Safe Haskell | None |
---|---|
Language | Haskell98 |
Panic
- data GhcException
- showGhcException :: GhcException -> String -> String
- throwGhcException :: GhcException -> a
- throwGhcExceptionIO :: GhcException -> IO a
- handleGhcException :: ExceptionMonad m => (GhcException -> m a) -> m a -> m a
- progName :: String
- pgmError :: String -> a
- panic :: String -> a
- sorry :: String -> a
- panicFastInt :: String -> FastInt
- assertPanic :: String -> Int -> a
- trace :: String -> a -> a
- panicDoc :: String -> SDoc -> a
- sorryDoc :: String -> SDoc -> a
- panicDocFastInt :: String -> SDoc -> FastInt
- pgmErrorDoc :: String -> SDoc -> a
- class (Typeable * e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- showException :: Exception e => e -> String
- safeShowException :: Exception e => e -> IO String
- try :: Exception e => IO a -> IO (Either e a)
- tryMost :: IO a -> IO (Either SomeException a)
- throwTo :: Exception e => ThreadId -> e -> IO ()
- installSignalHandlers :: IO ()
- pushInterruptTargetThread :: ThreadId -> IO ()
- popInterruptTargetThread :: IO ()
Documentation
data GhcExceptionSource
GHC's own exception type error messages all take the form:
location: error
If the location is on the command line, or in GHC itself, then location="ghc". All of the error types below correspond to a location of "ghc", except for ProgramError (where the string is assumed to contain a location already, so we don't print one).
Constructors
PhaseFailed String ExitCode | |
Signal Int | Some other fatal signal (SIGHUP,SIGTERM) |
UsageError String | Prints the short usage msg after the error |
CmdLineError String | A problem with the command line arguments, but don't print usage. |
Panic String | The |
PprPanic String SDoc | |
Sorry String | The user tickled something that's known not to work yet, but we're not counting it as a bug. |
PprSorry String SDoc | |
InstallationError String | An installation problem. |
ProgramError String | An error in the user's code, probably. |
PprProgramError String SDoc |
Instances
showGhcException :: GhcException -> String -> StringSource
Append a description of the given exception to this string.
throwGhcException :: GhcException -> aSource
throwGhcExceptionIO :: GhcException -> IO aSource
handleGhcException :: ExceptionMonad m => (GhcException -> m a) -> m a -> m aSource
panicFastInt :: String -> FastIntSource
Panic while pretending to return an unboxed int. You can't use the regular panic functions in expressions producing unboxed ints because they have the wrong kind.
assertPanic :: String -> Int -> aSource
Throw an failed assertion exception for a given filename and line number.
trace :: String -> a -> aSource
The trace
function outputs the trace message given as its first argument,
before returning the second argument as its result.
For example, this returns the value of f x
but first outputs the message.
trace ("calling f with x = " ++ show x) (f x)
The trace
function should only be used for debugging, or for monitoring
execution. The function is not referentially transparent: its type indicates
that it is a pure function but it has the side effect of outputting the
trace message.
panicDocFastInt :: String -> SDoc -> FastIntSource
pgmErrorDoc :: String -> SDoc -> aSource
class (Typeable * e, Show e) => Exception e whereSource
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 MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
Instances
showException :: Exception e => e -> StringSource
Show an exception as a string.
safeShowException :: Exception e => e -> IO StringSource
Show an exception which can possibly throw other exceptions. Used when displaying exception thrown within TH code.
try :: Exception e => IO a -> IO (Either e a)Source
Similar to catch
, but returns an Either
result which is
(
if no exception of type Right
a)e
was raised, or (
if an exception of type Left
ex)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)
tryMost :: IO a -> IO (Either SomeException a)Source
Like try, but pass through UserInterrupt and Panic exceptions. Used when we want soft failures when reading interface files, for example. TODO: I'm not entirely sure if this is catching what we really want to catch
throwTo :: Exception e => ThreadId -> e -> IO ()Source
throwTo
raises an arbitrary exception in the target thread (GHC only).
Exception delivery synchronizes between the source and target thread:
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. Exception delivery is also atomic
with respect to other exceptions. Atomicity is a useful property to have
when dealing with race conditions: e.g. 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 mask
or not. However, in GHC a foreign call
can be annotated as interruptible
, in which case a throwTo
will
cause the RTS to attempt to cause the call to return; see the GHC
documentation for more details.
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). Unlike other interruptible operations, however, throwTo
is always interruptible, even if it does not actually block.
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
.
If the target of throwTo
is the calling thread, then the behaviour
is the same as throwIO
, except that the exception
is thrown as an asynchronous exception. This means that if there is
an enclosing pure computation, which would be the case if the current
IO operation is inside unsafePerformIO
or unsafeInterleaveIO
, that
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
Note that if throwTo
is called with the current thread as the
target, the exception will be thrown even if the thread is currently
inside mask
or uninterruptibleMask
.
installSignalHandlers :: IO ()Source
Install standard signal handlers for catching ^C, which just throw an exception in the target thread. The current target thread is the thread at the head of the list in the MVar passed to installSignalHandlers.
pushInterruptTargetThread :: ThreadId -> IO ()Source