Data.Iteratee.Exception

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.

Exception types

data IFException

Root of the Iteratee exception hierarchy. IFException derives from Control.Exception.SomeException. EnumException, IterException, and all inheritants are descendents of IFException.

Constructors

forall e . Exception e => IFException e

Instances

Show IFException

Typeable IFException

Exception IFException

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 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

fromException :: SomeException -> Maybe e

Instances

Exception BlockedIndefinitelyOnMVar

Exception BlockedIndefinitelyOnSTM

Exception Deadlock

Exception AssertionFailed

Exception AsyncException

Exception ArrayException

Exception ExitCode

Exception IOException

Exception SomeException

Exception ErrorCall

Exception ArithException

Exception IterStringException

Exception EofException

Exception SeekException

Exception IterException

Exception EnumUnhandledIterException

Exception EnumStringException

Exception DivergentException

Exception EnumException

Exception IFException

Enumerator exceptions

data EnumException

Constructors

forall e . Exception e => EnumException e

Instances

Show EnumException

Typeable EnumException

Exception EnumException

data DivergentException

The iteratee diverged upon receiving EOF.

Constructors

DivergentException

Instances

Show DivergentException

Typeable DivergentException

Exception DivergentException

data EnumStringException

Create an enumerator exception from a String.

Constructors

EnumStringException String

Instances

Show EnumStringException

Typeable EnumStringException

Exception EnumStringException

data EnumUnhandledIterException

The enumerator received an IterException it could not handle.

Constructors

EnumUnhandledIterException IterException

Instances

Show EnumUnhandledIterException

Typeable EnumUnhandledIterException

Exception EnumUnhandledIterException

Iteratee exceptions

class Exception e => IException e where

A class for iteratee exceptions. Only inheritants of IterException should be instances of this class.

Methods

toIterException :: e -> IterException

fromIterException :: IterException -> Maybe e

Instances

IException IterStringException

IException EofException

IException SeekException

IException IterException

data IterException

Root of iteratee exceptions.

Constructors

forall e . Exception e => IterException e

Instances

Show IterException

Typeable IterException

Exception IterException

IException IterException

data SeekException

A seek request within an Iteratee.

Constructors

SeekException FileOffset

Instances

Show SeekException

Typeable SeekException

Exception SeekException

IException SeekException

data EofException

The Iteratee needs more data but received EOF.

Constructors

EofException

Instances

Show EofException

Typeable EofException

Exception EofException

IException EofException

data IterStringException

An Iteratee exception specified by a String.

Constructors

IterStringException String

Instances

Show IterStringException

Typeable IterStringException

Exception IterStringException

IException IterStringException

Functions

enStrExc :: String -> EnumException

Create an EnumException from a string.

iterStrExc :: String -> SomeException

Create an iteratee exception from a string. This convenience function wraps IterStringException and toException.

wrapIterExc :: IterException -> EnumException

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

iterExceptionFromException :: Exception e => SomeException -> Maybe e