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

Catch and exception and observe the stack trace. If on top of the IO monad, this will also capture asynchronous exceptions

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.