-% ------------------------------------------------------------------------------
-% $Id: IOBase.lhs,v 1.7 2002/03/14 12:09:50 simonmar Exp $
-%
-% (c) The University of Glasgow, 1994-2001
-%
-
-% Definitions for the @IO@ monad and its friends. Everything is exported
-% concretely; the @IO@ module itself exports abstractly.
-
\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
-module GHC.IOBase where
-
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module : GHC.IOBase
+-- Copyright : (c) The University of Glasgow 1994-2002
+-- License : see libraries/base/LICENSE
+--
+-- Maintainer : cvs-ghc@haskell.org
+-- Stability : internal
+-- Portability : non-portable (GHC Extensions)
+--
+-- Definitions for the 'IO' monad and its friends.
+--
+-----------------------------------------------------------------------------
+
+-- #hide
+module GHC.IOBase(
+ IO(..), unIO, failIO, liftIO, bindIO, thenIO, returnIO,
+ unsafePerformIO, unsafeInterleaveIO,
+ unsafeDupablePerformIO, unsafeDupableInterleaveIO,
+ noDuplicate,
+
+ -- To and from from ST
+ stToIO, ioToST, unsafeIOToST, unsafeSTToIO,
+
+ -- References
+ IORef(..), newIORef, readIORef, writeIORef,
+ IOArray(..), newIOArray, readIOArray, writeIOArray, unsafeReadIOArray, unsafeWriteIOArray,
+ MVar(..),
+
+ -- Handles, file descriptors,
+ FilePath,
+ Handle(..), Handle__(..), HandleType(..), IOMode(..), FD,
+ isReadableHandleType, isWritableHandleType, isReadWriteHandleType, showHandle,
+
+ -- Buffers
+ Buffer(..), RawBuffer, BufferState(..), BufferList(..), BufferMode(..),
+ bufferIsWritable, bufferEmpty, bufferFull,
+
+ -- Exceptions
+ Exception(..), ArithException(..), AsyncException(..), ArrayException(..),
+ stackOverflow, heapOverflow, throw, throwIO, ioException,
+ IOError, IOException(..), IOErrorType(..), ioError, userError,
+ ExitCode(..)
+ ) where
+
import GHC.ST
+import GHC.Arr -- to derive Ix class
+import GHC.Enum -- to derive Enum class
import GHC.STRef
import GHC.Base
-import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
+-- import GHC.Num -- To get fromInteger etc, needed because of -fno-implicit-prelude
import Data.Maybe ( Maybe(..) )
import GHC.Show
import GHC.List
import GHC.Read
-import {-# SOURCE #-} Data.Dynamic
+import Foreign.C.Types (CInt)
+
+#ifndef __HADDOCK__
+import {-# SOURCE #-} Data.Typeable ( showsTypeRep )
+import {-# SOURCE #-} Data.Dynamic ( Dynamic, dynTypeRep )
+#endif
-- ---------------------------------------------------------------------------
-- The IO Monad
--SDM
-}
+{-|
+A value of type @'IO' a@ is a computation which, when performed,
+does some I\/O before returning a value of type @a@.
+
+There is really only one way to \"perform\" an I\/O action: bind it to
+@Main.main@ in your program. When your program is run, the I\/O will
+be performed. It isn't possible to perform I\/O from an arbitrary
+function, unless that function is itself in the 'IO' monad and called
+at some point, directly or indirectly, from @Main.main@.
+
+'IO' is a monad, so 'IO' actions can be combined using either the do-notation
+or the '>>' and '>>=' operations from the 'Monad' class.
+-}
newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))
unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
(# new_s, a #) -> unIO (k a) new_s
)
+thenIO :: IO a -> IO b -> IO b
+thenIO (IO m) k = IO ( \ s ->
+ case m s of
+ (# new_s, a #) -> unIO k new_s
+ )
+
returnIO :: a -> IO a
returnIO x = IO (\ s -> (# s, x #))
-- ---------------------------------------------------------------------------
-- Coercions between IO and ST
---stToIO :: (forall s. ST s a) -> IO a
+-- | A monad transformer embedding strict state transformers in the 'IO'
+-- monad. The 'RealWorld' parameter indicates that the internal state
+-- used by the 'ST' computation is a special one supplied by the 'IO'
+-- monad, and thus distinct from those used by invocations of 'runST'.
stToIO :: ST RealWorld a -> IO a
stToIO (ST m) = IO m
ioToST :: IO a -> ST RealWorld a
ioToST (IO m) = (ST m)
+-- This relies on IO and ST having the same representation modulo the
+-- constraint on the type of the state
+--
+unsafeIOToST :: IO a -> ST s a
+unsafeIOToST (IO io) = ST $ \ s -> (unsafeCoerce# io) s
+
+unsafeSTToIO :: ST s a -> IO a
+unsafeSTToIO (ST m) = IO (unsafeCoerce# m)
+
-- ---------------------------------------------------------------------------
-- Unsafe IO operations
-{-# NOINLINE unsafePerformIO #-}
+{-|
+This is the \"back door\" into the 'IO' monad, allowing
+'IO' computation to be performed at any time. For
+this to be safe, the 'IO' computation should be
+free of side effects and independent of its environment.
+
+If the I\/O computation wrapped in 'unsafePerformIO'
+performs side effects, then the relative order in which those side
+effects take place (relative to the main I\/O trunk, or other calls to
+'unsafePerformIO') is indeterminate. You have to be careful when
+writing and compiling modules that use 'unsafePerformIO':
+
+ * Use @{\-\# NOINLINE foo \#-\}@ as a pragma on any function @foo@
+ that calls 'unsafePerformIO'. If the call is inlined,
+ the I\/O may be performed more than once.
+
+ * Use the compiler flag @-fno-cse@ to prevent common sub-expression
+ elimination being performed on the module, which might combine
+ two side effects that were meant to be separate. A good example
+ is using multiple global variables (like @test@ in the example below).
+
+ * Make sure that the either you switch off let-floating, or that the
+ call to 'unsafePerformIO' cannot float outside a lambda. For example,
+ if you say:
+ @
+ f x = unsafePerformIO (newIORef [])
+ @
+ you may get only one reference cell shared between all calls to @f@.
+ Better would be
+ @
+ f x = unsafePerformIO (newIORef [x])
+ @
+ because now it can't float outside the lambda.
+
+It is less well known that
+'unsafePerformIO' is not type safe. For example:
+
+> test :: IORef [a]
+> test = unsafePerformIO $ newIORef []
+>
+> main = do
+> writeIORef test [42]
+> bang <- readIORef test
+> print (bang :: [Char])
+
+This program will core dump. This problem with polymorphic references
+is well known in the ML community, and does not arise with normal
+monadic use of references. There is no easy way to make it impossible
+once you use 'unsafePerformIO'. Indeed, it is
+possible to write @coerce :: a -> b@ with the
+help of 'unsafePerformIO'. So be careful!
+-}
unsafePerformIO :: IO a -> a
-unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
-
-{-# NOINLINE unsafeInterleaveIO #-}
+unsafePerformIO m = unsafeDupablePerformIO (noDuplicate >> m)
+
+{-|
+This version of 'unsafePerformIO' is slightly more efficient,
+because it omits the check that the IO is only being performed by a
+single thread. Hence, when you write 'unsafeDupablePerformIO',
+there is a possibility that the IO action may be performed multiple
+times (on a multiprocessor), and you should therefore ensure that
+it gives the same results each time.
+-}
+{-# NOINLINE unsafeDupablePerformIO #-}
+unsafeDupablePerformIO :: IO a -> a
+unsafeDupablePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
+
+-- Why do we NOINLINE unsafeDupablePerformIO? See the comment with
+-- GHC.ST.runST. Essentially the issue is that the IO computation
+-- inside unsafePerformIO must be atomic: it must either all run, or
+-- not at all. If we let the compiler see the application of the IO
+-- to realWorld#, it might float out part of the IO.
+
+-- Why is there a call to 'lazy' in unsafeDupablePerformIO?
+-- If we don't have it, the demand analyser discovers the following strictness
+-- for unsafeDupablePerformIO: C(U(AV))
+-- But then consider
+-- unsafeDupablePerformIO (\s -> let r = f x in
+-- case writeIORef v r s of (# s1, _ #) ->
+-- (# s1, r #)
+-- The strictness analyser will find that the binding for r is strict,
+-- (becuase of uPIO's strictness sig), and so it'll evaluate it before
+-- doing the writeIORef. This actually makes tests/lib/should_run/memo002
+-- get a deadlock!
+--
+-- Solution: don't expose the strictness of unsafeDupablePerformIO,
+-- by hiding it with 'lazy'
+
+{-|
+'unsafeInterleaveIO' allows 'IO' computation to be deferred lazily.
+When passed a value of type @IO a@, the 'IO' will only be performed
+when the value of the @a@ is demanded. This is used to implement lazy
+file reading, see 'System.IO.hGetContents'.
+-}
+{-# INLINE unsafeInterleaveIO #-}
unsafeInterleaveIO :: IO a -> IO a
-unsafeInterleaveIO (IO m)
+unsafeInterleaveIO m = unsafeDupableInterleaveIO (noDuplicate >> m)
+
+-- We believe that INLINE on unsafeInterleaveIO is safe, because the
+-- state from this IO thread is passed explicitly to the interleaved
+-- IO, so it cannot be floated out and shared.
+
+{-# INLINE unsafeDupableInterleaveIO #-}
+unsafeDupableInterleaveIO :: IO a -> IO a
+unsafeDupableInterleaveIO (IO m)
= IO ( \ s -> let
r = case m s of (# _, res #) -> res
in
(# s, r #))
+{-|
+Ensures that the suspensions under evaluation by the current thread
+are unique; that is, the current thread is not evaluating anything
+that is also under evaluation by another thread that has also executed
+'noDuplicate'.
+
+This operation is used in the definition of 'unsafePerformIO' to
+prevent the IO action from being executed multiple times, which is usually
+undesirable.
+-}
+noDuplicate :: IO ()
+noDuplicate = IO $ \s -> case noDuplicate# s of s' -> (# s', () #)
+
-- ---------------------------------------------------------------------------
-- Handle type
data MVar a = MVar (MVar# RealWorld a)
+{- ^
+An 'MVar' (pronounced \"em-var\") is a synchronising variable, used
+for communication between concurrent threads. It can be thought of
+as a a box, which may be empty or full.
+-}
-- pull in Eq (Mvar a) too, to avoid GHC.Conc being an orphan-instance module
instance Eq (MVar a) where
-- Note: when a Handle is garbage collected, we want to flush its buffer
-- and close the OS file handle, so as to free up a (precious) resource.
+-- | Haskell defines operations to read and write characters from and to files,
+-- represented by values of type @Handle@. Each value of this type is a
+-- /handle/: a record used by the Haskell run-time system to /manage/ I\/O
+-- with file system objects. A handle has at least the following properties:
+--
+-- * whether it manages input or output or both;
+--
+-- * whether it is /open/, /closed/ or /semi-closed/;
+--
+-- * whether the object is seekable;
+--
+-- * whether buffering is disabled, or enabled on a line or block basis;
+--
+-- * a buffer (whose length may be zero).
+--
+-- Most handles will also have a current I\/O position indicating where the next
+-- input or output operation will occur. A handle is /readable/ if it
+-- manages only input or both input and output; likewise, it is /writable/ if
+-- it manages only output or both input and output. A handle is /open/ when
+-- first allocated.
+-- Once it is closed it can no longer be used for either input or output,
+-- though an implementation cannot re-use its storage while references
+-- remain to it. Handles are in the 'Show' and 'Eq' classes. The string
+-- produced by showing a handle is system dependent; it should include
+-- enough information to identify the handle for debugging. A handle is
+-- equal according to '==' only to itself; no attempt
+-- is made to compare the internal state of different handles for equality.
+--
+-- GHC note: a 'Handle' will be automatically closed when the garbage
+-- collector detects that it has become unreferenced by the program.
+-- However, relying on this behaviour is not generally recommended:
+-- the garbage collector is unpredictable. If possible, use explicit
+-- an explicit 'hClose' to close 'Handle's when they are no longer
+-- required. GHC does not currently attempt to free up file
+-- descriptors when they have run out, it is your responsibility to
+-- ensure that this doesn't happen.
+
data Handle
= FileHandle -- A normal handle to a file
+ FilePath -- the file (invariant)
!(MVar Handle__)
| DuplexHandle -- A handle to a read/write stream
+ FilePath -- file for a FIFO, otherwise some
+ -- descriptive string.
!(MVar Handle__) -- The read side
!(MVar Handle__) -- The write side
-- seekable.
instance Eq Handle where
- (FileHandle h1) == (FileHandle h2) = h1 == h2
- (DuplexHandle h1 _) == (DuplexHandle h2 _) = h1 == h2
+ (FileHandle _ h1) == (FileHandle _ h2) = h1 == h2
+ (DuplexHandle _ h1 _) == (DuplexHandle _ h2 _) = h1 == h2
_ == _ = False
-type FD = Int -- XXX ToDo: should be CInt
+type FD = CInt
data Handle__
= Handle__ {
haFD :: !FD, -- file descriptor
haType :: HandleType, -- type (read/write/append etc.)
haIsBin :: Bool, -- binary mode?
- haIsStream :: Bool, -- is this a stream handle?
+ haIsStream :: Bool, -- Windows : is this a socket?
+ -- Unix : is O_NONBLOCK set?
haBufferMode :: BufferMode, -- buffer contains read/write data?
- haFilePath :: FilePath, -- file name, possibly
haBuffer :: !(IORef Buffer), -- the current buffer
haBuffers :: !(IORef BufferList), -- spare buffers
haOtherSide :: Maybe (MVar Handle__) -- ptr to the write side of a
isWritableHandleType ReadWriteHandle = True
isWritableHandleType _ = False
--- File names are specified using @FilePath@, a OS-dependent
--- string that (hopefully, I guess) maps to an accessible file/object.
+isReadWriteHandleType ReadWriteHandle{} = True
+isReadWriteHandleType _ = False
+
+-- | File and directory names are values of type 'String', whose precise
+-- meaning is operating system dependent. Files can be opened, yielding a
+-- handle which can then be used to operate on the contents of that file.
type FilePath = String
-- ---------------------------------------------------------------------------
-- Buffering modes
--- Three kinds of buffering are supported: line-buffering,
+-- | Three kinds of buffering are supported: line-buffering,
-- block-buffering or no-buffering. These modes have the following
--- effects. For output, items are written out from the internal
--- buffer according to the buffer mode:
+-- effects. For output, items are written out, or /flushed/,
+-- from the internal buffer according to the buffer mode:
--
--- * line-buffering the entire output buffer is written
--- out whenever a newline is output, the output buffer overflows,
--- a flush is issued, or the handle is closed.
+-- * /line-buffering/: the entire output buffer is flushed
+-- whenever a newline is output, the buffer overflows,
+-- a 'System.IO.hFlush' is issued, or the handle is closed.
--
--- * block-buffering the entire output buffer is written out whenever
--- it overflows, a flush is issued, or the handle
--- is closed.
+-- * /block-buffering/: the entire buffer is written out whenever it
+-- overflows, a 'System.IO.hFlush' is issued, or the handle is closed.
--
--- * no-buffering output is written immediately, and never stored
--- in the output buffer.
+-- * /no-buffering/: output is written immediately, and never stored
+-- in the buffer.
--
+-- An implementation is free to flush the buffer more frequently,
+-- but not less frequently, than specified above.
-- The output buffer is emptied as soon as it has been written out.
-
--- Similarly, input occurs according to the buffer mode for handle {\em hdl}.
-
--- * line-buffering when the input buffer for the handle is not empty,
--- the next item is obtained from the buffer;
--- otherwise, when the input buffer is empty,
--- characters up to and including the next newline
--- character are read into the buffer. No characters
--- are available until the newline character is
--- available.
--
--- * block-buffering when the input buffer for the handle becomes empty,
--- the next block of data is read into this buffer.
+-- Similarly, input occurs according to the buffer mode for the handle:
--
--- * no-buffering the next input item is read and returned.
-
+-- * /line-buffering/: when the buffer for the handle is not empty,
+-- the next item is obtained from the buffer; otherwise, when the
+-- buffer is empty, characters up to and including the next newline
+-- character are read into the buffer. No characters are available
+-- until the newline character is available or the buffer is full.
+--
+-- * /block-buffering/: when the buffer for the handle becomes empty,
+-- the next block of data is read into the buffer.
+--
+-- * /no-buffering/: the next input item is read and returned.
+-- The 'System.IO.hLookAhead' operation implies that even a no-buffered
+-- handle may require a one-character buffer.
+--
+-- The default buffering mode when a handle is opened is
+-- implementation-dependent and may depend on the file system object
+-- which is attached to that handle.
-- For most implementations, physical files will normally be block-buffered
--- and terminals will normally be line-buffered. (the IO interface provides
--- operations for changing the default buffering of a handle tho.)
+-- and terminals will normally be line-buffered.
data BufferMode
- = NoBuffering | LineBuffering | BlockBuffering (Maybe Int)
+ = NoBuffering -- ^ buffering is disabled if possible.
+ | LineBuffering
+ -- ^ line-buffering should be enabled if possible.
+ | BlockBuffering (Maybe Int)
+ -- ^ block-buffering should be enabled if possible.
+ -- The size of the buffer is @n@ items if the argument
+ -- is 'Just' @n@ and is otherwise implementation-dependent.
deriving (Eq, Ord, Read, Show)
-- ---------------------------------------------------------------------------
-- IORefs
-newtype IORef a = IORef (STRef RealWorld a) deriving Eq
+-- |A mutable variable in the 'IO' monad
+newtype IORef a = IORef (STRef RealWorld a)
+
+-- explicit instance because Haddock can't figure out a derived one
+instance Eq (IORef a) where
+ IORef x == IORef y = x == y
+-- |Build a new 'IORef'
newIORef :: a -> IO (IORef a)
newIORef v = stToIO (newSTRef v) >>= \ var -> return (IORef var)
+-- |Read the value of an 'IORef'
readIORef :: IORef a -> IO a
readIORef (IORef var) = stToIO (readSTRef var)
+-- |Write a new value into an 'IORef'
writeIORef :: IORef a -> a -> IO ()
writeIORef (IORef var) v = stToIO (writeSTRef var v)
-- ---------------------------------------------------------------------------
+-- | An 'IOArray' is a mutable, boxed, non-strict array in the 'IO' monad.
+-- The type arguments are as follows:
+--
+-- * @i@: the index type of the array (should be an instance of 'Ix')
+--
+-- * @e@: the element type of the array.
+--
+--
+
+newtype IOArray i e = IOArray (STArray RealWorld i e)
+
+-- explicit instance because Haddock can't figure out a derived one
+instance Eq (IOArray i e) where
+ IOArray x == IOArray y = x == y
+
+-- |Build a new 'IOArray'
+newIOArray :: Ix i => (i,i) -> e -> IO (IOArray i e)
+{-# INLINE newIOArray #-}
+newIOArray lu init = stToIO $ do {marr <- newSTArray lu init; return (IOArray marr)}
+
+-- | Read a value from an 'IOArray'
+unsafeReadIOArray :: Ix i => IOArray i e -> Int -> IO e
+{-# INLINE unsafeReadIOArray #-}
+unsafeReadIOArray (IOArray marr) i = stToIO (unsafeReadSTArray marr i)
+
+-- | Write a new value into an 'IOArray'
+unsafeWriteIOArray :: Ix i => IOArray i e -> Int -> e -> IO ()
+{-# INLINE unsafeWriteIOArray #-}
+unsafeWriteIOArray (IOArray marr) i e = stToIO (unsafeWriteSTArray marr i e)
+
+-- | Read a value from an 'IOArray'
+readIOArray :: Ix i => IOArray i e -> i -> IO e
+readIOArray (IOArray marr) i = stToIO (readSTArray marr i)
+
+-- | Write a new value into an 'IOArray'
+writeIOArray :: Ix i => IOArray i e -> i -> e -> IO ()
+writeIOArray (IOArray marr) i e = stToIO (writeSTArray marr i e)
+
+
+-- ---------------------------------------------------------------------------
-- Show instance for Handles
-- handle types are 'show'n when printing error msgs, so
ReadWriteHandle -> showString "read-writable"
instance Show Handle where
- showsPrec p (FileHandle h) = showHandle p h False
- showsPrec p (DuplexHandle _ h) = showHandle p h True
-
-showHandle p h duplex =
- let
- -- (Big) SIGH: unfolded defn of takeMVar to avoid
- -- an (oh-so) unfortunate module loop with GHC.Conc.
- hdl_ = unsafePerformIO (IO $ \ s# ->
- case h of { MVar h# ->
- case takeMVar# h# s# of { (# s2# , r #) ->
- case putMVar# h# r s2# of { s3# ->
- (# s3#, r #) }}})
-
- showType | duplex = showString "duplex (read-write)"
- | otherwise = showsPrec p (haType hdl_)
- in
- showChar '{' .
- showHdl (haType hdl_)
- (showString "loc=" . showString (haFilePath hdl_) . showChar ',' .
- showString "type=" . showType . showChar ',' .
- showString "binary=" . showsPrec p (haIsBin hdl_) . showChar ',' .
- showString "buffering=" . showBufMode (unsafePerformIO (readIORef (haBuffer hdl_))) (haBufferMode hdl_) . showString "}" )
- where
-
- showHdl :: HandleType -> ShowS -> ShowS
- showHdl ht cont =
- case ht of
- ClosedHandle -> showsPrec p ht . showString "}"
- _ -> cont
-
- showBufMode :: Buffer -> BufferMode -> ShowS
- showBufMode buf bmo =
- case bmo of
- NoBuffering -> showString "none"
- LineBuffering -> showString "line"
- BlockBuffering (Just n) -> showString "block " . showParen True (showsPrec p n)
- BlockBuffering Nothing -> showString "block " . showParen True (showsPrec p def)
- where
- def :: Int
- def = bufSize buf
+ showsPrec p (FileHandle file _) = showHandle file
+ showsPrec p (DuplexHandle file _ _) = showHandle file
+
+showHandle file = showString "{handle: " . showString file . showString "}"
-- ------------------------------------------------------------------------
-- Exception datatype and operations
+-- |The type of exceptions. Every kind of system-generated exception
+-- has a constructor in the 'Exception' type, and values of other
+-- types may be injected into 'Exception' by coercing them to
+-- 'Data.Dynamic.Dynamic' (see the section on Dynamic Exceptions:
+-- "Control.Exception\#DynamicExceptions").
data Exception
- = IOException IOException -- IO exceptions
- | ArithException ArithException -- Arithmetic exceptions
- | ArrayException ArrayException -- Array-related exceptions
- | ErrorCall String -- Calls to 'error'
- | ExitException ExitCode -- Call to System.exitWith
- | NoMethodError String -- A non-existent method was invoked
- | PatternMatchFail String -- A pattern match / guard failure
- | RecSelError String -- Selecting a non-existent field
- | RecConError String -- Field missing in record construction
- | RecUpdError String -- Record doesn't contain updated field
- | AssertionFailed String -- Assertions
- | DynException Dynamic -- Dynamic exceptions
- | AsyncException AsyncException -- Externally generated errors
- | BlockedOnDeadMVar -- Blocking on a dead MVar
- | Deadlock -- no threads can run (raised in main thread)
+ = ArithException ArithException
+ -- ^Exceptions raised by arithmetic
+ -- operations. (NOTE: GHC currently does not throw
+ -- 'ArithException's except for 'DivideByZero').
+ | ArrayException ArrayException
+ -- ^Exceptions raised by array-related
+ -- operations. (NOTE: GHC currently does not throw
+ -- 'ArrayException's).
+ | AssertionFailed String
+ -- ^This exception is thrown by the
+ -- 'assert' operation when the condition
+ -- fails. The 'String' argument contains the
+ -- location of the assertion in the source program.
+ | AsyncException AsyncException
+ -- ^Asynchronous exceptions (see section on Asynchronous Exceptions: "Control.Exception\#AsynchronousExceptions").
+ | BlockedOnDeadMVar
+ -- ^The current thread was executing a call to
+ -- 'Control.Concurrent.MVar.takeMVar' that could never return,
+ -- because there are no other references to this 'MVar'.
+ | BlockedIndefinitely
+ -- ^The current thread was waiting to retry an atomic memory transaction
+ -- that could never become possible to complete because there are no other
+ -- threads referring to any of teh TVars involved.
+ | NestedAtomically
+ -- ^The runtime detected an attempt to nest one STM transaction
+ -- inside another one, presumably due to the use of
+ -- 'unsafePeformIO' with 'atomically'.
+ | Deadlock
+ -- ^There are no runnable threads, so the program is
+ -- deadlocked. The 'Deadlock' exception is
+ -- raised in the main thread only (see also: "Control.Concurrent").
+ | DynException Dynamic
+ -- ^Dynamically typed exceptions (see section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
+ | ErrorCall String
+ -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
+ -- argument of 'ErrorCall' is the string passed to 'error' when it was
+ -- called.
+ | ExitException ExitCode
+ -- ^The 'ExitException' exception is thrown by 'System.Exit.exitWith' (and
+ -- 'System.Exit.exitFailure'). The 'ExitCode' argument is the value passed
+ -- to 'System.Exit.exitWith'. An unhandled 'ExitException' exception in the
+ -- main thread will cause the program to be terminated with the given
+ -- exit code.
+ | IOException IOException
+ -- ^These are the standard IO exceptions generated by
+ -- Haskell\'s @IO@ operations. See also "System.IO.Error".
+ | NoMethodError String
+ -- ^An attempt was made to invoke a class method which has
+ -- no definition in this instance, and there was no default
+ -- definition given in the class declaration. GHC issues a
+ -- warning when you compile an instance which has missing
+ -- methods.
| NonTermination
-
+ -- ^The current thread is stuck in an infinite loop. This
+ -- exception may or may not be thrown when the program is
+ -- non-terminating.
+ | PatternMatchFail String
+ -- ^A pattern matching failure. The 'String' argument should contain a
+ -- descriptive message including the function name, source file
+ -- and line number.
+ | RecConError String
+ -- ^An attempt was made to evaluate a field of a record
+ -- for which no value was given at construction time. The
+ -- 'String' argument gives the location of the
+ -- record construction in the source program.
+ | RecSelError String
+ -- ^A field selection was attempted on a constructor that
+ -- doesn\'t have the requested field. This can happen with
+ -- multi-constructor records when one or more fields are
+ -- missing from some of the constructors. The
+ -- 'String' argument gives the location of the
+ -- record selection in the source program.
+ | RecUpdError String
+ -- ^An attempt was made to update a field in a record,
+ -- where the record doesn\'t have the requested field. This can
+ -- only occur with multi-constructor records, when one or more
+ -- fields are missing from some of the constructors. The
+ -- 'String' argument gives the location of the
+ -- record update in the source program.
+
+-- |The type of arithmetic exceptions
data ArithException
= Overflow
| Underflow
| Denormal
deriving (Eq, Ord)
+
+-- |Asynchronous exceptions
data AsyncException
= StackOverflow
+ -- ^The current thread\'s stack exceeded its limit.
+ -- Since an exception has been raised, the thread\'s stack
+ -- will certainly be below its limit again, but the
+ -- programmer should take remedial action
+ -- immediately.
| HeapOverflow
+ -- ^The program\'s heap is reaching its limit, and
+ -- the program should take action to reduce the amount of
+ -- live data it has. Notes:
+ --
+ -- * It is undefined which thread receives this exception.
+ --
+ -- * GHC currently does not throw 'HeapOverflow' exceptions.
| ThreadKilled
+ -- ^This exception is raised by another thread
+ -- calling 'Control.Concurrent.killThread', or by the system
+ -- if it needs to terminate the thread for some
+ -- reason.
deriving (Eq, Ord)
+-- | Exceptions generated by array operations
data ArrayException
- = IndexOutOfBounds String -- out-of-range array access
- | UndefinedElement String -- evaluating an undefined element
+ = IndexOutOfBounds String
+ -- ^An attempt was made to index an array outside
+ -- its declared bounds.
+ | UndefinedElement String
+ -- ^An attempt was made to evaluate an element of an
+ -- array that had not been initialized.
deriving (Eq, Ord)
stackOverflow, heapOverflow :: Exception -- for the RTS
showsPrec _ (RecConError err) = showString err
showsPrec _ (RecUpdError err) = showString err
showsPrec _ (AssertionFailed err) = showString err
- showsPrec _ (DynException _err) = showString "unknown exception"
+ showsPrec _ (DynException err) = showString "exception :: " . showsTypeRep (dynTypeRep err)
showsPrec _ (AsyncException e) = shows e
showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
+ showsPrec _ (BlockedIndefinitely) = showString "thread blocked indefinitely"
+ showsPrec _ (NestedAtomically) = showString "Control.Concurrent.STM.atomically was nested"
showsPrec _ (NonTermination) = showString "<<loop>>"
showsPrec _ (Deadlock) = showString "<<deadlock>>"
AsyncException e1 == AsyncException e2 = e1 == e2
BlockedOnDeadMVar == BlockedOnDeadMVar = True
NonTermination == NonTermination = True
+ NestedAtomically == NestedAtomically = True
Deadlock == Deadlock = True
+ _ == _ = False
-- -----------------------------------------------------------------------------
-- The ExitCode type
--- The `ExitCode' type defines the exit codes that a program
--- can return. `ExitSuccess' indicates successful termination;
--- and `ExitFailure code' indicates program failure
--- with value `code'. The exact interpretation of `code'
--- is operating-system dependent. In particular, some values of
--- `code' may be prohibited (e.g. 0 on a POSIX-compliant system).
-
-- We need it here because it is used in ExitException in the
-- Exception datatype (above).
-data ExitCode = ExitSuccess | ExitFailure Int
- deriving (Eq, Ord, Read, Show)
+data ExitCode
+ = ExitSuccess -- ^ indicates successful termination;
+ | ExitFailure Int
+ -- ^ indicates program failure with an exit code.
+ -- The exact interpretation of the code is
+ -- operating-system dependent. In particular, some values
+ -- may be prohibited (e.g. 0 on a POSIX-compliant system).
+ deriving (Eq, Ord, Read, Show)
-- --------------------------------------------------------------------------
-- Primitive throw
+-- | Throw an exception. Exceptions may be thrown from purely
+-- functional code, but may only be caught within the 'IO' monad.
throw :: Exception -> a
throw exception = raise# exception
-ioError :: Exception -> IO a
-ioError err = IO $ \s -> throw err s
+-- | A variant of 'throw' that can be used within the 'IO' monad.
+--
+-- Although 'throwIO' has a type that is an instance of the type of 'throw', the
+-- two functions are subtly different:
+--
+-- > throw e `seq` x ===> throw e
+-- > throwIO e `seq` x ===> x
+--
+-- The first example will cause the exception @e@ to be raised,
+-- whereas the second one won\'t. In fact, 'throwIO' will only cause
+-- an exception to be raised when it is used within the 'IO' monad.
+-- The 'throwIO' variant should be used in preference to 'throw' to
+-- raise an exception within the 'IO' monad because it guarantees
+-- ordering with respect to other 'IO' operations, whereas 'throw'
+-- does not.
+throwIO :: Exception -> IO a
+throwIO err = IO $ raiseIO# err
ioException :: IOException -> IO a
-ioException err = IO $ \s -> throw (IOException err) s
+ioException err = IO $ raiseIO# (IOException err)
+
+-- | Raise an 'IOError' in the 'IO' monad.
+ioError :: IOError -> IO a
+ioError = ioException
-- ---------------------------------------------------------------------------
-- IOError type
--- A value @IOError@ encode errors occurred in the @IO@ monad.
--- An @IOError@ records a more specific error type, a descriptive
+-- | The Haskell 98 type for exceptions in the 'IO' monad.
+-- Any I\/O operation may raise an 'IOError' instead of returning a result.
+-- For a more general type of exception, including also those that arise
+-- in pure code, see 'Control.Exception.Exception'.
+--
+-- In Haskell 98, this is an opaque type.
+type IOError = IOException
+
+-- |Exceptions that occur in the @IO@ monad.
+-- An @IOException@ records a more specific error type, a descriptive
-- string and maybe the handle that was used when the error was
-- flagged.
-
-type IOError = Exception
-
data IOException
= IOError {
ioe_handle :: Maybe Handle, -- the handle used by the action flagging
-- the error.
ioe_type :: IOErrorType, -- what it was.
ioe_location :: String, -- location.
- ioe_descr :: String, -- error type specific information.
+ ioe_description :: String, -- error type specific information.
ioe_filename :: Maybe FilePath -- filename the error is related to.
}
(IOError h1 e1 loc1 str1 fn1) == (IOError h2 e2 loc2 str2 fn2) =
e1==e2 && str1==str2 && h1==h2 && loc1==loc2 && fn1==fn2
+-- | An abstract type that contains a value for each variant of 'IOError'.
data IOErrorType
-- Haskell 98:
= AlreadyExists
x == y =
case x of
DynIOError{} -> False -- from a strictness POV, compatible with a derived Eq inst?
- _ -> getTag# x ==# getTag# y
+ _ -> getTag x ==# getTag y
instance Show IOErrorType where
showsPrec _ e =
UnsupportedOperation -> "unsupported operation"
DynIOError{} -> "unknown IO error"
+-- | Construct an 'IOError' value with a string describing the error.
+-- The 'fail' method of the 'IO' instance of the 'Monad' class raises a
+-- 'userError', thus:
+--
+-- > instance Monad IO where
+-- > ...
+-- > fail s = ioError (userError s)
+--
userError :: String -> IOError
-userError str = IOException (IOError Nothing UserError "" str Nothing)
+userError str = IOError Nothing UserError "" str Nothing
-- ---------------------------------------------------------------------------
-- Showing IOErrors
instance Show IOException where
showsPrec p (IOError hdl iot loc s fn) =
- showsPrec p iot .
+ (case fn of
+ Nothing -> case hdl of
+ Nothing -> id
+ Just h -> showsPrec p h . showString ": "
+ Just name -> showString name . showString ": ") .
(case loc of
"" -> id
- _ -> showString "\nAction: " . showString loc) .
- (case hdl of
- Nothing -> id
- Just h -> showString "\nHandle: " . showsPrec p h) .
+ _ -> showString loc . showString ": ") .
+ showsPrec p iot .
(case s of
"" -> id
- _ -> showString "\nReason: " . showString s) .
- (case fn of
- Nothing -> id
- Just name -> showString "\nFile: " . showString name)
+ _ -> showString " (" . showString s . showString ")")
+
+-- -----------------------------------------------------------------------------
+-- IOMode type
+
+data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
+ deriving (Eq, Ord, Ix, Enum, Read, Show)
\end{code}
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