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| Synopsis |
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| Documentation |
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| Constructors | |  Instances | |
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| When called, trace outputs the string in its first argument, before
returning the second argument as its result. The trace function is not
referentially transparent, and should only be used for debugging, or for
monitoring execution. Some implementations of trace may decorate the string
that's output to indicate that you're tracing. The function is implemented on
top of putTraceMsg.
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Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException
deriving (Show, Typeable)
instance Exception MyException
The default method definitions in the Exception class do what we need
in this case. You can now throw and catch ThisException and
ThatException as exceptions:
*Main> throw ThisException catch e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy
of exceptions:
---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
deriving Typeable
instance Show SomeCompilerException where
show (SomeCompilerException e) = show e
instance Exception SomeCompilerException
compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException
compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
SomeCompilerException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
deriving Typeable
instance Show SomeFrontendException where
show (SomeFrontendException e) = show e
instance Exception SomeFrontendException where
toException = compilerExceptionToException
fromException = compilerExceptionFromException
frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException
frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
SomeFrontendException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception
data MismatchedParentheses = MismatchedParentheses
deriving (Typeable, Show)
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromException
We can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses catch e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses
| | | Methods | | | | Instances | |
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Similar to catch, but returns an Either result which is
(Right a) if no exception of type e was raised, or (Left ex)
if an exception of type e was raised and its value is ex.
If any other type of exception is raised than it will be propogated
up to the next enclosing exception handler.
try a = catch (Right `liftM` a) (return . Left)
Note that System.IO.Error also exports a function called
System.IO.Error.try with a similar type to Control.Exception.try,
except that it catches only the IO and user families of exceptions
(as required by the Haskell 98 IO module).
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| tryMost is like try, but passes through Interrupted and Panic
exceptions. Used when we want soft failures when reading interface
files, for example.
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throwTo raises an arbitrary exception in the target thread (GHC only).
throwTo does not return until the exception has been raised in the
target thread.
The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.
Whatever work the target thread was doing when the exception was
raised is not lost: the computation is suspended until required by
another thread.
If the target thread is currently making a foreign call, then the
exception will not be raised (and hence throwTo will not return)
until the call has completed. This is the case regardless of whether
the call is inside a block or not.
Important note: the behaviour of throwTo differs from that described in
the paper "Asynchronous exceptions in Haskell"
(http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm).
In the paper, throwTo is non-blocking; but the library implementation adopts
a more synchronous design in which throwTo does not return until the exception
is received by the target thread. The trade-off is discussed in Section 9 of the paper.
Like any blocking operation, throwTo is therefore interruptible (see Section 5.3 of
the paper).
There is no guarantee that the exception will be delivered promptly,
although the runtime will endeavour to ensure that arbitrary
delays don't occur. In GHC, an exception can only be raised when a
thread reaches a safe point, where a safe point is where memory
allocation occurs. Some loops do not perform any memory allocation
inside the loop and therefore cannot be interrupted by a throwTo.
Blocked throwTo is fair: if multiple threads are trying to throw an
exception to the same target thread, they will succeed in FIFO order.
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| Produced by Haddock version 2.6.1 |
