertionFailed err) = showString err
736 showsPrec _ (DynException _err) = showString "unknown exception"
737 showsPrec _ (AsyncException e) = shows e
738 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
739 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
740 showsPrec _ (NonTermination) = showString "<<loop>>"
741 showsPrec _ (Deadlock) = showString "<<deadlock>>"
743 instance Eq Exception where
744 IOException e1 == IOException e2 = e1 == e2
745 ArithException e1 == ArithException e2 = e1 == e2
746 ArrayException e1 == ArrayException e2 = e1 == e2
747 ErrorCall e1 == ErrorCall e2 = e1 == e2
748 ExitException e1 == ExitException e2 = e1 == e2
749 NoMethodError e1 == NoMethodError e2 = e1 == e2
750 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
751 RecSelError e1 == RecSelError e2 = e1 == e2
752 RecConError e1 == RecConError e2 = e1 == e2
753 RecUpdError e1 == RecUpdError e2 = e1 == e2
754 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
755 DynException _ == DynException _ = False -- incomparable
756 AsyncException e1 == AsyncException e2 = e1 == e2
757 BlockedOnDeadMVar == BlockedOnDeadMVar = True
758 NonTermination == NonTermination = True
759 Deadlock == Deadlock = True
762 -- -----------------------------------------------------------------------------
765 -- We need it here because it is used in ExitException in the
766 -- Exception datatype (above).
769 = ExitSuccess -- ^ indicates successful termination;
771 -- ^ indicates program failure with an exit code.
772 -- The exact interpretation of the code is
773 -- operating-system dependent. In particular, some values
774 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
775 deriving (Eq, Ord, Read, Show)
777 -- --------------------------------------------------------------------------
780 -- | Throw an exception. Exceptions may be thrown from purely
781 -- functional code, but may only be caught within the 'IO' monad.
782 throw :: Exception -> a
783 throw exception = raise# exception
785 -- | A variant of 'throw' that can be used within the 'IO' monad.
787 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
788 -- two functions are subtly different:
790 -- > throw e `seq` return () ===> throw e
791 -- > throwIO e `seq` return () ===> return ()
793 -- The first example will cause the exception @e@ to be raised,
794 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
795 -- an exception to be raised when it is used within the 'IO' monad.
796 -- The 'throwIO' variant should be used in preference to 'throw' to
797 -- raise an exception within the 'IO' monad because it guarantees
798 -- ordering with respect to other 'IO' operations, whereas 'throw'
800 throwIO :: Exception -> IO a
801 throwIO err = IO $ raiseIO# err
803 ioException :: IOException -> IO a
804 ioException err = IO $ raiseIO# (IOException err)
806 -- | Raise an 'IOError' in the 'IO' monad.
807 ioError :: IOError -> IO a
808 ioError = ioException
810 -- ---------------------------------------------------------------------------
813 -- | The Haskell 98 type for exceptions in the 'IO' monad.
814 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
815 -- For a more general type of exception, including also those that arise
816 -- in pure code, see 'Control.Exception.Exception'.
818 -- In Haskell 98, this is an opaque type.
819 type IOError = IOException
821 -- |Exceptions that occur in the @IO@ monad.
822 -- An @IOException@ records a more specific error type, a descriptive
823 -- string and maybe the handle that was used when the error was
827 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
829 ioe_type :: IOErrorType, -- what it was.
830 ioe_location :: String, -- location.
831 ioe_description :: String, -- error type specific information.
832 ioe_filename :: Maybe FilePath -- filename the error is related to.
835 instance Eq IOException where
836 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
837 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
839 -- | An abstract type that contains a value for each variant of 'IOError'.
851 | UnsatisfiedConstraints
858 | UnsupportedOperation
862 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
864 instance Eq IOErrorType where
867 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
868 _ -> getTag x ==# getTag y
870 instance Show IOErrorType where
874 AlreadyExists -> "already exists"
875 NoSuchThing -> "does not exist"
876 ResourceBusy -> "resource busy"
877 ResourceExhausted -> "resource exhausted"
879 IllegalOperation -> "illegal operation"
880 PermissionDenied -> "permission denied"
881 UserError -> "user error"
882 HardwareFault -> "hardware fault"
883 InappropriateType -> "inappropriate type"
884 Interrupted -> "interrupted"
885 InvalidArgument -> "invalid argument"
886 OtherError -> "failed"
887 ProtocolError -> "protocol error"
888 ResourceVanished -> "resource vanished"
889 SystemError -> "system error"
890 TimeExpired -> "timeout"
891 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
892 UnsupportedOperation -> "unsupported operation"
893 DynIOError{} -> "unknown IO error"
895 -- | Construct an 'IOError' value with a string describing the error.
896 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
897 -- 'userError', thus:
899 -- > instance Monad IO where
901 -- > fail s = ioError (userError s)
903 userError :: String -> IOError
904 userError str = IOError Nothing UserError "" str Nothing
906 -- ---------------------------------------------------------------------------
909 instance Show IOException where
910 showsPrec p (IOError hdl iot loc s fn) =
912 Nothing -> case hdl of
914 Just h -> showsPrec p h . showString ": "
915 Just name -> showString name . showString ": ") .
918 _ -> showString loc . showString ": ") .
922 _ -> showString " (" . showString s . showString ")")
924 -- -----------------------------------------------------------------------------
927 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
928 deriving (Eq, Ord, Ix, Enum, Read, Show)