| Safe Haskell | Safe |
|---|---|
| Language | Haskell2010 |
Bio.Iteratee.Exception
Description
Monadic and General Iteratees: Messaging and exception handling.
Iteratees use an internal exception handling mechanism that is parallel to
that provided by Exception. This allows the iteratee framework
to handle its own exceptions outside IO.
Iteratee exceptions are divided into two categories, IterException and
EnumException. IterExceptions are exceptions within an iteratee, and
EnumExceptions are exceptions within an enumerator.
Enumerators can be constructed to handle an IterException with
Data.Iteratee.Iteratee.enumFromCallbackCatch. If the enumerator detects
an iteratee exception, the enumerator calls the provided exception handler.
The enumerator is then able to continue feeding data to the iteratee,
provided the exception was successfully handled. If the handler could
not handle the exception, the IterException is converted to an
EnumException and processing aborts.
Exceptions can also be cleared by Data.Iteratee.Iteratee.checkErr,
although in this case the iteratee continuation cannot be recovered.
When viewed as Resumable Exceptions, iteratee exceptions provide a means
for iteratees to send control messages to enumerators. The seek
implementation provides an example. Data.Iteratee.Iteratee.seek stores
the current iteratee continuation and throws a SeekException, which
inherits from IterException. Data.Iteratee.IO.enumHandleRandom is
constructed with enumFromCallbackCatch and a handler that performs
an hSeek. Upon receiving the SeekException, enumHandleRandom calls
the handler, checks that it executed properly, and then continues with
the stored continuation.
As the exception hierarchy is open, users can extend it with custom exceptions and exception handlers to implement sophisticated messaging systems based upon resumable exceptions.
- data IFException = Exception e => IFException e
- class (Typeable * e, Show e) => Exception e where
- data EnumException = Exception e => EnumException e
- data DivergentException = DivergentException
- data EnumStringException = EnumStringException String
- data EnumUnhandledIterException = EnumUnhandledIterException IterException
- class Exception e => IException e where
- data IterException = Exception e => IterException e
- data SeekException = SeekException FileOffset
- data EofException = EofException
- data IterStringException = IterStringException String
- enStrExc :: String -> EnumException
- iterStrExc :: String -> SomeException
- wrapIterExc :: IterException -> EnumException
- iterExceptionToException :: Exception e => e -> SomeException
- iterExceptionFromException :: Exception e => SomeException -> Maybe e
Exception types
data IFException Source #
Root of the Iteratee exception hierarchy. IFException derives from
Control.Exception.SomeException. EnumException, IterException,
and all inheritants are descendents of IFException.
Constructors
| Exception e => IFException e |
Instances
class (Typeable * e, Show e) => Exception e where #
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 MyExceptionThe 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 = frontendExceptionFromExceptionWe can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatche -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
Methods
toException :: e -> SomeException #
fromException :: SomeException -> Maybe e #
displayException :: e -> String #
Instances
Enumerator exceptions
data DivergentException Source #
The iteratee diverged upon receiving EOF.
Constructors
| DivergentException |
Instances
data EnumStringException Source #
Create an enumerator exception from a String.
Constructors
| EnumStringException String |
Instances
data EnumUnhandledIterException Source #
The enumerator received an IterException it could not handle.
Constructors
| EnumUnhandledIterException IterException |
Iteratee exceptions
class Exception e => IException e where Source #
A class for iteratee exceptions. Only inheritants of IterException
should be instances of this class.
Methods
toIterException :: e -> IterException Source #
fromIterException :: IterException -> Maybe e Source #
data IterException Source #
Root of iteratee exceptions.
Constructors
| Exception e => IterException e |
Instances
data SeekException Source #
A seek request within an Iteratee.
Constructors
| SeekException FileOffset |
Instances
data EofException Source #
The Iteratee needs more data but received EOF.
Constructors
| EofException |
Instances
data IterStringException Source #
An Iteratee exception specified by a String.
Constructors
| IterStringException String |
Functions
enStrExc :: String -> EnumException Source #
Create an EnumException from a string.
iterStrExc :: String -> SomeException Source #
Create an iteratee exception from a string.
This convenience function wraps IterStringException and toException.
wrapIterExc :: IterException -> EnumException Source #
Convert an IterException to an EnumException. Meant to be used
within an Enumerator to signify that it could not handle the
IterException.
iterExceptionToException :: Exception e => e -> SomeException Source #
iterExceptionFromException :: Exception e => SomeException -> Maybe e Source #