| Safe Haskell | Trustworthy |
|---|---|
| Language | Haskell2010 |
LIO.Exception
Description
Exception routines much like the IO ones in Control.Exception (we
duplicate the documentation below). There are two differences,
however. First, LIO does not allow masking of asynchronous exceptions
(since these are relied upon to kill a misbehaving thread). Hence,
routines like onException are not guaranteed to run if a thread is
unconditionally killed. Second, in a few cases (such as lWait) it
is possible for the current label to be raised above the current
clearance as an exception is thrown, in which case these functions do
not catch the exception, either, since code cannot run under such
circumstances.
Synopsis
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data SomeException where
- SomeException :: forall e. Exception e => e -> SomeException
- throwLIO :: Exception e => e -> LIO l a
- catch :: (Label l, Exception e) => LIO l a -> (e -> LIO l a) -> LIO l a
- handle :: (Label l, Exception e) => (e -> LIO l a) -> LIO l a -> LIO l a
- try :: (Label l, Exception a1) => LIO l a -> LIO l (Either a1 a)
- onException :: Label l => LIO l a -> LIO l b -> LIO l a
- finally :: Label l => LIO l a -> LIO l b -> LIO l a
- bracket :: Label l => LIO l a -> (a -> LIO l c) -> (a -> LIO l b) -> LIO l b
- evaluate :: a -> LIO l a
Documentation
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 MyExceptionThe 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 = frontendExceptionFromExceptionWe 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
Minimal complete definition
Nothing
Methods
toException :: e -> SomeException #
fromException :: SomeException -> Maybe e #
displayException :: e -> String #
Render this exception value in a human-friendly manner.
Default implementation: show
Since: base-4.8.0.0
Instances
data SomeException where #
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.
Constructors
| SomeException :: forall e. Exception e => e -> SomeException |
Instances
| Show SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
| Exception SomeException | Since: base-3.0 |
Defined in GHC.Exception.Type Methods toException :: SomeException -> SomeException # fromException :: SomeException -> Maybe SomeException # displayException :: SomeException -> String # | |
catch :: (Label l, Exception e) => LIO l a -> (e -> LIO l a) -> LIO l a Source #
A simple wrapper around IO catch. The only subtlety is that code
is not allowed to run unless the current label can flow to the
current clearance. Hence, if the label exceeds the clearance, the
exception is not caught. (Only a few conditions such as lWait or
raising the clearance within scopeClearance can lead to the label
exceeding the clarance, and an exception is always thrown at the
time this happens.)
handle :: (Label l, Exception e) => (e -> LIO l a) -> LIO l a -> LIO l a Source #
A version of catch with the arguments swapped around.
onException :: Label l => LIO l a -> LIO l b -> LIO l a Source #
Like finally, but only performs the final action if there was
an exception raised by the computation.
finally :: Label l => LIO l a -> LIO l b -> LIO l a Source #
A variant of bracket where the return value from the first
computation is not required.
Arguments
| :: Label l | |
| => LIO l a | Computation to run first |
| -> (a -> LIO l c) | Computation to run last |
| -> (a -> LIO l b) | Computation to run in-between |
| -> LIO l b |
When you want to acquire a resource, do some work with it, and
then release the resource, it is a good idea to use bracket,
because bracket will install the necessary exception handler to
release the resource in the event that an exception is raised
during the computation. If an exception is raised, then bracket
will re-raise the exception (after performing the release).