- data GhcException
- showGhcException :: GhcException -> String -> String
- throwGhcException :: GhcException -> a
- handleGhcException :: ExceptionMonad m => (GhcException -> m a) -> m a -> m a
- ghcError :: GhcException -> a
- progName :: String
- panic, pgmError, sorry :: String -> a
- panicFastInt :: String -> FastInt
- assertPanic :: String -> Int -> a
- trace :: String -> a -> a
- class (Typeable e, Show e) => Exception e where
- showException :: Exception e => e -> 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 ()
- interruptTargetThread :: MVar [ThreadId]
GHC's own exception type error messages all take the form:
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).
|PhaseFailed String ExitCode|
Some other fatal signal (SIGHUP,SIGTERM)
Prints the short usage msg after the error
A problem with the command line arguments, but don't print usage.
The user tickled something that's known not to work yet, but we're not counting it as a bug.
An installation problem.
An error in the user's code, probably.
Append a description of the given exception to this string.
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.
Throw an failed assertion exception for a given filename and line number.
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)
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
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
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
SomeCompilerException, but not other types, e.g.
*Main> throw MismatchedParentheses
catche -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParentheses
catche -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParentheses
catche -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParentheses
catche -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
catch, but returns an
Either result which is
( if no exception of type
e was raised, or
if an exception of type
e was raised and its value is
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)
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 raises an arbitrary exception in the target thread (GHC only).
throwTo does not return until the exception has been raised in the
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
mask or not. However, in GHC a foreign call
can be annotated as
interruptible, in which case a
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"
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,
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
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
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
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.