ghc-7.8.4: The GHC API

Safe Haskell | None
Language | Haskell98

Panic

Synopsis

- data GhcException
  - = PhaseFailed String ExitCode
  - | Signal Int
  - | UsageError String
  - | CmdLineError String
  - | Panic String
  - | PprPanic String SDoc
  - | Sorry String
  - | PprSorry String SDoc
  - | InstallationError String
  - | ProgramError String
  - | PprProgramError String SDoc
- 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 GhcException

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 `impossible` happened.
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

Show GhcException |
Exception GhcException |
Typeable * GhcException |

showGhcException :: GhcException -> String -> String

Append a description of the given exception to this string.

throwGhcException :: GhcException -> a

throwGhcExceptionIO :: GhcException -> IO a

handleGhcException :: ExceptionMonad m => (GhcException -> m a) -> m a -> m a

progName :: String

The name of this GHC.

pgmError :: String -> a

Panics and asserts.

panic :: String -> a

Panics and asserts.

sorry :: String -> a

Panics and asserts.

panicFastInt :: String -> FastInt

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 -> a

Throw an failed assertion exception for a given filename and line number.

trace :: String -> a -> a Source

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.

panicDoc :: String -> SDoc -> a

sorryDoc :: String -> SDoc -> a

panicDocFastInt :: String -> SDoc -> FastInt

pgmErrorDoc :: String -> SDoc -> a

class (Typeable * e, Show e) => Exception e where Source

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

Minimal complete definition

Nothing

Methods

toException :: e -> SomeException Source

fromException :: SomeException -> Maybe e Source

Instances

Exception PatternMatchFail |
Exception RecSelError |
Exception RecConError |
Exception RecUpdError |
Exception NoMethodError |
Exception NonTermination |
Exception NestedAtomically |
Exception BlockedIndefinitelyOnMVar |
Exception BlockedIndefinitelyOnSTM |
Exception Deadlock |
Exception AssertionFailed |
Exception SomeAsyncException |
Exception AsyncException |
Exception ArrayException |
Exception ExitCode |
Exception Dynamic |
Exception IOException |
Exception ErrorCall |
Exception ArithException |
Exception SomeException |
Exception GhcException |
Exception IOEnvFailure |
Exception GhcApiError |
Exception SourceError |

showException :: Exception e => e -> String

Show an exception as a string.

safeShowException :: Exception e => e -> IO String

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 `(`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)

tryMost :: IO a -> IO (Either SomeException a)

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 ()

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 ()

popInterruptTargetThread :: IO ()