ghc-7.0.3: The GHC API

Panic

Synopsis

- data GhcException
  - = PhaseFailed String ExitCode
  - | Signal Int
  - | UsageError String
  - | CmdLineError String
  - | Panic String
  - | Sorry String
  - | InstallationError String
  - | ProgramError String
- showGhcException :: GhcException -> String -> String
- throwGhcException :: GhcException -> a
- handleGhcException :: ExceptionMonad m => (GhcException -> m a) -> m a -> m a
- ghcError :: GhcException -> a
- progName :: String
- pgmError :: String -> a
- panic :: String -> a
- sorry :: String -> a
- panicFastInt :: String -> FastInt
- assertPanic :: String -> Int -> a
- trace :: String -> a -> a
- class (Typeable e, Show e) => Exception e where
  - toException :: e -> SomeException
  - fromException :: SomeException -> Maybe e
- 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]

# 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.
Sorry String | The user tickled something that's known not to work yet, but we're not counting it as a bug.
InstallationError String | An installation problem.
ProgramError String | An error in the user's code, probably.

Instances

Eq GhcException |
Show GhcException |
Typeable GhcException |
Exception GhcException |

showGhcException :: GhcException -> String -> String

Append a description of the given exception to this string.

throwGhcException :: GhcException -> a

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

ghcError :: GhcException -> a

Alias for `throwGhcException`

progName :: String

The name of this GHC.

pgmError :: String -> a

panic :: String -> a

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 -> aSource

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`.

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

Methods

toException :: e -> SomeException Source

fromException :: SomeException -> Maybe eSource

Instances

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

showException :: Exception e => e -> String

Show an exception as a string.

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)

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).

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).

`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 `mask` 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). 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`.

Blocked `throwTo` is fair: if multiple threads are trying to throw an exception to the same target thread, they will succeed in FIFO order.

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.

interruptTargetThread :: MVar [ThreadId]