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 -----------------------------------------------------------------------------
18 IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
19 unsafePerformIO, unsafeInterleaveIO,
21 -- To and from from ST
22 stToIO, ioToST, unsafeIOToST,
25 IORef(..), newIORef, readIORef, writeIORef,
26 IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
29 -- Handles, file descriptors,
31 Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
32 isReadableHandleType, isWritableHandleType, showHandle,
35 Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
36 bufferIsWritable, bufferEmpty, bufferFull,
39 Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
40 stackOverflow, heapOverflow, throw, throwIO, ioException,
41 IOError, IOException(..), IOErrorType(..), ioError, userError,
46 import GHC.Arr -- to derive Ix class
47 import GHC.Enum -- to derive Enum class
50 -- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
51 import Data.Maybe ( Maybe(..) )
57 import {-# SOURCE #-} Data.Dynamic
60 -- ---------------------------------------------------------------------------
64 The IO Monad is just an instance of the ST monad, where the state is
65 the real world. We use the exception mechanism (in GHC.Exception) to
66 implement IO exceptions.
68 NOTE: The IO representation is deeply wired in to various parts of the
69 system. The following list may or may not be exhaustive:
71 Compiler - types of various primitives in PrimOp.lhs
73 RTS - forceIO (StgMiscClosures.hc)
74 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
76 - raiseAsync (Schedule.c)
78 Prelude - GHC.IOBase.lhs, and several other places including
81 Libraries - parts of hslibs/lang.
87 A value of type @'IO' a@ is a computation which, when performed,
88 does some I\/O before returning a value of type @a@.
90 There is really only one way to \"perform\" an I\/O action: bind it to
91 @Main.main@ in your program. When your program is run, the I\/O will
92 be performed. It isn't possible to perform I\/O from an arbitrary
93 function, unless that function is itself in the 'IO' monad and called
94 at some point, directly or indirectly, from @Main.main@.
96 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
97 or the '>>' and '>>=' operations from the 'Monad' class.
99 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
101 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
104 instance Functor IO where
105 fmap f x = x >>= (return . f)
107 instance Monad IO where
108 {-# INLINE return #-}
111 m >> k = m >>= \ _ -> k
112 return x = returnIO x
117 failIO :: String -> IO a
118 failIO s = ioError (userError s)
120 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
121 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
123 bindIO :: IO a -> (a -> IO b) -> IO b
124 bindIO (IO m) k = IO ( \ s ->
126 (# new_s, a #) -> unIO (k a) new_s
129 thenIO :: IO a -> IO b -> IO b
130 thenIO (IO m) k = IO ( \ s ->
132 (# new_s, a #) -> unIO k new_s
135 returnIO :: a -> IO a
136 returnIO x = IO (\ s -> (# s, x #))
138 -- ---------------------------------------------------------------------------
139 -- Coercions between IO and ST
141 -- | A monad transformer embedding strict state transformers in the 'IO'
142 -- monad. The 'RealWorld' parameter indicates that the internal state
143 -- used by the 'ST' computation is a special one supplied by the 'IO'
144 -- monad, and thus distinct from those used by invocations of 'runST'.
145 stToIO :: ST RealWorld a -> IO a
148 ioToST :: IO a -> ST RealWorld a
149 ioToST (IO m) = (ST m)
151 -- This relies on IO and ST having the same representation modulo the
152 -- constraint on the type of the state
154 unsafeIOToST :: IO a -> ST s a
155 unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
157 -- ---------------------------------------------------------------------------
158 -- Unsafe IO operations
161 This is the \"back door\" into the 'IO' monad, allowing
162 'IO' computation to be performed at any time. For
163 this to be safe, the 'IO' computation should be
164 free of side effects and independent of its environment.
166 If the I\/O computation wrapped in 'unsafePerformIO'
167 performs side effects, then the relative order in which those side
168 effects take place (relative to the main I\/O trunk, or other calls to
169 'unsafePerformIO') is indeterminate. You have to be careful when
170 writing and compiling modules that use 'unsafePerformIO':
172 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
173 that calls 'unsafePerformIO'. If the call is inlined,
174 the I\/O may be performed more than once.
176 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
177 elimination being performed on the module, which might combine
178 two side effects that were meant to be separate. A good example
179 is using multiple global variables (like @test@ in the example below).
181 * Make sure that the either you switch off let-floating, or that the
182 call to 'unsafePerformIO' cannot float outside a lambda. For example,
185 f x = unsafePerformIO (newIORef [])
187 you may get only one reference cell shared between all calls to @f@.
190 f x = unsafePerformIO (newIORef [x])
192 because now it can't float outside the lambda.
194 It is less well known that
195 'unsafePerformIO' is not type safe. For example:
198 > test = unsafePerformIO $ newIORef []
201 > writeIORef test [42]
202 > bang <- readIORef test
203 > print (bang :: [Char])
205 This program will core dump. This problem with polymorphic references
206 is well known in the ML community, and does not arise with normal
207 monadic use of references. There is no easy way to make it impossible
208 once you use 'unsafePerformIO'. Indeed, it is
209 possible to write @coerce :: a -> b@ with the
210 help of 'unsafePerformIO'. So be careful!
212 {-# NOINLINE unsafePerformIO #-}
213 unsafePerformIO :: IO a -> a
214 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
216 -- Why do we NOINLINE unsafePerformIO? See the comment with
217 -- GHC.ST.runST. Essentially the issue is that the IO computation
218 -- inside unsafePerformIO must be atomic: it must either all run, or
219 -- not at all. If we let the compiler see the application of the IO
220 -- to realWorld#, it might float out part of the IO.
223 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
224 When passed a value of type @IO a@, the 'IO' will only be performed
225 when the value of the @a@ is demanded. This is used to implement lazy
226 file reading, see 'System.IO.hGetContents'.
228 {-# INLINE unsafeInterleaveIO #-}
229 unsafeInterleaveIO :: IO a -> IO a
230 unsafeInterleaveIO (IO m)
232 r = case m s of (# _, res #) -> res
236 -- We believe that INLINE on unsafeInterleaveIO is safe, because the
237 -- state from this IO thread is passed explicitly to the interleaved
238 -- IO, so it cannot be floated out and shared.
240 -- ---------------------------------------------------------------------------
243 data MVar a = MVar (MVar# RealWorld a)
245 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
246 for communication between concurrent threads. It can be thought of
247 as a a box, which may be empty or full.
250 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
251 instance Eq (MVar a) where
252 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
254 -- A Handle is represented by (a reference to) a record
255 -- containing the state of the I/O port/device. We record
256 -- the following pieces of info:
258 -- * type (read,write,closed etc.)
259 -- * the underlying file descriptor
261 -- * buffer, and spare buffers
262 -- * user-friendly name (usually the
263 -- FilePath used when IO.openFile was called)
265 -- Note: when a Handle is garbage collected, we want to flush its buffer
266 -- and close the OS file handle, so as to free up a (precious) resource.
268 -- | Haskell defines operations to read and write characters from and to files,
269 -- represented by values of type @Handle@. Each value of this type is a
270 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
271 -- with file system objects. A handle has at least the following properties:
273 -- * whether it manages input or output or both;
275 -- * whether it is /open/, /closed/ or /semi-closed/;
277 -- * whether the object is seekable;
279 -- * whether buffering is disabled, or enabled on a line or block basis;
281 -- * a buffer (whose length may be zero).
283 -- Most handles will also have a current I\/O position indicating where the next
284 -- input or output operation will occur. A handle is /readable/ if it
285 -- manages only input or both input and output; likewise, it is /writable/ if
286 -- it manages only output or both input and output. A handle is /open/ when
288 -- Once it is closed it can no longer be used for either input or output,
289 -- though an implementation cannot re-use its storage while references
290 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
291 -- produced by showing a handle is system dependent; it should include
292 -- enough information to identify the handle for debugging. A handle is
293 -- equal according to '==' only to itself; no attempt
294 -- is made to compare the internal state of different handles for equality.
296 -- GHC note: a 'Handle' will be automatically closed when the garbage
297 -- collector detects that it has become unreferenced by the program.
298 -- However, relying on this behaviour is not generally recommended:
299 -- the garbage collector is unpredictable. If possible, use explicit
300 -- an explicit 'hClose' to close 'Handle's when they are no longer
301 -- required. GHC does not currently attempt to free up file
302 -- descriptors when they have run out, it is your responsibility to
303 -- ensure that this doesn't happen.
306 = FileHandle -- A normal handle to a file
307 FilePath -- the file (invariant)
310 | DuplexHandle -- A handle to a read/write stream
311 FilePath -- file for a FIFO, otherwise some
312 -- descriptive string.
313 !(MVar Handle__) -- The read side
314 !(MVar Handle__) -- The write side
317 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
320 instance Eq Handle where
321 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
322 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
325 type FD = Int -- XXX ToDo: should be CInt
329 haFD :: !FD, -- file descriptor
330 haType :: HandleType, -- type (read/write/append etc.)
331 haIsBin :: Bool, -- binary mode?
332 haIsStream :: Bool, -- is this a stream handle?
333 haBufferMode :: BufferMode, -- buffer contains read/write data?
334 haBuffer :: !(IORef Buffer), -- the current buffer
335 haBuffers :: !(IORef BufferList), -- spare buffers
336 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
340 -- ---------------------------------------------------------------------------
343 -- The buffer is represented by a mutable variable containing a
344 -- record, where the record contains the raw buffer and the start/end
345 -- points of the filled portion. We use a mutable variable so that
346 -- the common operation of writing (or reading) some data from (to)
347 -- the buffer doesn't need to modify, and hence copy, the handle
348 -- itself, it just updates the buffer.
350 -- There will be some allocation involved in a simple hPutChar in
351 -- order to create the new Buffer structure (below), but this is
352 -- relatively small, and this only has to be done once per write
355 -- The buffer contains its size - we could also get the size by
356 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
357 -- to be rounded up to the nearest Word.
359 type RawBuffer = MutableByteArray# RealWorld
361 -- INVARIANTS on a Buffer:
363 -- * A handle *always* has a buffer, even if it is only 1 character long
364 -- (an unbuffered handle needs a 1 character buffer in order to support
365 -- hLookAhead and hIsEOF).
367 -- * if r == w, then r == 0 && w == 0
368 -- * if state == WriteBuffer, then r == 0
369 -- * a write buffer is never full. If an operation
370 -- fills up the buffer, it will always flush it before
372 -- * a read buffer may be full as a result of hLookAhead. In normal
373 -- operation, a read buffer always has at least one character of space.
381 bufState :: BufferState
384 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
386 -- we keep a few spare buffers around in a handle to avoid allocating
387 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
388 -- same size as the main buffer.
391 | BufferListCons RawBuffer BufferList
394 bufferIsWritable :: Buffer -> Bool
395 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
396 bufferIsWritable _other = False
398 bufferEmpty :: Buffer -> Bool
399 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
401 -- only makes sense for a write buffer
402 bufferFull :: Buffer -> Bool
403 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
405 -- Internally, we classify handles as being one
416 isReadableHandleType ReadHandle = True
417 isReadableHandleType ReadWriteHandle = True
418 isReadableHandleType _ = False
420 isWritableHandleType AppendHandle = True
421 isWritableHandleType WriteHandle = True
422 isWritableHandleType ReadWriteHandle = True
423 isWritableHandleType _ = False
425 -- | File and directory names are values of type 'String', whose precise
426 -- meaning is operating system dependent. Files can be opened, yielding a
427 -- handle which can then be used to operate on the contents of that file.
429 type FilePath = String
431 -- ---------------------------------------------------------------------------
434 -- | Three kinds of buffering are supported: line-buffering,
435 -- block-buffering or no-buffering. These modes have the following
436 -- effects. For output, items are written out, or /flushed/,
437 -- from the internal buffer according to the buffer mode:
439 -- * /line-buffering/: the entire output buffer is flushed
440 -- whenever a newline is output, the buffer overflows,
441 -- a 'System.IO.hFlush' is issued, or the handle is closed.
443 -- * /block-buffering/: the entire buffer is written out whenever it
444 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
446 -- * /no-buffering/: output is written immediately, and never stored
449 -- An implementation is free to flush the buffer more frequently,
450 -- but not less frequently, than specified above.
451 -- The output buffer is emptied as soon as it has been written out.
453 -- Similarly, input occurs according to the buffer mode for the handle:
455 -- * /line-buffering/: when the buffer for the handle is not empty,
456 -- the next item is obtained from the buffer; otherwise, when the
457 -- buffer is empty, characters up to and including the next newline
458 -- character are read into the buffer. No characters are available
459 -- until the newline character is available or the buffer is full.
461 -- * /block-buffering/: when the buffer for the handle becomes empty,
462 -- the next block of data is read into the buffer.
464 -- * /no-buffering/: the next input item is read and returned.
465 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
466 -- handle may require a one-character buffer.
468 -- The default buffering mode when a handle is opened is
469 -- implementation-dependent and may depend on the file system object
470 -- which is attached to that handle.
471 -- For most implementations, physical files will normally be block-buffered
472 -- and terminals will normally be line-buffered.
475 = NoBuffering -- ^ buffering is disabled if possible.
477 -- ^ line-buffering should be enabled if possible.
478 | BlockBuffering (Maybe Int)
479 -- ^ block-buffering should be enabled if possible.
480 -- The size of the buffer is @n@ items if the argument
481 -- is 'Just' @n@ and is otherwise implementation-dependent.
482 deriving (Eq, Ord, Read, Show)
484 -- ---------------------------------------------------------------------------
487 -- |A mutable variable in the 'IO' monad
488 newtype IORef a = IORef (STRef RealWorld a)
490 -- explicit instance because Haddock can't figure out a derived one
491 instance Eq (IORef a) where
492 IORef x == IORef y = x == y
494 -- |Build a new 'IORef'
495 newIORef :: a -> IO (IORef a)
496 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
498 -- |Read the value of an 'IORef'
499 readIORef :: IORef a -> IO a
500 readIORef (IORef var) = stToIO (readSTRef var)
502 -- |Write a new value into an 'IORef'
503 writeIORef :: IORef a -> a -> IO ()
504 writeIORef (IORef var) v = stToIO (writeSTRef var v)
506 -- ---------------------------------------------------------------------------
507 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
508 -- The type arguments are as follows:
510 -- * @i@: the index type of the array (should be an instance of 'Ix')
512 -- * @e@: the element type of the array.
516 newtype IOArray i e = IOArray (STArray RealWorld i e)
518 -- explicit instance because Haddock can't figure out a derived one
519 instance Eq (IOArray i e) where
520 IOArray x == IOArray y = x == y
522 -- |Build a new 'IOArray'
523 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
524 {-# INLINE newIOArray #-}
525 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
527 -- | Read a value from an 'IOArray'
528 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
529 {-# INLINE unsafeReadIOArray #-}
530 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
532 -- | Write a new value into an 'IOArray'
533 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
534 {-# INLINE unsafeWriteIOArray #-}
535 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
537 -- | Read a value from an 'IOArray'
538 readIOArray :: Ix i => IOArray i e -> i -> IO e
539 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
541 -- | Write a new value into an 'IOArray'
542 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
543 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
546 -- ---------------------------------------------------------------------------
547 -- Show instance for Handles
549 -- handle types are 'show'n when printing error msgs, so
550 -- we provide a more user-friendly Show instance for it
551 -- than the derived one.
553 instance Show HandleType where
556 ClosedHandle -> showString "closed"
557 SemiClosedHandle -> showString "semi-closed"
558 ReadHandle -> showString "readable"
559 WriteHandle -> showString "writable"
560 AppendHandle -> showString "writable (append)"
561 ReadWriteHandle -> showString "read-writable"
563 instance Show Handle where
564 showsPrec p (FileHandle file _) = showHandle file
565 showsPrec p (DuplexHandle file _ _) = showHandle file
567 showHandle file = showString "{handle: " . showString file . showString "}"
569 -- ------------------------------------------------------------------------
570 -- Exception datatype and operations
572 -- |The type of exceptions. Every kind of system-generated exception
573 -- has a constructor in the 'Exception' type, and values of other
574 -- types may be injected into 'Exception' by coercing them to
575 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
576 -- "Control.Exception\#DynamicExceptions").
578 = ArithException ArithException
579 -- ^Exceptions raised by arithmetic
580 -- operations. (NOTE: GHC currently does not throw
581 -- 'ArithException's except for 'DivideByZero').
582 | ArrayException ArrayException
583 -- ^Exceptions raised by array-related
584 -- operations. (NOTE: GHC currently does not throw
585 -- 'ArrayException's).
586 | AssertionFailed String
587 -- ^This exception is thrown by the
588 -- 'assert' operation when the condition
589 -- fails. The 'String' argument contains the
590 -- location of the assertion in the source program.
591 | AsyncException AsyncException
592 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
594 -- ^The current thread was executing a call to
595 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
596 -- because there are no other references to this 'MVar'.
597 | BlockedIndefinitely
598 -- ^The current thread was waiting to retry an atomic memory transaction
599 -- that could never become possible to complete because there are no other
600 -- threads referring to any of teh TVars involved.
602 -- ^There are no runnable threads, so the program is
603 -- deadlocked. The 'Deadlock' exception is
604 -- raised in the main thread only (see also: "Control.Concurrent").
605 | DynException Dynamic
606 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
608 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
609 -- argument of 'ErrorCall' is the string passed to 'error' when it was
611 | ExitException ExitCode
612 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
613 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
614 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
615 -- main thread will cause the program to be terminated with the given
617 | IOException IOException
618 -- ^These are the standard IO exceptions generated by
619 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
620 | NoMethodError String
621 -- ^An attempt was made to invoke a class method which has
622 -- no definition in this instance, and there was no default
623 -- definition given in the class declaration. GHC issues a
624 -- warning when you compile an instance which has missing
627 -- ^The current thread is stuck in an infinite loop. This
628 -- exception may or may not be thrown when the program is
630 | PatternMatchFail String
631 -- ^A pattern matching failure. The 'String' argument should contain a
632 -- descriptive message including the function name, source file
635 -- ^An attempt was made to evaluate a field of a record
636 -- for which no value was given at construction time. The
637 -- 'String' argument gives the location of the
638 -- record construction in the source program.
640 -- ^A field selection was attempted on a constructor that
641 -- doesn\'t have the requested field. This can happen with
642 -- multi-constructor records when one or more fields are
643 -- missing from some of the constructors. The
644 -- 'String' argument gives the location of the
645 -- record selection in the source program.
647 -- ^An attempt was made to update a field in a record,
648 -- where the record doesn\'t have the requested field. This can
649 -- only occur with multi-constructor records, when one or more
650 -- fields are missing from some of the constructors. The
651 -- 'String' argument gives the location of the
652 -- record update in the source program.
654 -- |The type of arithmetic exceptions
664 -- |Asynchronous exceptions
667 -- ^The current thread\'s stack exceeded its limit.
668 -- Since an exception has been raised, the thread\'s stack
669 -- will certainly be below its limit again, but the
670 -- programmer should take remedial action
673 -- ^The program\'s heap is reaching its limit, and
674 -- the program should take action to reduce the amount of
675 -- live data it has. Notes:
677 -- * It is undefined which thread receives this exception.
679 -- * GHC currently does not throw 'HeapOverflow' exceptions.
681 -- ^This exception is raised by another thread
682 -- calling 'Control.Concurrent.killThread', or by the system
683 -- if it needs to terminate the thread for some
687 -- | Exceptions generated by array operations
689 = IndexOutOfBounds String
690 -- ^An attempt was made to index an array outside
691 -- its declared bounds.
692 | UndefinedElement String
693 -- ^An attempt was made to evaluate an element of an
694 -- array that had not been initialized.
697 stackOverflow, heapOverflow :: Exception -- for the RTS
698 stackOverflow = AsyncException StackOverflow
699 heapOverflow = AsyncException HeapOverflow
701 instance Show ArithException where
702 showsPrec _ Overflow = showString "arithmetic overflow"
703 showsPrec _ Underflow = showString "arithmetic underflow"
704 showsPrec _ LossOfPrecision = showString "loss of precision"
705 showsPrec _ DivideByZero = showString "divide by zero"
706 showsPrec _ Denormal = showString "denormal"
708 instance Show AsyncException where
709 showsPrec _ StackOverflow = showString "stack overflow"
710 showsPrec _ HeapOverflow = showString "heap overflow"
711 showsPrec _ ThreadKilled = showString "thread killed"
713 instance Show ArrayException where
714 showsPrec _ (IndexOutOfBounds s)
715 = showString "array index out of range"
716 . (if not (null s) then showString ": " . showString s
718 showsPrec _ (UndefinedElement s)
719 = showString "undefined array element"
720 . (if not (null s) then showString ": " . showString s
723 instance Show Exception where
724 showsPrec _ (IOException err) = shows err
725 showsPrec _ (ArithException err) = shows err
726 showsPrec _ (ArrayException err) = shows err
727 showsPrec _ (ErrorCall err) = showString err
728 showsPrec _ (ExitException err) = showString "exit: " . shows err
729 showsPrec _ (NoMethodError err) = showString err
730 showsPrec _ (PatternMatchFail err) = showString err
731 showsPrec _ (RecSelError err) = showString err
732 showsPrec _ (RecConError err) = showString err
733 showsPrec _ (RecUpdError err) = showString err
734 showsPrec _ (AssertionFailed err) = showString err
735 showsPrec _ (DynException _err) = showString "unknown exception"
736 showsPrec _ (AsyncException e) = shows e
737 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
738 showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
739 showsPrec _ (NonTermination) = showString "<<loop>>"
740 showsPrec _ (Deadlock) = showString "<<deadlock>>"
742 instance Eq Exception where
743 IOException e1 == IOException e2 = e1 == e2
744 ArithException e1 == ArithException e2 = e1 == e2
745 ArrayException e1 == ArrayException e2 = e1 == e2
746 ErrorCall e1 == ErrorCall e2 = e1 == e2
747 ExitException e1 == ExitException e2 = e1 == e2
748 NoMethodError e1 == NoMethodError e2 = e1 == e2
749 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
750 RecSelError e1 == RecSelError e2 = e1 == e2
751 RecConError e1 == RecConError e2 = e1 == e2
752 RecUpdError e1 == RecUpdError e2 = e1 == e2
753 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
754 DynException _ == DynException _ = False -- incomparable
755 AsyncException e1 == AsyncException e2 = e1 == e2
756 BlockedOnDeadMVar == BlockedOnDeadMVar = True
757 NonTermination == NonTermination = True
758 Deadlock == Deadlock = True
761 -- -----------------------------------------------------------------------------
764 -- We need it here because it is used in ExitException in the
765 -- Exception datatype (above).
768 = ExitSuccess -- ^ indicates successful termination;
770 -- ^ indicates program failure with an exit code.
771 -- The exact interpretation of the code is
772 -- operating-system dependent. In particular, some values
773 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
774 deriving (Eq, Ord, Read, Show)
776 -- --------------------------------------------------------------------------
779 -- | Throw an exception. Exceptions may be thrown from purely
780 -- functional code, but may only be caught within the 'IO' monad.
781 throw :: Exception -> a
782 throw exception = raise# exception
784 -- | A variant of 'throw' that can be used within the 'IO' monad.
786 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
787 -- two functions are subtly different:
789 -- > throw e `seq` return () ===> throw e
790 -- > throwIO e `seq` return () ===> return ()
792 -- The first example will cause the exception @e@ to be raised,
793 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
794 -- an exception to be raised when it is used within the 'IO' monad.
795 -- The 'throwIO' variant should be used in preference to 'throw' to
796 -- raise an exception within the 'IO' monad because it guarantees
797 -- ordering with respect to other 'IO' operations, whereas 'throw'
799 throwIO :: Exception -> IO a
800 throwIO err = IO $ raiseIO# err
802 ioException :: IOException -> IO a
803 ioException err = IO $ raiseIO# (IOException err)
805 -- | Raise an 'IOError' in the 'IO' monad.
806 ioError :: IOError -> IO a
807 ioError = ioException
809 -- ---------------------------------------------------------------------------
812 -- | The Haskell 98 type for exceptions in the 'IO' monad.
813 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
814 -- For a more general type of exception, including also those that arise
815 -- in pure code, see 'Control.Exception.Exception'.
817 -- In Haskell 98, this is an opaque type.
818 type IOError = IOException
820 -- |Exceptions that occur in the @IO@ monad.
821 -- An @IOException@ records a more specific error type, a descriptive
822 -- string and maybe the handle that was used when the error was
826 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
828 ioe_type :: IOErrorType, -- what it was.
829 ioe_location :: String, -- location.
830 ioe_description :: String, -- error type specific information.
831 ioe_filename :: Maybe FilePath -- filename the error is related to.
834 instance Eq IOException where
835 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
836 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
838 -- | An abstract type that contains a value for each variant of 'IOError'.
850 | UnsatisfiedConstraints
857 | UnsupportedOperation
861 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
863 instance Eq IOErrorType where
866 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
867 _ -> getTag x ==# getTag y
869 instance Show IOErrorType where
873 AlreadyExists -> "already exists"
874 NoSuchThing -> "does not exist"
875 ResourceBusy -> "resource busy"
876 ResourceExhausted -> "resource exhausted"
878 IllegalOperation -> "illegal operation"
879 PermissionDenied -> "permission denied"
880 UserError -> "user error"
881 HardwareFault -> "hardware fault"
882 InappropriateType -> "inappropriate type"
883 Interrupted -> "interrupted"
884 InvalidArgument -> "invalid argument"
885 OtherError -> "failed"
886 ProtocolError -> "protocol error"
887 ResourceVanished -> "resource vanished"
888 SystemError -> "system error"
889 TimeExpired -> "timeout"
890 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
891 UnsupportedOperation -> "unsupported operation"
892 DynIOError{} -> "unknown IO error"
894 -- | Construct an 'IOError' value with a string describing the error.
895 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
896 -- 'userError', thus:
898 -- > instance Monad IO where
900 -- > fail s = ioError (userError s)
902 userError :: String -> IOError
903 userError str = IOError Nothing UserError "" str Nothing
905 -- ---------------------------------------------------------------------------
908 instance Show IOException where
909 showsPrec p (IOError hdl iot loc s fn) =
911 Nothing -> case hdl of
913 Just h -> showsPrec p h . showString ": "
914 Just name -> showString name . showString ": ") .
917 _ -> showString loc . showString ": ") .
921 _ -> showString " (" . showString s . showString ")")
923 -- -----------------------------------------------------------------------------
926 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
927 deriving (Eq, Ord, Ix, Enum, Read, Show)