biohazard-1.0.1: bioinformatics support library

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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. 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.


Exception e => IFException e 

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

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

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

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 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


toException :: e -> SomeException #

fromException :: SomeException -> Maybe e #

displayException :: e -> String #

Render this exception value in a human-friendly manner.

Default implementation: show.



Exception Void


Exception PatternMatchFail

Since: 4.0

Exception RecSelError

Since: 4.0

Exception RecConError

Since: 4.0

Exception RecUpdError

Since: 4.0

Exception NoMethodError

Since: 4.0

Exception TypeError


Exception NonTermination

Since: 4.0

Exception NestedAtomically

Since: 4.0

Exception BlockedIndefinitelyOnMVar


Exception BlockedIndefinitelyOnSTM


Exception Deadlock


Exception AllocationLimitExceeded


Exception CompactionFailed


Exception AssertionFailed


Exception SomeAsyncException


Exception AsyncException


Exception ArrayException


Exception ExitCode


Exception IOException


Exception ErrorCall


Exception ArithException


Exception SomeException

Since: 3.0

Exception UnicodeException 
Exception DecompressError 
Exception IterStringException # 
Exception EofException # 
Exception SeekException # 
Exception IterException # 
Exception EnumUnhandledIterException # 
Exception EnumStringException # 
Exception DivergentException # 
Exception EnumException # 
Exception IFException # 
Exception ParseError # 
Exception ZLibException # 
Exception ZLibParamsException # 

Enumerator exceptions

Iteratee exceptions


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.