iteratee-mtl- Iteratee-based I/O




Monadic and General Iteratees: Messaging and exception handling.

Iteratees use an internal exception handling mechanism that is parallel to that provided by Control.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. 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 Source

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


forall e . Exception e => IFException e 

Enumerator exceptions

data DivergentException Source

The iteratee diverged upon receiving EOF.



Iteratee exceptions

class Exception e => IException e whereSource

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

data IterException Source

Root of iteratee exceptions.


forall e . Exception e => IterException e 

data EofException Source

The Iteratee needs more data but received EOF.




enStrExc :: String -> EnumExceptionSource

Create an EnumException from a string.

iterStrExc :: String -> SomeExceptionSource

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

wrapIterExc :: IterException -> EnumExceptionSource

Convert an IterException to an EnumException. Meant to be used within an Enumerator to signify that it could not handle the IterException.