A monad of explicitly typed, checked exceptions

Run a computation explicitly handling exceptions

Run a safe computation

Run a computation checking even unchecked (UncaughtExceptions) exceptions

A Monad Transformer for explicitly typed checked exceptions.

Run a computation explicitly handling exceptions

Run a safe computation

Run a safe computation checking even unchecked (UncaughtException) exceptions

UncaughtException models unchecked exceptions

In order to declare an unchecked exception E, all that is needed is to make e an instance of UncaughtException

 instance UncaughtException E

Note that declaring an exception E as unchecked does not automatically turn its children unchecked too. This is a shortcoming of the current encoding.

Throws is a type level binary relationship used to model a list of exceptions.

There is only one case in which the user must add further instances to Throws. If your sets of exceptions are hierarchical then you need to teach Throws about the hierarchy.

Subtyping

As there is no way to automatically infer the subcases of an exception, they have to be encoded manually mirroring the hierarchy defined in the defined Exception instances. For example, the following instance encodes that MyFileNotFoundException is a subexception of MyIOException :

 instance Throws MyFileNotFoundException (Caught MyIOException l)

Throws is not a transitive relation and every ancestor relation must be explicitly encoded.

                                                            --   TopException
                                                            --         |
   instance Throws MidException   (Caught TopException l)   --         |
                                                            --   MidException
   instance Throws ChildException (Caught MidException l)   --         |
   instance Throws ChildException (Caught TopException l)   --         |
                                                            --  ChildException

Note that SomeException is automatically an ancestor of every other exception type.

A type level witness of a exception handler.

The throw primitive

Rethrow an exception keeping the call trace

The catch primitive

Like catch but makes the call trace available

Sequence two computations discarding the result of the second one. If the first computation rises an exception, the second computation is run and then the exception is rethrown.

Like finally, but performs the second computation only when the first one rises an exception

Capture an exception e, wrap it, and rethrow. Keeps the original monadic call trace.

FailException is thrown by Monad fail

MonadZeroException is thrown by MonadPlus mzero

This function may be used as a value for mplus in MonadPlus

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

The SomeException type is the root of the exception type hierarchy. When an exception of type e is thrown, behind the scenes it is encapsulated in a SomeException.

The class Typeable allows a concrete representation of a type to be calculated.

Generating stack traces for failures

withLocTH is a convenient TH macro which expands to withLoc <source location> It should only be used when the MonadLoc preprocessor is not available. Usage:

 f x = $withLocTH $ do
          $withLocTH $ something
          x < -$withLocTH $ something-else
          ...

NOTE: unfortunately type signatures are necessary when using withLocTH