2 {-# OPTIONS_GHC -fno-implicit-prelude #-}
3 -----------------------------------------------------------------------------
5 -- Module : GHC.Exception
6 -- Copyright : (c) The University of Glasgow, 1998-2002
7 -- License : see libraries/base/LICENSE
9 -- Maintainer : cvs-ghc@haskell.org
10 -- Stability : internal
11 -- Portability : non-portable (GHC extensions)
13 -- Exceptions and exception-handling functions.
15 -----------------------------------------------------------------------------
19 ( module GHC.Exception,
20 Exception(..), AsyncException(..),
21 IOException(..), ArithException(..), ArrayException(..),
22 throw, throwIO, ioError )
29 %*********************************************************
31 \subsection{Primitive catch}
33 %*********************************************************
35 catchException used to handle the passing around of the state to the
36 action and the handler. This turned out to be a bad idea - it meant
37 that we had to wrap both arguments in thunks so they could be entered
38 as normal (remember IO returns an unboxed pair...).
42 catch# :: IO a -> (b -> IO a) -> IO a
44 (well almost; the compiler doesn't know about the IO newtype so we
45 have to work around that in the definition of catchException below).
48 catchException :: IO a -> (Exception -> IO a) -> IO a
49 catchException (IO m) k = IO $ \s -> catch# m (\ex -> unIO (k ex)) s
51 -- | The 'catch' function establishes a handler that receives any 'IOError'
52 -- raised in the action protected by 'catch'. An 'IOError' is caught by
53 -- the most recent handler established by 'catch'. These handlers are
54 -- not selective: all 'IOError's are caught. Exception propagation
55 -- must be explicitly provided in a handler by re-raising any unwanted
56 -- exceptions. For example, in
58 -- > f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)
60 -- the function @f@ returns @[]@ when an end-of-file exception
61 -- (cf. 'System.IO.Error.isEOFError') occurs in @g@; otherwise, the
62 -- exception is propagated to the next outer handler.
64 -- When an exception propagates outside the main program, the Haskell
65 -- system prints the associated 'IOError' value and exits the program.
67 -- Non-I\/O exceptions are not caught by this variant; to catch all
68 -- exceptions, use 'Control.Exception.catch' from "Control.Exception".
69 catch :: IO a -> (IOError -> IO a) -> IO a
70 catch m k = catchException m handler
71 where handler (IOException err) = k err
72 handler other = throw other
76 %*********************************************************
78 \subsection{Controlling asynchronous exception delivery}
80 %*********************************************************
83 -- | Applying 'block' to a computation will
84 -- execute that computation with asynchronous exceptions
85 -- /blocked/. That is, any thread which
86 -- attempts to raise an exception in the current thread will be
87 -- blocked until asynchronous exceptions are enabled again. There\'s
88 -- no need to worry about re-enabling asynchronous exceptions; that is
89 -- done automatically on exiting the scope of
93 -- | To re-enable asynchronous exceptions inside the scope of
94 -- 'block', 'unblock' can be
95 -- used. It scopes in exactly the same way, so on exit from
96 -- 'unblock' asynchronous exception delivery will
98 unblock :: IO a -> IO a
100 block (IO io) = IO $ blockAsyncExceptions# io
101 unblock (IO io) = IO $ unblockAsyncExceptions# io
105 -- | Forces its argument to be evaluated, and returns the result in
106 -- the 'IO' monad. It can be used to order evaluation with respect to
107 -- other 'IO' operations; its semantics are given by
109 -- > evaluate x `seq` y ==> y
110 -- > evaluate x `catch` f ==> (return $! x) `catch` f
111 -- > evaluate x >>= f ==> (return $! x) >>= f
113 -- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
114 -- same as @(return $! x)@.
115 evaluate :: a -> IO a
116 evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
118 -- a `seq` (# s, a #)
119 -- because we can't have an unboxed tuple as a function argument