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.
224 = FileHandle -- A normal handle to a file
227 | DuplexHandle -- A handle to a read/write stream
228 !(MVar Handle__) -- The read side
229 !(MVar Handle__) -- The write side
232 -- * A 'FileHandle' is seekable. A 'DuplexHandle' may or may not be
235 instance Eq Handle where
236 (FileHandle h1) == (FileHandle h2) = h1 == h2
237 (DuplexHandle h1 _) == (DuplexHandle h2 _) = h1 == h2
240 type FD = Int -- XXX ToDo: should be CInt
244 haFD :: !FD, -- file descriptor
245 haType :: HandleType, -- type (read/write/append etc.)
246 haIsBin :: Bool, -- binary mode?
247 haIsStream :: Bool, -- is this a stream handle?
248 haBufferMode :: BufferMode, -- buffer contains read/write data?
249 haFilePath :: FilePath, -- file name, possibly
250 haBuffer :: !(IORef Buffer), -- the current buffer
251 haBuffers :: !(IORef BufferList), -- spare buffers
252 haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
256 -- ---------------------------------------------------------------------------
259 -- The buffer is represented by a mutable variable containing a
260 -- record, where the record contains the raw buffer and the start/end
261 -- points of the filled portion. We use a mutable variable so that
262 -- the common operation of writing (or reading) some data from (to)
263 -- the buffer doesn't need to modify, and hence copy, the handle
264 -- itself, it just updates the buffer.
266 -- There will be some allocation involved in a simple hPutChar in
267 -- order to create the new Buffer structure (below), but this is
268 -- relatively small, and this only has to be done once per write
271 -- The buffer contains its size - we could also get the size by
272 -- calling sizeOfMutableByteArray# on the raw buffer, but that tends
273 -- to be rounded up to the nearest Word.
275 type RawBuffer = MutableByteArray# RealWorld
277 -- INVARIANTS on a Buffer:
279 -- * A handle *always* has a buffer, even if it is only 1 character long
280 -- (an unbuffered handle needs a 1 character buffer in order to support
281 -- hLookAhead and hIsEOF).
283 -- * if r == w, then r == 0 && w == 0
284 -- * if state == WriteBuffer, then r == 0
285 -- * a write buffer is never full. If an operation
286 -- fills up the buffer, it will always flush it before
288 -- * a read buffer may be full as a result of hLookAhead. In normal
289 -- operation, a read buffer always has at least one character of space.
297 bufState :: BufferState
300 data BufferState = ReadBuffer | WriteBuffer deriving (Eq)
302 -- we keep a few spare buffers around in a handle to avoid allocating
303 -- a new one for each hPutStr. These buffers are *guaranteed* to be the
304 -- same size as the main buffer.
307 | BufferListCons RawBuffer BufferList
310 bufferIsWritable :: Buffer -> Bool
311 bufferIsWritable Buffer{ bufState=WriteBuffer } = True
312 bufferIsWritable _other = False
314 bufferEmpty :: Buffer -> Bool
315 bufferEmpty Buffer{ bufRPtr=r, bufWPtr=w } = r == w
317 -- only makes sense for a write buffer
318 bufferFull :: Buffer -> Bool
319 bufferFull b@Buffer{ bufWPtr=w } = w >= bufSize b
321 -- Internally, we classify handles as being one
332 isReadableHandleType ReadHandle = True
333 isReadableHandleType ReadWriteHandle = True
334 isReadableHandleType _ = False
336 isWritableHandleType AppendHandle = True
337 isWritableHandleType WriteHandle = True
338 isWritableHandleType ReadWriteHandle = True
339 isWritableHandleType _ = False
341 -- File names are specified using @FilePath@, a OS-dependent
342 -- string that (hopefully, I guess) maps to an accessible file/object.
344 type FilePath = String
346 -- ---------------------------------------------------------------------------
349 -- Three kinds of buffering are supported: line-buffering,
350 -- block-buffering or no-buffering. These modes have the following
351 -- effects. For output, items are written out from the internal
352 -- buffer according to the buffer mode:
354 -- o line-buffering the entire output buffer is written
355 -- out whenever a newline is output, the output buffer overflows,
356 -- a flush is issued, or the handle is closed.
358 -- o block-buffering the entire output buffer is written out whenever
359 -- it overflows, a flush is issued, or the handle
362 -- o no-buffering output is written immediately, and never stored
363 -- in the output buffer.
365 -- The output buffer is emptied as soon as it has been written out.
367 -- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
369 -- o line-buffering when the input buffer for the handle is not empty,
370 -- the next item is obtained from the buffer;
371 -- otherwise, when the input buffer is empty,
372 -- characters up to and including the next newline
373 -- character are read into the buffer. No characters
374 -- are available until the newline character is
377 -- o block-buffering when the input buffer for the handle becomes empty,
378 -- the next block of data is read into this buffer.
380 -- o no-buffering the next input item is read and returned.
382 -- For most implementations, physical files will normally be block-buffered
383 -- and terminals will normally be line-buffered. (the IO interface provides
384 -- operations for changing the default buffering of a handle tho.)
387 = NoBuffering | LineBuffering | BlockBuffering (Maybe Int)
388 deriving (Eq, Ord, Read, Show)
390 -- ---------------------------------------------------------------------------
393 -- |A mutable variable in the 'IO' monad
394 newtype IORef a = IORef (STRef RealWorld a) deriving Eq
396 -- |Build a new 'IORef'
397 newIORef :: a -> IO (IORef a)
398 newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
400 -- |Read the value of an 'IORef'
401 readIORef :: IORef a -> IO a
402 readIORef (IORef var) = stToIO (readSTRef var)
404 -- |Write a new value into an 'IORef'
405 writeIORef :: IORef a -> a -> IO ()
406 writeIORef (IORef var) v = stToIO (writeSTRef var v)
408 -- ---------------------------------------------------------------------------
409 -- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
410 -- The type arguments are as follows:
412 -- * @i@: the index type of the array (should be an instance of @Ix@)
414 -- * @e@: the element type of the array.
418 newtype IOArray i e = IOArray (STArray RealWorld i e) deriving Eq
420 -- |Build a new 'IOArray'
421 newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
422 {-# INLINE newIOArray #-}
423 newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
425 -- | Read a value from an 'IOArray'
426 unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
427 {-# INLINE unsafeReadIOArray #-}
428 unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
430 -- | Write a new value into an 'IOArray'
431 unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
432 {-# INLINE unsafeWriteIOArray #-}
433 unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
435 -- | Read a value from an 'IOArray'
436 readIOArray :: Ix i => IOArray i e -> i -> IO e
437 readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
439 -- | Write a new value into an 'IOArray'
440 writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
441 writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
444 -- ---------------------------------------------------------------------------
445 -- Show instance for Handles
447 -- handle types are 'show'n when printing error msgs, so
448 -- we provide a more user-friendly Show instance for it
449 -- than the derived one.
451 instance Show HandleType where
454 ClosedHandle -> showString "closed"
455 SemiClosedHandle -> showString "semi-closed"
456 ReadHandle -> showString "readable"
457 WriteHandle -> showString "writable"
458 AppendHandle -> showString "writable (append)"
459 ReadWriteHandle -> showString "read-writable"
461 instance Show Handle where
462 showsPrec p (FileHandle h) = showHandle p h False
463 showsPrec p (DuplexHandle _ h) = showHandle p h True
465 showHandle p h duplex =
467 -- (Big) SIGH: unfolded defn of takeMVar to avoid
468 -- an (oh-so) unfortunate module loop with GHC.Conc.
469 hdl_ = unsafePerformIO (IO $ \ s# ->
470 case h of { MVar h# ->
471 case takeMVar# h# s# of { (# s2# , r #) ->
472 case putMVar# h# r s2# of { s3# ->
475 showType | duplex = showString "duplex (read-write)"
476 | otherwise = showsPrec p (haType hdl_)
479 showHdl (haType hdl_)
480 (showString "loc=" . showString (haFilePath hdl_) . showChar ',' .
481 showString "type=" . showType . showChar ',' .
482 showString "binary=" . showsPrec p (haIsBin hdl_) . showChar ',' .
483 showString "buffering=" . showBufMode (unsafePerformIO (readIORef (haBuffer hdl_))) (haBufferMode hdl_) . showString "}" )
486 showHdl :: HandleType -> ShowS -> ShowS
489 ClosedHandle -> showsPrec p ht . showString "}"
492 showBufMode :: Buffer -> BufferMode -> ShowS
493 showBufMode buf bmo =
495 NoBuffering -> showString "none"
496 LineBuffering -> showString "line"
497 BlockBuffering (Just n) -> showString "block " . showParen True (showsPrec p n)
498 BlockBuffering Nothing -> showString "block " . showParen True (showsPrec p def)
503 -- ------------------------------------------------------------------------
504 -- Exception datatype and operations
506 -- |The type of exceptions. Every kind of system-generated exception
507 -- has a constructor in the 'Exception' type, and values of other
508 -- types may be injected into 'Exception' by coercing them to
509 -- 'Dynamic' (see the section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
511 = ArithException ArithException
512 -- ^Exceptions raised by arithmetic
513 -- operations. (NOTE: GHC currently does not throw
514 -- 'ArithException's except for 'DivideByZero').
515 | ArrayException ArrayException
516 -- ^Exceptions raised by array-related
517 -- operations. (NOTE: GHC currently does not throw
518 -- 'ArrayException's).
519 | AssertionFailed String
520 -- ^This exception is thrown by the
521 -- 'assert' operation when the condition
522 -- fails. The 'String' argument contains the
523 -- location of the assertion in the source program.
524 | AsyncException AsyncException
525 -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
527 -- ^The current thread was executing a call to
528 -- 'takeMVar' that could never return, because there are no other
529 -- references to this 'MVar'.
531 -- ^There are no runnable threads, so the program is
532 -- deadlocked. The 'Deadlock' exception is
533 -- raised in the main thread only (see also: "Control.Concurrent").
534 | DynException Dynamic
535 -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
537 -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
538 -- argument of 'ErrorCall' is the string passed to 'error' when it was
540 | ExitException ExitCode
541 -- ^The 'ExitException' exception is thrown by 'System.exitWith' (and
542 -- 'System.exitFailure'). The 'ExitCode' argument is the value passed
543 -- to 'System.exitWith'. An unhandled 'ExitException' exception in the
544 -- main thread will cause the program to be terminated with the given
546 | IOException IOException
547 -- ^These are the standard IO exceptions generated by
548 -- Haskell\'s @IO@ operations. See also "System.IO.Error".
549 | NoMethodError String
550 -- ^An attempt was made to invoke a class method which has
551 -- no definition in this instance, and there was no default
552 -- definition given in the class declaration. GHC issues a
553 -- warning when you compile an instance which has missing
556 -- ^The current thread is stuck in an infinite loop. This
557 -- exception may or may not be thrown when the program is
559 | PatternMatchFail String
560 -- ^A pattern matching failure. The 'String' argument should contain a
561 -- descriptive message including the function name, source file
564 -- ^An attempt was made to evaluate a field of a record
565 -- for which no value was given at construction time. The
566 -- 'String' argument gives the location of the
567 -- record construction in the source program.
569 -- ^A field selection was attempted on a constructor that
570 -- doesn\'t have the requested field. This can happen with
571 -- multi-constructor records when one or more fields are
572 -- missing from some of the constructors. The
573 -- 'String' argument gives the location of the
574 -- record selection in the source program.
576 -- ^An attempt was made to update a field in a record,
577 -- where the record doesn\'t have the requested field. This can
578 -- only occur with multi-constructor records, when one or more
579 -- fields are missing from some of the constructors. The
580 -- 'String' argument gives the location of the
581 -- record update in the source program.
583 -- |The type of arithmetic exceptions
593 -- |Asynchronous exceptions
596 -- ^The current thread\'s stack exceeded its limit.
597 -- Since an exception has been raised, the thread\'s stack
598 -- will certainly be below its limit again, but the
599 -- programmer should take remedial action
602 -- ^The program\'s heap is reaching its limit, and
603 -- the program should take action to reduce the amount of
604 -- live data it has. Notes:
606 -- * It is undefined which thread receives this exception.
608 -- * GHC currently does not throw 'HeapOverflow' exceptions.
610 -- ^This exception is raised by another thread
611 -- calling 'killThread', or by the system
612 -- if it needs to terminate the thread for some
616 -- | Exceptions generated by array operations
618 = IndexOutOfBounds String
619 -- ^An attempt was made to index an array outside
620 -- its declared bounds.
621 | UndefinedElement String
622 -- ^An attempt was made to evaluate an element of an
623 -- array that had not been initialized.
626 stackOverflow, heapOverflow :: Exception -- for the RTS
627 stackOverflow = AsyncException StackOverflow
628 heapOverflow = AsyncException HeapOverflow
630 instance Show ArithException where
631 showsPrec _ Overflow = showString "arithmetic overflow"
632 showsPrec _ Underflow = showString "arithmetic underflow"
633 showsPrec _ LossOfPrecision = showString "loss of precision"
634 showsPrec _ DivideByZero = showString "divide by zero"
635 showsPrec _ Denormal = showString "denormal"
637 instance Show AsyncException where
638 showsPrec _ StackOverflow = showString "stack overflow"
639 showsPrec _ HeapOverflow = showString "heap overflow"
640 showsPrec _ ThreadKilled = showString "thread killed"
642 instance Show ArrayException where
643 showsPrec _ (IndexOutOfBounds s)
644 = showString "array index out of range"
645 . (if not (null s) then showString ": " . showString s
647 showsPrec _ (UndefinedElement s)
648 = showString "undefined array element"
649 . (if not (null s) then showString ": " . showString s
652 instance Show Exception where
653 showsPrec _ (IOException err) = shows err
654 showsPrec _ (ArithException err) = shows err
655 showsPrec _ (ArrayException err) = shows err
656 showsPrec _ (ErrorCall err) = showString err
657 showsPrec _ (ExitException err) = showString "exit: " . shows err
658 showsPrec _ (NoMethodError err) = showString err
659 showsPrec _ (PatternMatchFail err) = showString err
660 showsPrec _ (RecSelError err) = showString err
661 showsPrec _ (RecConError err) = showString err
662 showsPrec _ (RecUpdError err) = showString err
663 showsPrec _ (AssertionFailed err) = showString err
664 showsPrec _ (DynException _err) = showString "unknown exception"
665 showsPrec _ (AsyncException e) = shows e
666 showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
667 showsPrec _ (NonTermination) = showString "<<loop>>"
668 showsPrec _ (Deadlock) = showString "<<deadlock>>"
670 instance Eq Exception where
671 IOException e1 == IOException e2 = e1 == e2
672 ArithException e1 == ArithException e2 = e1 == e2
673 ArrayException e1 == ArrayException e2 = e1 == e2
674 ErrorCall e1 == ErrorCall e2 = e1 == e2
675 ExitException e1 == ExitException e2 = e1 == e2
676 NoMethodError e1 == NoMethodError e2 = e1 == e2
677 PatternMatchFail e1 == PatternMatchFail e2 = e1 == e2
678 RecSelError e1 == RecSelError e2 = e1 == e2
679 RecConError e1 == RecConError e2 = e1 == e2
680 RecUpdError e1 == RecUpdError e2 = e1 == e2
681 AssertionFailed e1 == AssertionFailed e2 = e1 == e2
682 DynException _ == DynException _ = False -- incomparable
683 AsyncException e1 == AsyncException e2 = e1 == e2
684 BlockedOnDeadMVar == BlockedOnDeadMVar = True
685 NonTermination == NonTermination = True
686 Deadlock == Deadlock = True
689 -- -----------------------------------------------------------------------------
692 -- The `ExitCode' type defines the exit codes that a program
693 -- can return. `ExitSuccess' indicates successful termination;
694 -- and `ExitFailure code' indicates program failure
695 -- with value `code'. The exact interpretation of `code'
696 -- is operating-system dependent. In particular, some values of
697 -- `code' may be prohibited (e.g. 0 on a POSIX-compliant system).
699 -- We need it here because it is used in ExitException in the
700 -- Exception datatype (above).
702 data ExitCode = ExitSuccess | ExitFailure Int
703 deriving (Eq, Ord, Read, Show)
705 -- --------------------------------------------------------------------------
708 -- | Throw an exception. Exceptions may be thrown from purely
709 -- functional code, but may only be caught within the 'IO' monad.
710 throw :: Exception -> a
711 throw exception = raise# exception
713 -- | A variant of 'throw' that can be used within the 'IO' monad.
715 -- Although 'throwIO' has a type that is an instance of the type of 'throw', the
716 -- two functions are subtly different:
718 -- > throw e `seq` return () ===> throw e
719 -- > throwIO e `seq` return () ===> return ()
721 -- The first example will cause the exception @e@ to be raised,
722 -- whereas the second one won\'t. In fact, 'throwIO' will only cause
723 -- an exception to be raised when it is used within the 'IO' monad.
724 -- The 'throwIO' variant should be used in preference to 'throw' to
725 -- raise an exception within the 'IO' monad because it guarantees
726 -- ordering with respect to other 'IO' operations, whereas 'throw'
728 throwIO :: Exception -> IO a
729 throwIO err = IO $ \s -> throw err s
731 ioException :: IOException -> IO a
732 ioException err = IO $ \s -> throw (IOException err) s
734 ioError :: IOError -> IO a
735 ioError = ioException
737 -- ---------------------------------------------------------------------------
740 -- | The Haskell 98 type for exceptions in the @IO@ monad.
741 -- In Haskell 98, this is an opaque type.
742 type IOError = IOException
744 -- |Exceptions that occur in the @IO@ monad.
745 -- An @IOException@ records a more specific error type, a descriptive
746 -- string and maybe the handle that was used when the error was
750 ioe_handle :: Maybe Handle, -- the handle used by the action flagging
752 ioe_type :: IOErrorType, -- what it was.
753 ioe_location :: String, -- location.
754 ioe_description :: String, -- error type specific information.
755 ioe_filename :: Maybe FilePath -- filename the error is related to.
758 instance Eq IOException where
759 (IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
760 e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
773 | UnsatisfiedConstraints
780 | UnsupportedOperation
784 | DynIOError Dynamic -- cheap&cheerful extensible IO error type.
786 instance Eq IOErrorType where
789 DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
790 _ -> getTag x ==# getTag y
792 instance Show IOErrorType where
796 AlreadyExists -> "already exists"
797 NoSuchThing -> "does not exist"
798 ResourceBusy -> "resource busy"
799 ResourceExhausted -> "resource exhausted"
801 IllegalOperation -> "illegal operation"
802 PermissionDenied -> "permission denied"
803 UserError -> "user error"
804 HardwareFault -> "hardware fault"
805 InappropriateType -> "inappropriate type"
806 Interrupted -> "interrupted"
807 InvalidArgument -> "invalid argument"
808 OtherError -> "failed"
809 ProtocolError -> "protocol error"
810 ResourceVanished -> "resource vanished"
811 SystemError -> "system error"
812 TimeExpired -> "timeout"
813 UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
814 UnsupportedOperation -> "unsupported operation"
815 DynIOError{} -> "unknown IO error"
817 userError :: String -> IOError
818 userError str = IOError Nothing UserError "" str Nothing
820 -- ---------------------------------------------------------------------------
823 instance Show IOException where
824 showsPrec p (IOError hdl iot loc s fn) =
828 _ -> showString "\nAction: " . showString loc) .
831 Just h -> showString "\nHandle: " . showsPrec p h) .
834 _ -> showString "\nReason: " . showString s) .
837 Just name -> showString "\nFile: " . showString name)
839 -- -----------------------------------------------------------------------------
842 data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
843 deriving (Eq, Ord, Ix, Enum, Read, Show)