-% ------------------------------------------------------------------------------
-% $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
+-- 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.
+--
+-----------------------------------------------------------------------------
module GHC.IOBase 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.Show
import GHC.List
import GHC.Read
+
+#ifndef __HADDOCK__
import {-# SOURCE #-} Data.Dynamic
+#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 #))
-- ---------------------------------------------------------------------------
-- Unsafe IO operations
+{-|
+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!
+-}
{-# NOINLINE unsafePerformIO #-}
unsafePerformIO :: IO a -> a
unsafePerformIO (IO m) = case m realWorld# of (# _, r #) -> r
+{-|
+'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'.
+-}
{-# NOINLINE unsafeInterleaveIO #-}
unsafeInterleaveIO :: IO a -> IO a
unsafeInterleaveIO (IO m)
-- 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
-- effects. For output, items are written out from the internal
-- buffer according to the buffer mode:
--
--- * line-buffering the entire output buffer is written
+-- o 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.
--
--- * block-buffering the entire output buffer is written out whenever
+-- o block-buffering the entire output buffer is written out whenever
-- it overflows, a flush is issued, or the handle
-- is closed.
--
--- * no-buffering output is written immediately, and never stored
+-- o no-buffering output is written immediately, and never stored
-- in the output buffer.
--
-- 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,
+-- o 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
-- are available until the newline character is
-- available.
--
--- * block-buffering when the input buffer for the handle becomes empty,
+-- o block-buffering when the input buffer for the handle becomes empty,
-- the next block of data is read into this buffer.
--
--- * no-buffering the next input item is read and returned.
+-- o no-buffering the next input item is read and returned.
-- For most implementations, physical files will normally be block-buffered
-- and terminals will normally be line-buffered. (the IO interface provides
-- ---------------------------------------------------------------------------
-- IORefs
+-- |A mutable variable in the 'IO' monad
newtype IORef a = IORef (STRef RealWorld a) deriving Eq
+-- |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)
-- ------------------------------------------------------------------------
-- 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
+-- '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
+ -- 'takeMVar' that could never return, because there are no other
+ -- references to this 'MVar'.
+ | 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.exitWith' (and
+ -- 'System.exitFailure'). The 'ExitCode' argument is the value passed
+ -- to 'System.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 '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
BlockedOnDeadMVar == BlockedOnDeadMVar = True
NonTermination == NonTermination = True
Deadlock == Deadlock = True
+ _ == _ = False
-- -----------------------------------------------------------------------------
-- The ExitCode type
-- --------------------------------------------------------------------------
-- 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` return () ===> throw e
+-- > throwIO e `seq` return () ===> return ()
+--
+-- 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 $ \s -> throw err s
ioException :: IOException -> IO a
ioException err = IO $ \s -> throw (IOException err) s
+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.
+-- 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
DynIOError{} -> "unknown IO error"
userError :: String -> IOError
-userError str = IOException (IOError Nothing UserError "" str Nothing)
+userError str = IOError Nothing UserError "" str Nothing
-- ---------------------------------------------------------------------------
-- Showing IOErrors
(case fn of
Nothing -> id
Just name -> showString "\nFile: " . showString name)
+
+-- -----------------------------------------------------------------------------
+-- IOMode type
+
+data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode
+ deriving (Eq, Ord, Ix, Enum, Read, Show)
\end{code}
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