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 --stToIO :: (forall s. ST s a) -> IO a
116 stToIO :: ST RealWorld a -> IO a
119 ioToST :: IO a -> ST RealWorld a
120 ioToST (IO m) = (ST m)
122 -- ---------------------------------------------------------------------------
123 -- Unsafe IO operations
126 This is the \"back door\" into the 'IO' monad, allowing
127 'IO' computation to be performed at any time. For
128 this to be safe, the 'IO' computation should be
129 free of side effects and independent of its environment.
131 If the I\/O computation wrapped in 'unsafePerformIO'
132 performs side effects, then the relative order in which those side
133 effects take place (relative to the main I\/O trunk, or other calls to
134 'unsafePerformIO') is indeterminate. You have to be careful when
135 writing and compiling modules that use 'unsafePerformIO':
137 * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
138 that calls 'unsafePerformIO'. If the call is inlined,
139 the I\/O may be performed more than once.
141 * Use the compiler flag @-fno-cse@ to prevent common sub-expression
142 elimination being performed on the module, which might combine
143 two side effects that were meant to be separate. A good example
144 is using multiple global variables (like @test@ in the example below).
146 * Make sure that the either you switch off let-floating, or that the
147 call to 'unsafePerformIO' cannot float outside a lambda. For example,
150 f x = unsafePerformIO (newIORef [])
152 you may get only one reference cell shared between all calls to @f@.
155 f x = unsafePerformIO (newIORef [x])
157 because now it can't float outside the lambda.
159 It is less well known that
160 'unsafePerformIO' is not type safe. For example:
163 > test = unsafePerformIO $ newIORef []
166 > writeIORef test [42]
167 > bang \<- readIORef test
168 > print (bang :: [Char])
170 This program will core dump. This problem with polymorphic references
171 is well known in the ML community, and does not arise with normal
172 monadic use of references. There is no easy way to make it impossible
173 once you use 'unsafePerformIO'. Indeed, it is
174 possible to write @coerce :: a -> b@ with the
175 help of 'unsafePerformIO'. So be careful!
177 {-# NOINLINE unsafePerformIO #-}
178 unsafePerformIO :: IO a -> a
179 unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
182 'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
183 When passed a value of type @IO a@, the 'IO' will only be performed
184 when the value of the @a@ is demanded. This is used to implement lazy
185 file reading, see 'System.IO.hGetContents'.
187 {-# NOINLINE unsafeInterleaveIO #-}
188 unsafeInterleaveIO :: IO a -> IO a
189 unsafeInterleaveIO (IO m)
191 r = case m s of (# _, res #) -> res
195 -- ---------------------------------------------------------------------------
198 data MVar a = MVar (MVar# RealWorld a)
200 An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
201 for communication between concurrent threads. It can be thought of
202 as a a box, which may be empty or full.
205 -- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
206 instance Eq (MVar a) where
207 (MVar mvar1#) == (MVar mvar2#) = sameMVar# mvar1# mvar2#
209 -- A Handle is represented by (a reference to) a record
210 -- containing the state of the I/O port/device. We record
211 -- the following pieces of info:
213 -- * type (read,write,closed etc.)
214 -- * the underlying file descriptor
216 -- * buffer, and spare buffers
217 -- * user-friendly name (usually the
218 -- FilePath used when IO.openFile was called)
220 -- Note: when a Handle is garbage collected, we want to flush its buffer
221 -- and close the OS file handle, so as to free up a (precious) resource.
223 -- | Haskell defines operations to read and write characters from and to files,
224 -- represented by values of type @Handle@. Each value of this type is a
225 -- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
226 -- with file system objects. A handle has at least the following properties:
228 -- * whether it manages input or output or both;
230 -- * whether it is /open/, /closed/ or /semi-closed/;
232 -- * whether the object is seekable;
234 -- * whether buffering is disabled, or enabled on a line or block basis;
236 -- * a buffer (whose length may be zero).
238 -- Most handles will also have a current I\/O position indicating where the next
239 -- input or output operation will occur. A handle is /readable/ if it
240 -- manages only input or both input and output; likewise, it is /writable/ if
241 -- it manages only output or both input and output. A handle is /open/ when
243 -- Once it is closed it can no longer be used for either input or output,
244 -- though an implementation cannot re-use its storage while references
245 -- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
246 -- produced by showing a handle is system dependent; it should include
247 -- enough information to identify the handle for debugging. A handle is
248 -- equal according to '==' only to itself; no attempt
249 -- is made to compare the internal state of different handles for equality.
252 = FileHandle -- A normal handle to a file
253 FilePath -- the file (invariant)
256 | DuplexHandle -- A handle to a read/write stream
257 FilePath -- file for a FIFO, otherwise some
258 -- descriptive string.
259 !(MVar Handle__) -- The read side
260 !(MVar Handle__) -- The write side
263 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
266 instance Eq Handle where
267 (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
268 (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
271 type FD = Int -- XXX ToDo: should be CInt
275 haFD :: !FD, -- file descriptor
276 haType :: HandleType, -- type (read/write/append etc.)
277 haIsBin :: Bool, -- binary mode?
278 haIsStream :: Bool, -- is this a stream handle?
279 haBufferMode :: BufferMode, -- buffer contains read/write data?
280 haBuffer :: !(IORef Buffer), -- the current buffer
281 haBuffers :: !(IORef BufferList), -- spare buffers
282 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
286 -- ---------------------------------------------------------------------------
289 -- The buffer is represented by a mutable variable containing a
290 -- record, where the record contains the raw buffer and the start/end
291 -- points of the filled portion. We use a mutable variable so that
292 -- the common operation of writing (or reading) some data from (to)
293 -- the buffer doesn't need to modify, and hence copy, the handle
294 -- itself, it just updates the buffer.
296 -- There will be some allocation involved in a simple hPutChar in
297 -- order to create the new Buffer structure (below), but this is
298 -- relatively small, and this only has to be done once per write
301 -- The buffer contains its size - we could also get the size by
302 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
303 -- to be rounded up to the nearest Word.
305 type RawBuffer = MutableByteArray# RealWorld
307 -- INVARIANTS on a Buffer:
309 -- * A handle *always* has a buffer, even if it is only 1 character long
310 -- (an unbuffered handle needs a 1 character buffer in order to support
311 -- hLookAhead and hIsEOF).
313 -- * if r == w, then r == 0 && w == 0
314 -- * if state == WriteBuffer, then r == 0
315 -- * a write buffer is never full. If an operation
316 -- fills up the buffer, it will always flush it before
318 -- * a read buffer may be full as a result of hLookAhead. In normal
319 -- operation, a read buffer always has at least one character of space.
327 bufState :: BufferState
330 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
332 -- we keep a few spare buffers around in a handle to avoid allocating
333 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
334 -- same size as the main buffer.
337 | BufferListCons RawBuffer BufferList
340 bufferIsWritable :: Buffer -> Bool
341 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
342 bufferIsWritable _other = False
344 bufferEmpty :: Buffer -> Bool
345 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
347 -- only makes sense for a write buffer
348 bufferFull :: Buffer -> Bool
349 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
351 -- Internally, we classify handles as being one
362 isReadableHandleType ReadHandle = True
363 isReadableHandleType ReadWriteHandle = True
364 isReadableHandleType _ = False
366 isWritableHandleType AppendHandle = True
367 isWritableHandleType WriteHandle = True
368 isWritableHandleType ReadWriteHandle = True
369 isWritableHandleType _ = False
371 -- | File and directory names are values of type 'String', whose precise
372 -- meaning is operating system dependent. Files can be opened, yielding a
373 -- handle which can then be used to operate on the contents of that file.
375 type FilePath = String
377 -- ---------------------------------------------------------------------------
380 -- | Three kinds of buffering are supported: line-buffering,
381 -- block-buffering or no-buffering. These modes have the following
382 -- effects. For output, items are written out, or /flushed/,
383 -- from the internal buffer according to the buffer mode:
385 -- * /line-buffering/: the entire output buffer is flushed
386 -- whenever a newline is output, the buffer overflows,
387 -- a 'System.IO.hFlush' is issued, or the handle is closed.
389 -- * /block-buffering/: the entire buffer is written out whenever it
390 -- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
392 -- * /no-buffering/: output is written immediately, and never stored
395 -- An implementation is free to flush the buffer more frequently,
396 -- but not less frequently, than specified above.
397 -- The output buffer is emptied as soon as it has been written out.
399 -- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
401 -- * /line-buffering/: when the buffer for the handle is not empty,
402 -- the next item is obtained from the buffer; otherwise, when the
403 -- buffer is empty, characters up to and including the next newline
404 -- character are read into the buffer. No characters are available
405 -- until the newline character is available or the buffer is full.
407 -- * /block-buffering/: when the buffer for the handle becomes empty,
408 -- the next block of data is read into the buffer.
410 -- * /no-buffering/: the next input item is read and returned.
411 -- The 'hLookAhead' operation implies that even a no-buffered handle
412 -- may require a one-character buffer.
414 -- The default buffering mode when a handle is opened is
415 -- implementation-dependent and may depend on the file system object
416 -- which is attached to that handle.
417 -- For most implementations, physical files will normally be block-buffered
418 -- and terminals will normally be line-buffered.
421 = NoBuffering -- ^ buffering is disabled if possible.
423 -- ^ line-buffering should be enabled if possible.
424 | BlockBuffering (Maybe Int)
425 -- ^ block-buffering should be enabled if possible.
426 -- The size of the buffer is @n@ items if the argument
427 -- is 'Just' @n@ and is otherwise implementation-dependent.
428 deriving (Eq, Ord, Read, Show)
430 -- ---------------------------------------------------------------------------
433 -- |A mutable variable in the 'IO' monad
434 newtype IORef a = IORef (STRef RealWorld a)
436 -- explicit instance because Haddock can't figure out a derived one
437 instance Eq (IORef a) where
438 IORef x == IORef y = x == y
440 -- |Build a new 'IORef'
441 newIORef :: a -> IO (IORef a)
442 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
444 -- |Read the value of an 'IORef'
445 readIORef :: IORef a -> IO a
446 readIORef (IORef var) = stToIO (readSTRef var)
448 -- |Write a new value into an 'IORef'
449 writeIORef :: IORef a -> a -> IO ()
450 writeIORef (IORef var) v = stToIO (writeSTRef var v)
452 -- ---------------------------------------------------------------------------
453 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
454 -- The type arguments are as follows:
456 -- * @i@: the index type of the array (should be an instance of 'Ix')
458 -- * @e@: the element type of the array.
462 newtype IOArray i e = IOArray (STArray RealWorld i e)
464 -- explicit instance because Haddock can't figure out a derived one
465 instance Eq (IOArray i e) where
466 IOArray x == IOArray y = x == y
468 -- |Build a new 'IOArray'
469 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
470 {-# INLINE newIOArray #-}
471 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
473 -- | Read a value from an 'IOArray'
474 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
475 {-# INLINE unsafeReadIOArray #-}
476 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
478 -- | Write a new value into an 'IOArray'
479 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
480 {-# INLINE unsafeWriteIOArray #-}
481 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
483 -- | Read a value from an 'IOArray'
484 readIOArray :: Ix i => IOArray i e -> i -> IO e
485 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
487 -- | Write a new value into an 'IOArray'
488 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
489 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
492 -- ---------------------------------------------------------------------------
493 -- Show instance for Handles
495 -- handle types are 'show'n when printing error msgs, so
496 -- we provide a more user-friendly Show instance for it
497 -- than the derived one.
499 instance Show HandleType where
502 ClosedHandle -> showString "closed"
503 SemiClosedHandle -> showString "semi-closed"
504 ReadHandle -> showString "readable"
505 WriteHandle -> showString "writable"
506 AppendHandle -> showString "writable (append)"
507 ReadWriteHandle -> showString "read-writable"
509 instance Show Handle where
510 showsPrec p (FileHandle file _) = showHandle file
511 showsPrec p (DuplexHandle file _ _) = showHandle file
513 showHandle file = showString "{handle: " . showString file . showString "}"
515 -- ------------------------------------------------------------------------
516 -- Exception datatype and operations
518 -- |The type of exceptions. Every kind of system-generated exception
519 -- has a constructor in the 'Exception' type, and values of other
520 -- types may be injected into 'Exception' by coercing them to
521 -- 'Dynamic' (see the section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
523 = ArithException ArithException
524 -- ^Exceptions raised by arithmetic
525 -- operations. (NOTE: GHC currently does not throw
526 -- 'ArithException's except for 'DivideByZero').
527 | ArrayException ArrayException
528 -- ^Exceptions raised by array-related
529 -- operations. (NOTE: GHC currently does not throw
530 -- 'ArrayException's).
531 | AssertionFailed String
532 -- ^This exception is thrown by the
533 -- 'assert' operation when the condition
534 -- fails. The 'String' argument contains the
535 -- location of the assertion in the source program.
536 | AsyncException AsyncException
537 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
539 -- ^The current thread was executing a call to
540 -- 'takeMVar' that could never return, because there are no other
541 -- references to this 'MVar'.
543 -- ^There are no runnable threads, so the program is
544 -- deadlocked. The 'Deadlock' exception is
545 -- raised in the main thread only (see also: "Control.Concurrent").
546 | DynException Dynamic
547 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
549 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
550 -- argument of 'ErrorCall' is the string passed to 'error' when it was
552 | ExitException ExitCode
553 -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
554 -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
555 -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
556 -- main thread will cause the program to be terminated with the given
558 | IOException IOException
559 -- ^These are the standard IO exceptions generated by
560 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
561 | NoMethodError String
562 -- ^An attempt was made to invoke a class method which has
563 -- no definition in this instance, and there was no default
564 -- definition given in the class declaration. GHC issues a
565 -- warning when you compile an instance which has missing
568 -- ^The current thread is stuck in an infinite loop. This
569 -- exception may or may not be thrown when the program is
571 | PatternMatchFail String
572 -- ^A pattern matching failure. The 'String' argument should contain a
573 -- descriptive message including the function name, source file
576 -- ^An attempt was made to evaluate a field of a record
577 -- for which no value was given at construction time. The
578 -- 'String' argument gives the location of the
579 -- record construction in the source program.
581 -- ^A field selection was attempted on a constructor that
582 -- doesn\'t have the requested field. This can happen with
583 -- multi-constructor records when one or more fields are
584 -- missing from some of the constructors. The
585 -- 'String' argument gives the location of the
586 -- record selection in the source program.
588 -- ^An attempt was made to update a field in a record,
589 -- where the record doesn\'t have the requested field. This can
590 -- only occur with multi-constructor records, when one or more
591 -- fields are missing from some of the constructors. The
592 -- 'String' argument gives the location of the
593 -- record update in the source program.
595 -- |The type of arithmetic exceptions
605 -- |Asynchronous exceptions
608 -- ^The current thread\'s stack exceeded its limit.
609 -- Since an exception has been raised, the thread\'s stack
610 -- will certainly be below its limit again, but the
611 -- programmer should take remedial action
614 -- ^The program\'s heap is reaching its limit, and
615 -- the program should take action to reduce the amount of
616 -- live data it has. Notes:
618 -- * It is undefined which thread receives this exception.
620 -- * GHC currently does not throw 'HeapOverflow' exceptions.
622 -- ^This exception is raised by another thread
623 -- calling 'killThread', or by the system
624 -- if it needs to terminate the thread for some
628 -- | Exceptions generated by array operations
630 = IndexOutOfBounds String
631 -- ^An attempt was made to index an array outside
632 -- its declared bounds.
633 | UndefinedElement String
634 -- ^An attempt was made to evaluate an element of an
635 -- array that had not been initialized.
638 stackOverflow, heapOverflow :: Exception -- for the RTS
639 stackOverflow = AsyncException StackOverflow
640 heapOverflow = AsyncException HeapOverflow
642 instance Show ArithException where
643 showsPrec _ Overflow = showString "arithmetic overflow"
644 showsPrec _ Underflow = showString "arithmetic underflow"
645 showsPrec _ LossOfPrecision = showString "loss of precision"
646 showsPrec _ DivideByZero = showString "divide by zero"
647 showsPrec _ Denormal = showString "denormal"
649 instance Show AsyncException where
650 showsPrec _ StackOverflow = showString "stack overflow"
651 showsPrec _ HeapOverflow = showString "heap overflow"
652 showsPrec _ ThreadKilled = showString "thread killed"
654 instance Show ArrayException where
655 showsPrec _ (IndexOutOfBounds s)
656 = showString "array index out of range"
657 . (if not (null s) then showString ": " . showString s
659 showsPrec _ (UndefinedElement s)
660 = showString "undefined array element"
661 . (if not (null s) then showString ": " . showString s
664 instance Show Exception where
665 showsPrec _ (IOException err) = shows err
666 showsPrec _ (ArithException err) = shows err
667 showsPrec _ (ArrayException err) = shows err
668 showsPrec _ (ErrorCall err) = showString err
669 showsPrec _ (ExitException err) = showString "exit: " . shows err
670 showsPrec _ (NoMethodError err) = showString err
671 showsPrec _ (PatternMatchFail err) = showString err
672 showsPrec _ (RecSelError err) = showString err
673 showsPrec _ (RecConError err) = showString err
674 showsPrec _ (RecUpdError err) = showString err
675 showsPrec _ (AssertionFailed err) = showString err
676 showsPrec _ (DynException _err) = showString "unknown exception"
677 showsPrec _ (AsyncException e) = shows e
678 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
679 showsPrec _ (NonTermination) = showString "<<loop>>"
680 showsPrec _ (Deadlock) = showString "<<deadlock>>"
682 instance Eq Exception where
683 IOException e1 == IOException e2 = e1 == e2
684 ArithException e1 == ArithException e2 = e1 == e2
685 ArrayException e1 == ArrayException e2 = e1 == e2
686 ErrorCall e1 == ErrorCall e2 = e1 == e2
687 ExitException e1 == ExitException e2 = e1 == e2
688 NoMethodError e1 == NoMethodError e2 = e1 == e2
689 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
690 RecSelError e1 == RecSelError e2 = e1 == e2
691 RecConError e1 == RecConError e2 = e1 == e2
692 RecUpdError e1 == RecUpdError e2 = e1 == e2
693 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
694 DynException _ == DynException _ = False -- incomparable
695 AsyncException e1 == AsyncException e2 = e1 == e2
696 BlockedOnDeadMVar == BlockedOnDeadMVar = True
697 NonTermination == NonTermination = True
698 Deadlock == Deadlock = True
701 -- -----------------------------------------------------------------------------
704 -- The `ExitCode' type defines the exit codes that a program
705 -- can return. `ExitSuccess' indicates successful termination;
706 -- and `ExitFailure code' indicates program failure
707 -- with value `code'. The exact interpretation of `code'
708 -- is operating-system dependent. In particular, some values of
709 -- `code' may be prohibited (e.g. 0 on a POSIX-compliant system).
711 -- We need it here because it is used in ExitException in the
712 -- Exception datatype (above).
714 data ExitCode = ExitSuccess | ExitFailure Int
715 deriving (Eq, Ord, Read, Show)
717 -- --------------------------------------------------------------------------
720 -- | Throw an exception. Exceptions may be thrown from purely
721 -- functional code, but may only be caught within the 'IO' monad.
722 throw :: Exception -> a
723 throw exception = raise# exception
725 -- | A variant of 'throw' that can be used within the 'IO' monad.
727 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
728 -- two functions are subtly different:
730 -- > throw e `seq` return () ===> throw e
731 -- > throwIO e `seq` return () ===> return ()
733 -- The first example will cause the exception @e@ to be raised,
734 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
735 -- an exception to be raised when it is used within the 'IO' monad.
736 -- The 'throwIO' variant should be used in preference to 'throw' to
737 -- raise an exception within the 'IO' monad because it guarantees
738 -- ordering with respect to other 'IO' operations, whereas 'throw'
740 throwIO :: Exception -> IO a
741 throwIO err = IO $ raiseIO# err
743 ioException :: IOException -> IO a
744 ioException err = IO $ raiseIO# (IOException err)
746 ioError :: IOError -> IO a
747 ioError = ioException
749 -- ---------------------------------------------------------------------------
752 -- | The Haskell 98 type for exceptions in the @IO@ monad.
753 -- In Haskell 98, this is an opaque type.
754 type IOError = IOException
756 -- |Exceptions that occur in the @IO@ monad.
757 -- An @IOException@ records a more specific error type, a descriptive
758 -- string and maybe the handle that was used when the error was
762 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
764 ioe_type :: IOErrorType, -- what it was.
765 ioe_location :: String, -- location.
766 ioe_description :: String, -- error type specific information.
767 ioe_filename :: Maybe FilePath -- filename the error is related to.
770 instance Eq IOException where
771 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
772 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
785 | UnsatisfiedConstraints
792 | UnsupportedOperation
796 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
798 instance Eq IOErrorType where
801 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
802 _ -> getTag x ==# getTag y
804 instance Show IOErrorType where
808 AlreadyExists -> "already exists"
809 NoSuchThing -> "does not exist"
810 ResourceBusy -> "resource busy"
811 ResourceExhausted -> "resource exhausted"
813 IllegalOperation -> "illegal operation"
814 PermissionDenied -> "permission denied"
815 UserError -> "user error"
816 HardwareFault -> "hardware fault"
817 InappropriateType -> "inappropriate type"
818 Interrupted -> "interrupted"
819 InvalidArgument -> "invalid argument"
820 OtherError -> "failed"
821 ProtocolError -> "protocol error"
822 ResourceVanished -> "resource vanished"
823 SystemError -> "system error"
824 TimeExpired -> "timeout"
825 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
826 UnsupportedOperation -> "unsupported operation"
827 DynIOError{} -> "unknown IO error"
829 userError :: String -> IOError
830 userError str = IOError Nothing UserError "" str Nothing
832 -- ---------------------------------------------------------------------------
835 instance Show IOException where
836 showsPrec p (IOError hdl iot loc s fn) =
838 Nothing -> case hdl of
840 Just h -> showsPrec p h . showString ": "
841 Just name -> showString name . showString ": ") .
844 _ -> showString loc . showString ": ") .
848 _ -> showString " (" . showString s . showString ")")
850 -- -----------------------------------------------------------------------------
853 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
854 deriving (Eq, Ord, Ix, Enum, Read, Show)