-- Hoogle documentation, generated by Haddock
-- See Hoogle, http://www.haskell.org/hoogle/
-- | Extensible optionally-pure exceptions
--
-- Extensible optionally-pure exceptions.
@package exceptions
@version 0.9.0
-- | This module supports monads that can throw extensible exceptions. The
-- exceptions are the very same from Control.Exception, and the
-- operations offered very similar, but here they are not limited to
-- IO.
--
-- This code is in the style of both transformers and mtl, and is
-- compatible with them, though doesn't mimic the module structure or
-- offer the complete range of features in those packages.
--
-- This is very similar to ErrorT and MonadError, but
-- based on features of Control.Exception. In particular, it
-- handles the complex case of asynchronous exceptions by including
-- mask in the typeclass. Note that the extensible exceptions
-- feature relies on the RankNTypes language extension.
module Control.Monad.Catch
-- | A class for monads in which exceptions may be thrown.
--
-- Instances should obey the following law:
--
--
-- throwM e >> x = throwM e
--
--
-- In other words, throwing an exception short-circuits the rest of the
-- monadic computation.
class Monad m => MonadThrow m
-- | Throw an exception. Note that this throws when this action is run in
-- the monad m, not when it is applied. It is a generalization
-- of Control.Exception's throwIO.
--
-- Should satisfy the law:
--
--
-- throwM e >> f = throwM e
--
throwM :: (MonadThrow m, Exception e) => e -> m a
-- | A class for monads which allow exceptions to be caught, in particular
-- exceptions which were thrown by throwM.
--
-- Instances should obey the following law:
--
--
-- catch (throwM e) f = f e
--
--
-- Note that the ability to catch an exception does not guarantee
-- that we can deal with all possible exit points from a computation.
-- Some monads, such as continuation-based stacks, allow for more than
-- just a success/failure strategy, and therefore catch
-- cannot be used by those monads to properly implement a function
-- such as finally. For more information, see MonadMask.
class MonadThrow m => MonadCatch m
-- | Provide a handler for exceptions thrown during execution of the first
-- action. Note that type of the type of the argument to the handler will
-- constrain which exceptions are caught. See Control.Exception's
-- catch.
catch :: (MonadCatch m, Exception e) => m a -> (e -> m a) -> m a
-- | A class for monads which provide for the ability to account for all
-- possible exit points from a computation, and to mask asynchronous
-- exceptions. Continuation-based monads are invalid instances of this
-- class.
--
-- Note that this package does provide a MonadMask
-- instance for CatchT. This instance is only valid if
-- the base monad provides no ability to provide multiple exit. For
-- example, IO or Either would be invalid base monads,
-- but Reader or State would be acceptable.
--
-- Instances should ensure that, in the following code:
--
--
-- f `finally` g
--
--
-- The action g is called regardless of what occurs within
-- f, including async exceptions.
class MonadCatch m => MonadMask m
-- | Runs an action with asynchronous exceptions disabled. The action is
-- provided a method for restoring the async. environment to what it was
-- at the mask call. See Control.Exception's mask.
mask :: MonadMask m => ((forall a. m a -> m a) -> m b) -> m b
-- | Like mask, but the masked computation is not interruptible (see
-- Control.Exception's uninterruptibleMask. WARNING: Only
-- use if you need to mask exceptions around an interruptible operation
-- AND you can guarantee the interruptible operation will only block for
-- a short period of time. Otherwise you render the program/thread
-- unresponsive and/or unkillable.
uninterruptibleMask :: MonadMask m => ((forall a. m a -> m a) -> m b) -> m b
-- | A generalized version of the standard bracket function which allows
-- distinguishing different exit cases. Instead of providing it a single
-- release action, this function takes two different actions: one for the
-- case of a successful run of the inner function, and one in the case of
-- an exception. The former function is provided the acquired value,
-- while the exception release function is provided both the acquired
-- value and the exception that was thrown. The result values of both of
-- these functions are ignored.
--
-- NOTE This method was added in version 0.9.0 of this library.
-- Previously, implementation of functions like bracket and
-- finally in this module were based on the mask and
-- uninterruptibleMask functions only, disallowing some classes of
-- tranformers from having MonadMask instances (notably
-- multi-exit-point transformers like ExceptT). If you are a
-- library author, you'll now need to provide an implementation for this
-- method. As two examples, here is a ReaderT implementation:
--
--
-- generalBracket acquire release cleanup use = ReaderT $ r ->
-- generalBracket
-- (runReaderT acquire r)
-- (resource -> runReaderT (release resource) r)
-- (resource e -> runReaderT (cleanup resource e) r)
-- (resource -> runReaderT (use resource) r)
--
--
-- This implementation reuses the base monad's generalBracket,
-- and simply uses the ReaderT environment to run the relevant
-- acquire, release, cleanup (for exceptions),
-- and use actions. A more complicated example is the
-- implementation for ExceptT, which must implement
-- ExceptT's short-circuit logic itself:
--
--
-- generalBracket acquire release cleanup use = ExceptT $
-- generalBracket
-- (runExceptT acquire)
-- (eresource ->
-- case eresource of
-- Left _ -> return ()
-- Right resource -> runExceptT (release resource) >> return ())
-- (eresource e ->
-- case eresource of
-- Left _ -> return ()
-- Right resource -> runExceptT (cleanup resource e) >> return ())
-- (either (return . Left) (runExceptT . use))
--
--
-- In this implementation, we need to deal with the potential that the
-- acquire action returned a Left (as opposed to
-- succeeding with a Right or throwing an exception via
-- throwM), and therefore have to handle the Left case
-- explicitly when provide release, cleanup, and
-- use actions to the base monad's implementation of
-- generalBracket.
--
-- You should ensure that in all cases of the acquire action
-- completing successfully, either the release or
-- cleanup actions are called, regardless of what occurs in
-- use.
generalBracket :: MonadMask m => m a -> (a -> m ignored1) -> (a -> SomeException -> m ignored2) -> (a -> m b) -> m b
-- | Like mask, but does not pass a restore action to the
-- argument.
mask_ :: MonadMask m => m a -> m a
-- | Like uninterruptibleMask, but does not pass a restore
-- action to the argument.
uninterruptibleMask_ :: MonadMask m => m a -> m a
-- | Catches all exceptions, and somewhat defeats the purpose of the
-- extensible exception system. Use sparingly.
catchAll :: MonadCatch m => m a -> (SomeException -> m a) -> m a
-- | Catch all IOError (eqv. IOException) exceptions. Still
-- somewhat too general, but better than using catchAll. See
-- catchIf for an easy way of catching specific IOErrors
-- based on the predicates in System.IO.Error.
catchIOError :: MonadCatch m => m a -> (IOError -> m a) -> m a
-- | A more generalized way of determining which exceptions to catch at run
-- time.
catchJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> (b -> m a) -> m a
-- | Catch exceptions only if they pass some predicate. Often useful with
-- the predicates for testing IOError values in
-- System.IO.Error.
catchIf :: (MonadCatch m, Exception e) => (e -> Bool) -> m a -> (e -> m a) -> m a
-- | Generalized version of Handler
data Handler m a
Handler :: (e -> m a) -> Handler m a
-- | Catches different sorts of exceptions. See Control.Exception's
-- catches
catches :: (Foldable f, MonadCatch m) => m a -> f (Handler m a) -> m a
-- | Flipped catch. See Control.Exception's handle.
handle :: (MonadCatch m, Exception e) => (e -> m a) -> m a -> m a
-- | Flipped catchAll
handleAll :: MonadCatch m => (SomeException -> m a) -> m a -> m a
-- | Flipped catchIOError
handleIOError :: MonadCatch m => (IOError -> m a) -> m a -> m a
-- | Flipped catchJust. See Control.Exception's
-- handleJust.
handleJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> (b -> m a) -> m a -> m a
-- | Flipped catchIf
handleIf :: (MonadCatch m, Exception e) => (e -> Bool) -> (e -> m a) -> m a -> m a
-- | Similar to catch, but returns an Either result. See
-- Control.Exception's try.
try :: (MonadCatch m, Exception e) => m a -> m (Either e a)
-- | A variant of try that takes an exception predicate to select
-- which exceptions are caught. See Control.Exception's
-- tryJust
tryJust :: (MonadCatch m, Exception e) => (e -> Maybe b) -> m a -> m (Either b a)
-- | Run an action only if an exception is thrown in the main action. The
-- exception is not caught, simply rethrown.
onException :: MonadCatch m => m a -> m b -> m a
-- | Generalized abstracted pattern of safe resource acquisition and
-- release in the face of exceptions. The first action "acquires" some
-- value, which is "released" by the second action at the end. The third
-- action "uses" the value and its result is the result of the
-- bracket.
--
-- If an exception occurs during the use, the release still happens
-- before the exception is rethrown.
--
-- Note that this is essentially a type-specialized version of
-- generalBracket. This function has a more common signature
-- (matching the signature from Control.Exception), and is often
-- more convenient to use. By contrast, generalBracket is more
-- expressive, allowing us to implement other functions like
-- bracketOnError.
bracket :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
-- | Version of bracket without any value being passed to the second
-- and third actions.
bracket_ :: MonadMask m => m a -> m b -> m c -> m c
-- | Perform an action with a finalizer action that is run, even if an
-- exception occurs.
finally :: MonadMask m => m a -> m b -> m a
-- | Like bracket, but only performs the final action if there was
-- an exception raised by the in-between computation.
bracketOnError :: MonadMask m => m a -> (a -> m b) -> (a -> m c) -> m c
-- | 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
--
class (Typeable * e, Show e) => Exception e
toException :: Exception e => e -> SomeException
fromException :: Exception e => SomeException -> Maybe e
-- | Render this exception value in a human-friendly manner.
--
-- Default implementation: show.
displayException :: Exception e => e -> String
-- | 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.
data SomeException :: *
[SomeException] :: SomeException
instance GHC.Base.Monad m => GHC.Base.Functor (Control.Monad.Catch.Handler m)
instance Control.Monad.Catch.MonadMask GHC.Types.IO
instance e ~ GHC.Exception.SomeException => Control.Monad.Catch.MonadMask (Data.Either.Either e)
instance Control.Monad.Catch.MonadMask m => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Identity.IdentityT m)
instance Control.Monad.Catch.MonadMask m => Control.Monad.Catch.MonadMask (Control.Monad.Trans.State.Lazy.StateT s m)
instance Control.Monad.Catch.MonadMask m => Control.Monad.Catch.MonadMask (Control.Monad.Trans.State.Strict.StateT s m)
instance Control.Monad.Catch.MonadMask m => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Reader.ReaderT r m)
instance (Control.Monad.Catch.MonadMask m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Writer.Strict.WriterT w m)
instance (Control.Monad.Catch.MonadMask m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Writer.Lazy.WriterT w m)
instance (Control.Monad.Catch.MonadMask m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadMask (Control.Monad.Trans.RWS.Lazy.RWST r w s m)
instance (Control.Monad.Catch.MonadMask m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadMask (Control.Monad.Trans.RWS.Strict.RWST r w s m)
instance (Control.Monad.Trans.Error.Error e, Control.Monad.Catch.MonadMask m) => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Error.ErrorT e m)
instance Control.Monad.Catch.MonadMask m => Control.Monad.Catch.MonadMask (Control.Monad.Trans.Except.ExceptT e m)
instance Control.Monad.Catch.MonadCatch GHC.Types.IO
instance Control.Monad.Catch.MonadCatch GHC.Conc.Sync.STM
instance e ~ GHC.Exception.SomeException => Control.Monad.Catch.MonadCatch (Data.Either.Either e)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Identity.IdentityT m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.State.Lazy.StateT s m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.State.Strict.StateT s m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Reader.ReaderT r m)
instance (Control.Monad.Catch.MonadCatch m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Writer.Strict.WriterT w m)
instance (Control.Monad.Catch.MonadCatch m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Writer.Lazy.WriterT w m)
instance (Control.Monad.Catch.MonadCatch m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.RWS.Lazy.RWST r w s m)
instance (Control.Monad.Catch.MonadCatch m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.RWS.Strict.RWST r w s m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.List.ListT m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Maybe.MaybeT m)
instance (Control.Monad.Trans.Error.Error e, Control.Monad.Catch.MonadCatch m) => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Error.ErrorT e m)
instance Control.Monad.Catch.MonadCatch m => Control.Monad.Catch.MonadCatch (Control.Monad.Trans.Except.ExceptT e m)
instance Control.Monad.Catch.MonadThrow []
instance Control.Monad.Catch.MonadThrow GHC.Base.Maybe
instance Control.Monad.Catch.MonadThrow Language.Haskell.TH.Syntax.Q
instance Control.Monad.Catch.MonadThrow GHC.Types.IO
instance Control.Monad.Catch.MonadThrow GHC.Conc.Sync.STM
instance e ~ GHC.Exception.SomeException => Control.Monad.Catch.MonadThrow (Data.Either.Either e)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Identity.IdentityT m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.State.Lazy.StateT s m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.State.Strict.StateT s m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Reader.ReaderT r m)
instance (Control.Monad.Catch.MonadThrow m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Writer.Strict.WriterT w m)
instance (Control.Monad.Catch.MonadThrow m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Writer.Lazy.WriterT w m)
instance (Control.Monad.Catch.MonadThrow m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.RWS.Lazy.RWST r w s m)
instance (Control.Monad.Catch.MonadThrow m, GHC.Base.Monoid w) => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.RWS.Strict.RWST r w s m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.List.ListT m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Maybe.MaybeT m)
instance (Control.Monad.Trans.Error.Error e, Control.Monad.Catch.MonadThrow m) => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Error.ErrorT e m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Except.ExceptT e m)
instance Control.Monad.Catch.MonadThrow m => Control.Monad.Catch.MonadThrow (Control.Monad.Trans.Cont.ContT r m)
-- | This module supplies a 'pure' monad transformer that can be used for
-- mock-testing code that throws exceptions, so long as those exceptions
-- are always thrown with throwM.
--
-- Do not mix CatchT with IO. Choose one or the other for
-- the bottom of your transformer stack!
module Control.Monad.Catch.Pure
-- | Add Exception handling abilities to a Monad.
--
-- This should never be used in combination with IO. Think
-- of CatchT as an alternative base monad for use with mocking
-- code that solely throws exceptions via throwM.
--
-- Note: that IO monad has these abilities already, so stacking
-- CatchT on top of it does not add any value and can possibly be
-- confusing:
--
--
-- >>> (error "Hello!" :: IO ()) `catch` (\(e :: ErrorCall) -> liftIO $ print e)
-- Hello!
--
--
--
-- >>> runCatchT $ (error "Hello!" :: CatchT IO ()) `catch` (\(e :: ErrorCall) -> liftIO $ print e)
-- *** Exception: Hello!
--
--
--
-- >>> runCatchT $ (throwM (ErrorCall "Hello!") :: CatchT IO ()) `catch` (\(e :: ErrorCall) -> liftIO $ print e)
-- Hello!
--
newtype CatchT m a
CatchT :: m (Either SomeException a) -> CatchT m a
[runCatchT] :: CatchT m a -> m (Either SomeException a)
type Catch = CatchT Identity
runCatch :: Catch a -> Either SomeException a
-- | Map the unwrapped computation using the given function.
--
--
-- runCatchT (mapCatchT f m) = f (runCatchT m)
--
mapCatchT :: (m (Either SomeException a) -> n (Either SomeException b)) -> CatchT m a -> CatchT n b
instance GHC.Base.Monad m => GHC.Base.Functor (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => GHC.Base.Applicative (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => GHC.Base.Monad (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.Fix.MonadFix m => Control.Monad.Fix.MonadFix (Control.Monad.Catch.Pure.CatchT m)
instance Data.Foldable.Foldable m => Data.Foldable.Foldable (Control.Monad.Catch.Pure.CatchT m)
instance (GHC.Base.Monad m, Data.Traversable.Traversable m) => Data.Traversable.Traversable (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => GHC.Base.Alternative (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => GHC.Base.MonadPlus (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.Trans.Class.MonadTrans Control.Monad.Catch.Pure.CatchT
instance Control.Monad.IO.Class.MonadIO m => Control.Monad.IO.Class.MonadIO (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => Control.Monad.Catch.MonadThrow (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => Control.Monad.Catch.MonadCatch (Control.Monad.Catch.Pure.CatchT m)
instance GHC.Base.Monad m => Control.Monad.Catch.MonadMask (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.State.Class.MonadState s m => Control.Monad.State.Class.MonadState s (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.Reader.Class.MonadReader e m => Control.Monad.Reader.Class.MonadReader e (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.Writer.Class.MonadWriter w m => Control.Monad.Writer.Class.MonadWriter w (Control.Monad.Catch.Pure.CatchT m)
instance Control.Monad.RWS.Class.MonadRWS r w s m => Control.Monad.RWS.Class.MonadRWS r w s (Control.Monad.Catch.Pure.CatchT m)