1 % -----------------------------------------------------------------------------
2 % $Id: PrelException.lhs,v 1.6 1999/05/10 16:52:10 sof Exp $
4 % (c) The GRAP/AQUA Project, Glasgow University, 1998
7 Exceptions and exception-handling functions.
10 {-# OPTIONS -fno-implicit-prelude #-}
13 module PrelException where
17 import PrelST ( STret(..) )
23 -----------------------------------------------------------------------------
24 Exception datatype and operations.
28 = IOException IOError -- IO exceptions (from 'ioError')
29 | ArithException ArithException -- Arithmetic exceptions
30 | ErrorCall String -- Calls to 'error'
31 | NoMethodError String -- A non-existent method was invoked
32 | PatternMatchFail String -- A pattern match failed
33 | NonExhaustiveGuards String -- A guard match failed
34 | RecSelError String -- Selecting a non-existent field
35 | RecConError String -- Field missing in record construction
36 | RecUpdError String -- Record doesn't contain updated field
37 | AssertionFailed String -- Assertions
38 | DynException Dynamic -- Dynamic exceptions
39 | AsyncException AsyncException -- Externally generated errors
55 stackOverflow, heapOverflow :: Exception -- for the RTS
56 stackOverflow = AsyncException StackOverflow
57 heapOverflow = AsyncException HeapOverflow
59 instance Show ArithException where
60 showsPrec _ Overflow = showString "arithmetic overflow"
61 showsPrec _ Underflow = showString "arithmetic underflow"
62 showsPrec _ LossOfPrecision = showString "loss of precision"
63 showsPrec _ DivideByZero = showString "divide by zero"
64 showsPrec _ Denormal = showString "denormal"
66 instance Show AsyncException where
67 showsPrec _ StackOverflow = showString "stack overflow"
68 showsPrec _ HeapOverflow = showString "heap overflow"
69 showsPrec _ ThreadKilled = showString "thread killed"
71 instance Show Exception where
72 showsPrec _ (IOException err) = shows err
73 showsPrec _ (ArithException err) = shows err
74 showsPrec _ (ErrorCall err) = showString err
75 showsPrec _ (NoMethodError err) = showString err
76 showsPrec _ (PatternMatchFail err) = showString err
77 showsPrec _ (NonExhaustiveGuards err) = showString err
78 showsPrec _ (RecSelError err) = showString err
79 showsPrec _ (RecConError err) = showString err
80 showsPrec _ (RecUpdError err) = showString err
81 showsPrec _ (AssertionFailed err) = showString err
82 showsPrec _ (AsyncException e) = shows e
83 showsPrec _ (DynException _err) = showString "unknown exception"
87 throw :: Exception -> a
92 throw exception = raise# exception
96 catch handles the passing around of the state in the IO monad; if we
97 don't actually apply (and hence run) an IO computation, we don't get
98 any exceptions! Hence a large mantrap to watch out for is
100 catch# (m :: IO ()) (handler :: NDSet Exception -> IO ())
102 since the computation 'm' won't actually be performed in the context
103 of the 'catch#'. In fact, don't use catch# at all.
106 catchException :: IO a -> (Exception -> IO a) -> IO a
108 catchException m k = ST (\s -> unST m s `primCatch'` \ err -> unST (k err) s)
110 catchException m k = IO $ \s -> case catch# (liftIO m s) (\exs -> liftIO (k exs) s)
111 of STret s1 r -> (# s1, r #)
114 catch :: IO a -> (IOError -> IO a) -> IO a
115 catch m k = catchException m handler
116 where handler (IOException err) = k err
117 handler other = throw other
119 catchNonIO :: IO a -> (Exception -> IO a) -> IO a
120 catchNonIO m k = catchException m handler
121 where handler (IOException err) = ioError err
122 handler other = k other
125 Why is this stuff here? To avoid recursive module dependencies of
129 ioError :: IOError -> IO a
130 ioError err = throw (IOException err)