2 {-# OPTIONS -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 -----------------------------------------------------------------------------
17 module GHC.IOBase where
20 import GHC.Arr -- to derive Ix class
21 import GHC.Enum -- to derive Enum class
24 import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
25 import Data.Maybe ( Maybe(..) )
31 import {-# SOURCE #-} Data.Dynamic
34 -- ---------------------------------------------------------------------------
38 The IO Monad is just an instance of the ST monad, where the state is
39 the real world. We use the exception mechanism (in GHC.Exception) to
40 implement IO exceptions.
42 NOTE: The IO representation is deeply wired in to various parts of the
43 system. The following list may or may not be exhaustive:
45 Compiler - types of various primitives in PrimOp.lhs
47 RTS - forceIO (StgMiscClosures.hc)
48 - catchzh_fast, (un)?blockAsyncExceptionszh_fast, raisezh_fast
50 - raiseAsync (Schedule.c)
52 Prelude - GHC.IOBase.lhs, and several other places including
55 Libraries - parts of hslibs/lang.
61 A value of type @'IO' a@ is a computation which, when performed,
62 does some I\/O before returning a value of type @a@.
64 There is really only one way to \"perform\" an I\/O action: bind it to
65 @Main.main@ in your program. When your program is run, the I\/O will
66 be performed. It isn't possible to perform I\/O from an arbitrary
67 function, unless that function is itself in the 'IO' monad and called
68 at some point, directly or indirectly, from @Main.main@.
70 'IO' is a monad, so 'IO' actions can be combined using either the do-notation
71 or the '>>' and '>>=' operations from the 'Monad' class.
73 newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
75 unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
78 instance Functor IO where
79 fmap f x = x >>= (return . f)
81 instance Monad IO where
85 m >> k = m >>= \ _ -> k
91 failIO :: String -> IO a
92 failIO s = ioError (userError s)
94 liftIO :: IO a -> State# RealWorld -> STret RealWorld a
95 liftIO (IO m) = \s -> case m s of (# s', r #) -> STret s' r
97 bindIO :: IO a -> (a -> IO b) -> IO b
98 bindIO (IO m) k = IO ( \ s ->
100 (# new_s, a #) -> unIO (k a) new_s
103 thenIO :: IO a -> IO b -> IO b
104 thenIO (IO m) k = IO ( \ s ->
106 (# new_s, a #) -> unIO k new_s
109 returnIO :: a -> IO a
110 returnIO x = IO (\ s -> (# s, x #))
112 -- ---------------------------------------------------------------------------
113 -- Coercions between IO and ST
115 -- | A monad transformer embedding strict state transformers in the 'IO'
116 -- monad. The 'RealWorld' parameter indicates that the internal state
117 -- used by the 'ST' computation is a special one supplied by the 'IO'
118 -- monad, and thus distinct from those used by invocations of 'runST'.
119 stToIO :: ST RealWorld a -> IO a
122 ioToST :: IO a -> ST RealWorld a
123 ioToST (IO m) = (ST m)
125 -- ---------------------------------------------------------------------------
126 -- Unsafe IO operations
129 This is the \"back door\" into the 'IO' monad, allowing
130 'IO' computation to be performed at any time. For
131 this to be safe, the 'IO' computation should be
132 free of side effects and independent of its environment.
134 If the I\/O computation wrapped in 'unsafePerformIO'
135 performs side effects, then the relative order in which those side
136 effects take place (relative to the main I\/O trunk, or other calls to
137 'unsafePerformIO') is indeterminate. You have to be careful when
138 writing and compiling modules that use 'unsafePerformIO':
140 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
141 that calls 'unsafePerformIO'. If the call is inlined,
142 the I\/O may be performed more than once.
144 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
145 elimination being performed on the module, which might combine
146 two side effects that were meant to be separate. A good example
147 is using multiple global variables (like @test@ in the example below).
149 * Make sure that the either you switch off let-floating, or that the
150 call to 'unsafePerformIO' cannot float outside a lambda. For example,
153 f x = unsafePerformIO (newIORef [])
155 you may get only one reference cell shared between all calls to @f@.
158 f x = unsafePerformIO (newIORef [x])
160 because now it can't float outside the lambda.
162 It is less well known that
163 'unsafePerformIO' is not type safe. For example:
166 > test = unsafePerformIO $ newIORef []
169 > writeIORef test [42]
170 > bang \<- readIORef test
171 > print (bang :: [Char])
173 This program will core dump. This problem with polymorphic references
174 is well known in the ML community, and does not arise with normal
175 monadic use of references. There is no easy way to make it impossible
176 once you use 'unsafePerformIO'. Indeed, it is
177 possible to write @coerce :: a -> b@ with the
178 help of 'unsafePerformIO'. So be careful!
180 {-# NOINLINE unsafePerformIO #-}
181 unsafePerformIO :: IO a -> a
182 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
185 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
186 When passed a value of type @IO a@, the 'IO' will only be performed
187 when the value of the @a@ is demanded. This is used to implement lazy
188 file reading, see 'System.IO.hGetContents'.
190 {-# NOINLINE unsafeInterleaveIO #-}
191 unsafeInterleaveIO :: IO a -> IO a
192 unsafeInterleaveIO (IO m)
194 r = case m s of (# _, res #) -> res
198 -- ---------------------------------------------------------------------------
201 data MVar a = MVar (MVar# RealWorld a)
203 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
204 for communication between concurrent threads. It can be thought of
205 as a a box, which may be empty or full.
208 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
209 instance Eq (MVar a) where
210 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
212 -- A Handle is represented by (a reference to) a record
213 -- containing the state of the I/O port/device. We record
214 -- the following pieces of info:
216 -- * type (read,write,closed etc.)
217 -- * the underlying file descriptor
219 -- * buffer, and spare buffers
220 -- * user-friendly name (usually the
221 -- FilePath used when IO.openFile was called)
223 -- Note: when a Handle is garbage collected, we want to flush its buffer
224 -- and close the OS file handle, so as to free up a (precious) resource.
226 -- | Haskell defines operations to read and write characters from and to files,
227 -- represented by values of type @Handle@. Each value of this type is a
228 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
229 -- with file system objects. A handle has at least the following properties:
231 -- * whether it manages input or output or both;
233 -- * whether it is /open/, /closed/ or /semi-closed/;
235 -- * whether the object is seekable;
237 -- * whether buffering is disabled, or enabled on a line or block basis;
239 -- * a buffer (whose length may be zero).
241 -- Most handles will also have a current I\/O position indicating where the next
242 -- input or output operation will occur. A handle is /readable/ if it
243 -- manages only input or both input and output; likewise, it is /writable/ if
244 -- it manages only output or both input and output. A handle is /open/ when
246 -- Once it is closed it can no longer be used for either input or output,
247 -- though an implementation cannot re-use its storage while references
248 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
249 -- produced by showing a handle is system dependent; it should include
250 -- enough information to identify the handle for debugging. A handle is
251 -- equal according to '==' only to itself; no attempt
252 -- is made to compare the internal state of different handles for equality.
255 = FileHandle -- A normal handle to a file
256 FilePath -- the file (invariant)
259 | DuplexHandle -- A handle to a read/write stream
260 FilePath -- file for a FIFO, otherwise some
261 -- descriptive string.
262 !(MVar Handle__) -- The read side
263 !(MVar Handle__) -- The write side
266 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
269 instance Eq Handle where
270 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
271 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
274 type FD = Int -- XXX ToDo: should be CInt
278 haFD :: !FD, -- file descriptor
279 haType :: HandleType, -- type (read/write/append etc.)
280 haIsBin :: Bool, -- binary mode?
281 haIsStream :: Bool, -- is this a stream handle?
282 haBufferMode :: BufferMode, -- buffer contains read/write data?
283 haBuffer :: !(IORef Buffer), -- the current buffer
284 haBuffers :: !(IORef BufferList), -- spare buffers
285 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
289 -- ---------------------------------------------------------------------------
292 -- The buffer is represented by a mutable variable containing a
293 -- record, where the record contains the raw buffer and the start/end
294 -- points of the filled portion. We use a mutable variable so that
295 -- the common operation of writing (or reading) some data from (to)
296 -- the buffer doesn't need to modify, and hence copy, the handle
297 -- itself, it just updates the buffer.
299 -- There will be some allocation involved in a simple hPutChar in
300 -- order to create the new Buffer structure (below), but this is
301 -- relatively small, and this only has to be done once per write
304 -- The buffer contains its size - we could also get the size by
305 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
306 -- to be rounded up to the nearest Word.
308 type RawBuffer = MutableByteArray# RealWorld
310 -- INVARIANTS on a Buffer:
312 -- * A handle *always* has a buffer, even if it is only 1 character long
313 -- (an unbuffered handle needs a 1 character buffer in order to support
314 -- hLookAhead and hIsEOF).
316 -- * if r == w, then r == 0 && w == 0
317 -- * if state == WriteBuffer, then r == 0
318 -- * a write buffer is never full. If an operation
319 -- fills up the buffer, it will always flush it before
321 -- * a read buffer may be full as a result of hLookAhead. In normal
322 -- operation, a read buffer always has at least one character of space.
330 bufState :: BufferState
333 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
335 -- we keep a few spare buffers around in a handle to avoid allocating
336 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
337 -- same size as the main buffer.
340 | BufferListCons RawBuffer BufferList
343 bufferIsWritable :: Buffer -> Bool
344 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
345 bufferIsWritable _other = False
347 bufferEmpty :: Buffer -> Bool
348 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
350 -- only makes sense for a write buffer
351 bufferFull :: Buffer -> Bool
352 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
354 -- Internally, we classify handles as being one
365 isReadableHandleType ReadHandle = True
366 isReadableHandleType ReadWriteHandle = True
367 isReadableHandleType _ = False
369 isWritableHandleType AppendHandle = True
370 isWritableHandleType WriteHandle = True
371 isWritableHandleType ReadWriteHandle = True
372 isWritableHandleType _ = False
374 -- | File and directory names are values of type 'String', whose precise
375 -- meaning is operating system dependent. Files can be opened, yielding a
376 -- handle which can then be used to operate on the contents of that file.
378 type FilePath = String
380 -- ---------------------------------------------------------------------------
383 -- | Three kinds of buffering are supported: line-buffering,
384 -- block-buffering or no-buffering. These modes have the following
385 -- effects. For output, items are written out, or /flushed/,
386 -- from the internal buffer according to the buffer mode:
388 -- * /line-buffering/: the entire output buffer is flushed
389 -- whenever a newline is output, the buffer overflows,
390 -- a 'System.IO.hFlush' is issued, or the handle is closed.
392 -- * /block-buffering/: the entire buffer is written out whenever it
393 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
395 -- * /no-buffering/: output is written immediately, and never stored
398 -- An implementation is free to flush the buffer more frequently,
399 -- but not less frequently, than specified above.
400 -- The output buffer is emptied as soon as it has been written out.
402 -- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
404 -- * /line-buffering/: when the buffer for the handle is not empty,
405 -- the next item is obtained from the buffer; otherwise, when the
406 -- buffer is empty, characters up to and including the next newline
407 -- character are read into the buffer. No characters are available
408 -- until the newline character is available or the buffer is full.
410 -- * /block-buffering/: when the buffer for the handle becomes empty,
411 -- the next block of data is read into the buffer.
413 -- * /no-buffering/: the next input item is read and returned.
414 -- The 'System.IO.hLookAhead' operation implies that even a no-buffered
415 -- handle may require a one-character buffer.
417 -- The default buffering mode when a handle is opened is
418 -- implementation-dependent and may depend on the file system object
419 -- which is attached to that handle.
420 -- For most implementations, physical files will normally be block-buffered
421 -- and terminals will normally be line-buffered.
424 = NoBuffering -- ^ buffering is disabled if possible.
426 -- ^ line-buffering should be enabled if possible.
427 | BlockBuffering (Maybe Int)
428 -- ^ block-buffering should be enabled if possible.
429 -- The size of the buffer is @n@ items if the argument
430 -- is 'Just' @n@ and is otherwise implementation-dependent.
431 deriving (Eq, Ord, Read, Show)
433 -- ---------------------------------------------------------------------------
436 -- |A mutable variable in the 'IO' monad
437 newtype IORef a = IORef (STRef RealWorld a)
439 -- explicit instance because Haddock can't figure out a derived one
440 instance Eq (IORef a) where
441 IORef x == IORef y = x == y
443 -- |Build a new 'IORef'
444 newIORef :: a -> IO (IORef a)
445 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
447 -- |Read the value of an 'IORef'
448 readIORef :: IORef a -> IO a
449 readIORef (IORef var) = stToIO (readSTRef var)
451 -- |Write a new value into an 'IORef'
452 writeIORef :: IORef a -> a -> IO ()
453 writeIORef (IORef var) v = stToIO (writeSTRef var v)
455 -- ---------------------------------------------------------------------------
456 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
457 -- The type arguments are as follows:
459 -- * @i@: the index type of the array (should be an instance of 'Ix')
461 -- * @e@: the element type of the array.
465 newtype IOArray i e = IOArray (STArray RealWorld i e)
467 -- explicit instance because Haddock can't figure out a derived one
468 instance Eq (IOArray i e) where
469 IOArray x == IOArray y = x == y
471 -- |Build a new 'IOArray'
472 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
473 {-# INLINE newIOArray #-}
474 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
476 -- | Read a value from an 'IOArray'
477 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
478 {-# INLINE unsafeReadIOArray #-}
479 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
481 -- | Write a new value into an 'IOArray'
482 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
483 {-# INLINE unsafeWriteIOArray #-}
484 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
486 -- | Read a value from an 'IOArray'
487 readIOArray :: Ix i => IOArray i e -> i -> IO e
488 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
490 -- | Write a new value into an 'IOArray'
491 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
492 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
495 -- ---------------------------------------------------------------------------
496 -- Show instance for Handles
498 -- handle types are 'show'n when printing error msgs, so
499 -- we provide a more user-friendly Show instance for it
500 -- than the derived one.
502 instance Show HandleType where
505 ClosedHandle -> showString "closed"
506 SemiClosedHandle -> showString "semi-closed"
507 ReadHandle -> showString "readable"
508 WriteHandle -> showString "writable"
509 AppendHandle -> showString "writable (append)"
510 ReadWriteHandle -> showString "read-writable"
512 instance Show Handle where
513 showsPrec p (FileHandle file _) = showHandle file
514 showsPrec p (DuplexHandle file _ _) = showHandle file
516 showHandle file = showString "{handle: " . showString file . showString "}"
518 -- ------------------------------------------------------------------------
519 -- Exception datatype and operations
521 -- |The type of exceptions. Every kind of system-generated exception
522 -- has a constructor in the 'Exception' type, and values of other
523 -- types may be injected into 'Exception' by coercing them to
524 -- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
525 -- "Control.Exception\#DynamicExceptions").
527 = ArithException ArithException
528 -- ^Exceptions raised by arithmetic
529 -- operations. (NOTE: GHC currently does not throw
530 -- 'ArithException's except for 'DivideByZero').
531 | ArrayException ArrayException
532 -- ^Exceptions raised by array-related
533 -- operations. (NOTE: GHC currently does not throw
534 -- 'ArrayException's).
535 | AssertionFailed String
536 -- ^This exception is thrown by the
537 -- 'assert' operation when the condition
538 -- fails. The 'String' argument contains the
539 -- location of the assertion in the source program.
540 | AsyncException AsyncException
541 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
543 -- ^The current thread was executing a call to
544 -- 'Control.Concurrent.MVar.takeMVar' that could never return,
545 -- because there are no other references to this 'MVar'.
547 -- ^There are no runnable threads, so the program is
548 -- deadlocked. The 'Deadlock' exception is
549 -- raised in the main thread only (see also: "Control.Concurrent").
550 | DynException Dynamic
551 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
553 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
554 -- argument of 'ErrorCall' is the string passed to 'error' when it was
556 | ExitException ExitCode
557 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
558 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
559 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
560 -- main thread will cause the program to be terminated with the given
562 | IOException IOException
563 -- ^These are the standard IO exceptions generated by
564 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
565 | NoMethodError String
566 -- ^An attempt was made to invoke a class method which has
567 -- no definition in this instance, and there was no default
568 -- definition given in the class declaration. GHC issues a
569 -- warning when you compile an instance which has missing
572 -- ^The current thread is stuck in an infinite loop. This
573 -- exception may or may not be thrown when the program is
575 | PatternMatchFail String
576 -- ^A pattern matching failure. The 'String' argument should contain a
577 -- descriptive message including the function name, source file
580 -- ^An attempt was made to evaluate a field of a record
581 -- for which no value was given at construction time. The
582 -- 'String' argument gives the location of the
583 -- record construction in the source program.
585 -- ^A field selection was attempted on a constructor that
586 -- doesn\'t have the requested field. This can happen with
587 -- multi-constructor records when one or more fields are
588 -- missing from some of the constructors. The
589 -- 'String' argument gives the location of the
590 -- record selection in the source program.
592 -- ^An attempt was made to update a field in a record,
593 -- where the record doesn\'t have the requested field. This can
594 -- only occur with multi-constructor records, when one or more
595 -- fields are missing from some of the constructors. The
596 -- 'String' argument gives the location of the
597 -- record update in the source program.
599 -- |The type of arithmetic exceptions
609 -- |Asynchronous exceptions
612 -- ^The current thread\'s stack exceeded its limit.
613 -- Since an exception has been raised, the thread\'s stack
614 -- will certainly be below its limit again, but the
615 -- programmer should take remedial action
618 -- ^The program\'s heap is reaching its limit, and
619 -- the program should take action to reduce the amount of
620 -- live data it has. Notes:
622 -- * It is undefined which thread receives this exception.
624 -- * GHC currently does not throw 'HeapOverflow' exceptions.
626 -- ^This exception is raised by another thread
627 -- calling 'Control.Concurrent.killThread', or by the system
628 -- if it needs to terminate the thread for some
632 -- | Exceptions generated by array operations
634 = IndexOutOfBounds String
635 -- ^An attempt was made to index an array outside
636 -- its declared bounds.
637 | UndefinedElement String
638 -- ^An attempt was made to evaluate an element of an
639 -- array that had not been initialized.
642 stackOverflow, heapOverflow :: Exception -- for the RTS
643 stackOverflow = AsyncException StackOverflow
644 heapOverflow = AsyncException HeapOverflow
646 instance Show ArithException where
647 showsPrec _ Overflow = showString "arithmetic overflow"
648 showsPrec _ Underflow = showString "arithmetic underflow"
649 showsPrec _ LossOfPrecision = showString "loss of precision"
650 showsPrec _ DivideByZero = showString "divide by zero"
651 showsPrec _ Denormal = showString "denormal"
653 instance Show AsyncException where
654 showsPrec _ StackOverflow = showString "stack overflow"
655 showsPrec _ HeapOverflow = showString "heap overflow"
656 showsPrec _ ThreadKilled = showString "thread killed"
658 instance Show ArrayException where
659 showsPrec _ (IndexOutOfBounds s)
660 = showString "array index out of range"
661 . (if not (null s) then showString ": " . showString s
663 showsPrec _ (UndefinedElement s)
664 = showString "undefined array element"
665 . (if not (null s) then showString ": " . showString s
668 instance Show Exception where
669 showsPrec _ (IOException err) = shows err
670 showsPrec _ (ArithException err) = shows err
671 showsPrec _ (ArrayException err) = shows err
672 showsPrec _ (ErrorCall err) = showString err
673 showsPrec _ (ExitException err) = showString "exit: " . shows err
674 showsPrec _ (NoMethodError err) = showString err
675 showsPrec _ (PatternMatchFail err) = showString err
676 showsPrec _ (RecSelError err) = showString err
677 showsPrec _ (RecConError err) = showString err
678 showsPrec _ (RecUpdError err) = showString err
679 showsPrec _ (AssertionFailed err) = showString err
680 showsPrec _ (DynException _err) = showString "unknown exception"
681 showsPrec _ (AsyncException e) = shows e
682 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
683 showsPrec _ (NonTermination) = showString "<<loop>>"
684 showsPrec _ (Deadlock) = showString "<<deadlock>>"
686 instance Eq Exception where
687 IOException e1 == IOException e2 = e1 == e2
688 ArithException e1 == ArithException e2 = e1 == e2
689 ArrayException e1 == ArrayException e2 = e1 == e2
690 ErrorCall e1 == ErrorCall e2 = e1 == e2
691 ExitException e1 == ExitException e2 = e1 == e2
692 NoMethodError e1 == NoMethodError e2 = e1 == e2
693 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
694 RecSelError e1 == RecSelError e2 = e1 == e2
695 RecConError e1 == RecConError e2 = e1 == e2
696 RecUpdError e1 == RecUpdError e2 = e1 == e2
697 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
698 DynException _ == DynException _ = False -- incomparable
699 AsyncException e1 == AsyncException e2 = e1 == e2
700 BlockedOnDeadMVar == BlockedOnDeadMVar = True
701 NonTermination == NonTermination = True
702 Deadlock == Deadlock = True
705 -- -----------------------------------------------------------------------------
708 -- We need it here because it is used in ExitException in the
709 -- Exception datatype (above).
712 = ExitSuccess -- ^ indicates successful termination;
714 -- ^ indicates program failure with an exit code.
715 -- The exact interpretation of the code is
716 -- operating-system dependent. In particular, some values
717 -- may be prohibited (e.g. 0 on a POSIX-compliant system).
718 deriving (Eq, Ord, Read, Show)
720 -- --------------------------------------------------------------------------
723 -- | Throw an exception. Exceptions may be thrown from purely
724 -- functional code, but may only be caught within the 'IO' monad.
725 throw :: Exception -> a
726 throw exception = raise# exception
728 -- | A variant of 'throw' that can be used within the 'IO' monad.
730 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
731 -- two functions are subtly different:
733 -- > throw e `seq` return () ===> throw e
734 -- > throwIO e `seq` return () ===> return ()
736 -- The first example will cause the exception @e@ to be raised,
737 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
738 -- an exception to be raised when it is used within the 'IO' monad.
739 -- The 'throwIO' variant should be used in preference to 'throw' to
740 -- raise an exception within the 'IO' monad because it guarantees
741 -- ordering with respect to other 'IO' operations, whereas 'throw'
743 throwIO :: Exception -> IO a
744 throwIO err = IO $ raiseIO# err
746 ioException :: IOException -> IO a
747 ioException err = IO $ raiseIO# (IOException err)
749 -- | Raise an 'IOError' in the 'IO' monad.
750 ioError :: IOError -> IO a
751 ioError = ioException
753 -- ---------------------------------------------------------------------------
756 -- | The Haskell 98 type for exceptions in the 'IO' monad.
757 -- Any I\/O operation may raise an 'IOError' instead of returning a result.
758 -- For a more general type of exception, including also those that arise
759 -- in pure code, see 'Control.Exception.Exception'.
761 -- In Haskell 98, this is an opaque type.
762 type IOError = IOException
764 -- |Exceptions that occur in the @IO@ monad.
765 -- An @IOException@ records a more specific error type, a descriptive
766 -- string and maybe the handle that was used when the error was
770 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
772 ioe_type :: IOErrorType, -- what it was.
773 ioe_location :: String, -- location.
774 ioe_description :: String, -- error type specific information.
775 ioe_filename :: Maybe FilePath -- filename the error is related to.
778 instance Eq IOException where
779 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
780 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
782 -- | An abstract type that contains a value for each variant of 'IOError'.
794 | UnsatisfiedConstraints
801 | UnsupportedOperation
805 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
807 instance Eq IOErrorType where
810 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
811 _ -> getTag x ==# getTag y
813 instance Show IOErrorType where
817 AlreadyExists -> "already exists"
818 NoSuchThing -> "does not exist"
819 ResourceBusy -> "resource busy"
820 ResourceExhausted -> "resource exhausted"
822 IllegalOperation -> "illegal operation"
823 PermissionDenied -> "permission denied"
824 UserError -> "user error"
825 HardwareFault -> "hardware fault"
826 InappropriateType -> "inappropriate type"
827 Interrupted -> "interrupted"
828 InvalidArgument -> "invalid argument"
829 OtherError -> "failed"
830 ProtocolError -> "protocol error"
831 ResourceVanished -> "resource vanished"
832 SystemError -> "system error"
833 TimeExpired -> "timeout"
834 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
835 UnsupportedOperation -> "unsupported operation"
836 DynIOError{} -> "unknown IO error"
838 -- | Construct an 'IOError' value with a string describing the error.
839 -- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
840 -- 'userError', thus:
842 -- > instance Monad IO where
844 -- > fail s = ioError (userError s)
846 userError :: String -> IOError
847 userError str = IOError Nothing UserError "" str Nothing
849 -- ---------------------------------------------------------------------------
852 instance Show IOException where
853 showsPrec p (IOError hdl iot loc s fn) =
855 Nothing -> case hdl of
857 Just h -> showsPrec p h . showString ": "
858 Just name -> showString name . showString ": ") .
861 _ -> showString loc . showString ": ") .
865 _ -> showString " (" . showString s . showString ")")
867 -- -----------------------------------------------------------------------------
870 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
871 deriving (Eq, Ord, Ix, Enum, Read, Show)