    -------------------------
    -- |
    -- Module      :  Control.Exception
    -- Copyright   :  (c) The University of Glasgow 2001
    -- License     :  BSD-style (see the file LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (uses ExistentialQuantification and DeriveDataTypeable)
    --
    -- This module provides the extensible exceptions API for raising and catching both
    -- built-in and user-defined exceptions.
    --
    -- For newer versions of GHC (>=6.9), this package re-exports 'Control.Exception'.
    -- Otherwise, it provides a compatibility layer around the previous version of the
    -- extensions API.

    #ifdef USE_NEW_EXCEPTIONS
    module Control.Exception.Extensible (module Control.Exception) where

    import Control.Exception hiding (blocked)

    #else
    module Control.Exception.Extensible (
        -- * The Exception type
        SomeException(..),
        Exception(..),
        E.IOException,
        E.ArithException(..),
        E.ArrayException(..),
        AssertionFailed(..),
        E.AsyncException(..),
        NonTermination(..),
        NestedAtomically(..),
        ExitCode(..),
        BlockedOnDeadMVar(..),
        BlockedIndefinitely(..),
        Deadlock(..),
        NoMethodError(..),
        PatternMatchFail(..),
        RecConError(..),
        RecSelError(..),
        RecUpdError(..),
        ErrorCall(..),

        -- * Throwing exceptions
        throwIO,
        throw,
        ioError,
        throwTo,
        -- * Catching Exceptions

        -- |There are several functions for catching and examining
        -- exceptions; all of them may only be used from within the
        -- 'IO' monad.

        -- ** The @catch@ functions
        catch,     -- :: IO a -> (Exception -> IO a) -> IO a
        catches, Handler(..),
        catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a

        -- ** The @handle@ functions
        handle,    -- :: (Exception -> IO a) -> IO a -> IO a
        handleJust,-- :: (Exception -> Maybe b) -> (b -> IO a) -> IO a -> IO a

        -- ** The @try@ functions
        try,       -- :: IO a -> IO (Either Exception a)
        tryJust,   -- :: (Exception -> Maybe b) -> a    -> IO (Either b a)
        onException,

        -- ** The @evaluate@ function
        E.evaluate,  -- :: a -> IO a

        -- ** The @mapException@ function
        mapException,           -- :: (Exception -> Exception) -> a -> a

        -- * Asynchronous Exceptions

        -- $async

        -- ** Asynchronous exception control

        -- |The following two functions allow a thread to control delivery of
        -- asynchronous exceptions during a critical region.

        E.block,
        E.unblock,

        -- *** Applying @block@ to an exception handler

        -- $block_handler

        -- *** Interruptible operations

        -- $interruptible

        -- * Assertions

        assert,

        -- * Utilities
        bracket,
        bracket_,
        bracketOnError,
        finally
        ) where

    import Prelude hiding (catch)
    import Control.Concurrent hiding (throwTo)
    import qualified Control.Exception as E
    import Data.Dynamic
    import Data.Typeable
    import System.Exit
    import System.IO.Unsafe(unsafePerformIO)

    class (Typeable e, Show e) => Exception e where
        toException   :: e -> SomeException
        fromException :: SomeException -> Maybe e

        toException = SomeException
        fromException (SomeException e) = cast e

    data SomeException = forall e . Exception e => SomeException e
        deriving Typeable

    instance Show SomeException where
        showsPrec p (SomeException e) = showsPrec p e

    instance Exception SomeException where
        toException se = se
        fromException = Just

    mkOldException :: Exception e => e -> E.Exception
    mkOldException e = let e' = toException e
              in case fromException e' of
                 Just e'' -> -- If the exception is actually a legacy exception
                             -- then throw it directly so the legacy functions
                             -- catch it as they expect
                             e''
                 Nothing -> -- Otherwise, throw it as a dynamic
                            E.DynException (toDyn e')

    throw :: Exception e => e -> a
    throw e = E.throw (mkOldException e)

    throwIO :: Exception e => e -> IO a
    throwIO e = E.throwIO (mkOldException e)

    throwTo :: Exception e => ThreadId -> e -> IO ()
    throwTo tid e = E.throwTo tid (mkOldException e)

    -----------------------------------------------------------------------------
    -- Catching exceptions

    -- |This is the simplest of the exception-catching functions.  It
    -- takes a single argument, runs it, and if an exception is raised
    -- the \"handler\" is executed, with the value of the exception passed as an
    -- argument.  Otherwise, the result is returned as normal.  For example:
    --
    -- >   catch (openFile f ReadMode)
    -- >       (\e -> hPutStr stderr ("Couldn't open "++f++": " ++ show e))
    --
    -- For catching exceptions in pure (non-'IO') expressions, see the
    -- function 'evaluate'.
    --
    -- Note that due to Haskell\'s unspecified evaluation order, an
    -- expression may return one of several possible exceptions: consider
    -- the expression @error \"urk\" + 1 \`div\` 0@.  Does
    -- 'catch' execute the handler passing
    -- @ErrorCall \"urk\"@, or @ArithError DivideByZero@?
    --
    -- The answer is \"either\": 'catch' makes a
    -- non-deterministic choice about which exception to catch.  If you
    -- call it again, you might get a different exception back.  This is
    -- ok, because 'catch' is an 'IO' computation.
    --
    -- Note that 'catch' catches all types of exceptions, and is generally
    -- used for \"cleaning up\" before passing on the exception using
    -- 'throwIO'.  It is not good practice to discard the exception and
    -- continue, without first checking the type of the exception (it
    -- might be a 'ThreadKilled', for example).  In this case it is usually better
    -- to use 'catchJust' and select the kinds of exceptions to catch.
    --
    -- Also note that the "Prelude" also exports a function called
    -- 'Prelude.catch' with a similar type to 'Control.Exception.catch',
    -- except that the "Prelude" version only catches the IO and user
    -- families of exceptions (as required by Haskell 98).
    --
    -- We recommend either hiding the "Prelude" version of 'Prelude.catch'
    -- when importing "Control.Exception":
    --
    -- > import Prelude hiding (catch)
    --
    -- or importing "Control.Exception" qualified, to avoid name-clashes:
    --
    -- > import qualified Control.Exception as C
    --
    -- and then using @C.catch@
    --
    catch   :: Exception e
            => IO a         -- ^ The computation to run
            -> (e -> IO a)  -- ^ Handler to invoke if an exception is raised
            -> IO a
    catch io poly_handler = io `E.catch` handler'
        where
              -- First look for "new style" exceptions, which are thrown
              -- as E.DynException (SomeException e)

              -- needs scoped TVs: handler' :: E.Exception -> IO a
              handler' e = case e of
                               E.DynException dyn ->
                                   case fromDynamic dyn of
                                   Just se@(SomeException _) ->
                                       case fromException se of
                                         Just e' -> poly_handler e'
                                         Nothing -> E.throw e
                                   Nothing -> try_old e
                               _ -> try_old e

              -- If it's a legacy exception (E.Exception or one of the
              -- types that make up E.Exception), check for a handler than
              -- can handle them:

              -- needs scoped TVs: try_old :: E.Exception -> IO a
              try_old e = case fromException (toException e) of
                           Just e' -> poly_handler e'
                           Nothing -> E.throw e

    -- | 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).
    --
    -- A common example is opening a file:
    --
    -- > bracket
    -- >   (openFile "filename" ReadMode)
    -- >   (hClose)
    -- >   (\handle -> do { ... })
    --
    -- The arguments to 'bracket' are in this order so that we can partially apply
    -- it, e.g.:
    --
    -- > withFile name mode = bracket (openFile name mode) hClose
    --
    bracket
            :: IO a         -- ^ computation to run first (\"acquire resource\")
            -> (a -> IO b)  -- ^ computation to run last (\"release resource\")
            -> (a -> IO c)  -- ^ computation to run in-between
            -> IO c         -- returns the value from the in-between computation
    bracket before after thing =
      E.block (do
        a <- before
        r <- E.unblock (thing a) `onException` after a
        after a
        return r
     )

    onException :: IO a -> IO b -> IO a
    onException io what = io `catch` \e -> do what
                                              throw (e :: SomeException)

    block, unblock :: IO a -> IO a
    block = E.block
    unblock = E.unblock

    -- | A specialised variant of 'bracket' with just a computation to run
    -- afterward.
    --
    finally :: IO a         -- ^ computation to run first
            -> IO b         -- ^ computation to run afterward (even if an exception
                            -- was raised)
            -> IO a         -- returns the value from the first computation
    a `finally` sequel =
      E.block (do
        r <- E.unblock a `onException` sequel
        sequel
        return r
      )

    -- | A variant of 'bracket' where the return value from the first computation
    -- is not required.
    bracket_ :: IO a -> IO b -> IO c -> IO c
    bracket_ before after thing = bracket before (const after) (const thing)

    -- | Like bracket, but only performs the final action if there was an
    -- exception raised by the in-between computation.
    bracketOnError
            :: IO a         -- ^ computation to run first (\"acquire resource\")
            -> (a -> IO b)  -- ^ computation to run last (\"release resource\")
            -> (a -> IO c)  -- ^ computation to run in-between
            -> IO c         -- returns the value from the in-between computation
    bracketOnError before after thing =
      block (do
        a <- before
        unblock (thing a) `onException` after a
      )

    assert :: Bool -> a -> a
    assert True x = x
    assert False _ = throw (AssertionFailed "")

    -- | The function 'catchJust' is like 'catch', but it takes an extra
    -- argument which is an /exception predicate/, a function which
    -- selects which type of exceptions we\'re interested in.
    --
    -- >   result <- catchJust errorCalls thing_to_try handler
    --
    -- Any other exceptions which are not matched by the predicate
    -- are re-raised, and may be caught by an enclosing
    -- 'catch' or 'catchJust'.
    catchJust
            :: Exception e
            => (e -> Maybe b)         -- ^ Predicate to select exceptions
            -> IO a                   -- ^ Computation to run
            -> (b -> IO a)            -- ^ Handler
            -> IO a
    catchJust p a handler = catch a handler'
      where handler' e = case p e of
                            Nothing -> throw e
                            Just b  -> handler b

    -- | A version of 'catch' with the arguments swapped around; useful in
    -- situations where the code for the handler is shorter.  For example:
    --
    -- >   do handle (\e -> exitWith (ExitFailure 1)) $
    -- >      ...
    handle     :: Exception e => (e -> IO a) -> IO a -> IO a
    handle     =  flip catch

    -- | A version of 'catchJust' with the arguments swapped around (see
    -- 'handle').
    handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
    handleJust p =  flip (catchJust p)

    -----------------------------------------------------------------------------
    -- 'mapException'

    -- | This function maps one exception into another as proposed in the
    -- paper \"A semantics for imprecise exceptions\".

    -- Notice that the usage of 'unsafePerformIO' is safe here.

    mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
    mapException f v = unsafePerformIO (catch (E.evaluate v)
                                              (\x -> throw (f x)))

    -----------------------------------------------------------------------------
    -- 'try' and variations.

    -- | Similar to 'catch', but returns an 'Either' result which is
    -- @('Right' a)@ if no exception was raised, or @('Left' e)@ if an
    -- exception was raised and its value is @e@.
    --
    -- >  try a = catch (Right `liftM` a) (return . Left)
    --
    -- Note: as with 'catch', it is only polite to use this variant if you intend
    -- to re-throw the exception after performing whatever cleanup is needed.
    -- Otherwise, 'tryJust' is generally considered to be better.
    --
    -- Also note that "System.IO.Error" also exports a function called
    -- 'System.IO.Error.try' with a similar type to 'Control.Exception.try',
    -- except that it catches only the IO and user families of exceptions
    -- (as required by the Haskell 98 @IO@ module).

    try :: Exception e => IO a -> IO (Either e a)
    try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))

    -- | A variant of 'try' that takes an exception predicate to select
    -- which exceptions are caught (c.f. 'catchJust').  If the exception
    -- does not match the predicate, it is re-thrown.
    tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
    tryJust p a = do
      r <- try a
      case r of
            Right v -> return (Right v)
            Left  e -> case p e of
                            Nothing -> throw e
                            Just b  -> return (Left b)

    -------------

    data Handler a = forall e . Exception e => Handler (e -> IO a)

    catches :: IO a -> [Handler a] -> IO a
    catches io handlers = io `catch` catchesHandler handlers

    catchesHandler :: [Handler a] -> SomeException -> IO a
    catchesHandler handlers e = foldr tryHandler (throw e) handlers
        where tryHandler (Handler handler) res
                  = case fromException e of
                    Just e' -> handler e'
                    Nothing -> res

    -- -----------------------------------------------------------------------------
    -- Asynchronous exceptions

    {- $async

     #AsynchronousExceptions# Asynchronous exceptions are so-called because they arise due to
    external influences, and can be raised at any point during execution.
    'StackOverflow' and 'HeapOverflow' are two examples of
    system-generated asynchronous exceptions.

    The primary source of asynchronous exceptions, however, is
    'throwTo':

    >  throwTo :: ThreadId -> Exception -> IO ()

    'throwTo' (also 'throwDynTo' and 'Control.Concurrent.killThread') allows one
    running thread to raise an arbitrary exception in another thread.  The
    exception is therefore asynchronous with respect to the target thread,
    which could be doing anything at the time it receives the exception.
    Great care should be taken with asynchronous exceptions; it is all too
    easy to introduce race conditions by the over zealous use of
    'throwTo'.
    -}

    {- $block_handler
    There\'s an implied 'block' around every exception handler in a call
    to one of the 'catch' family of functions.  This is because that is
    what you want most of the time - it eliminates a common race condition
    in starting an exception handler, because there may be no exception
    handler on the stack to handle another exception if one arrives
    immediately.  If asynchronous exceptions are blocked on entering the
    handler, though, we have time to install a new exception handler
    before being interrupted.  If this weren\'t the default, one would have
    to write something like

    >      block (
    >           catch (unblock (...))
    >                      (\e -> handler)
    >      )

    If you need to unblock asynchronous exceptions again in the exception
    handler, just use 'unblock' as normal.

    Note that 'try' and friends /do not/ have a similar default, because
    there is no exception handler in this case.  If you want to use 'try'
    in an asynchronous-exception-safe way, you will need to use
    'block'.
    -}

    {- $interruptible

    Some operations are /interruptible/, which means that they can receive
    asynchronous exceptions even in the scope of a 'block'.  Any function
    which may itself block is defined as interruptible; this includes
    'Control.Concurrent.MVar.takeMVar'
    (but not 'Control.Concurrent.MVar.tryTakeMVar'),
    and most operations which perform
    some I\/O with the outside world.  The reason for having
    interruptible operations is so that we can write things like

    >      block (
    >         a <- takeMVar m
    >         catch (unblock (...))
    >               (\e -> ...)
    >      )

    if the 'Control.Concurrent.MVar.takeMVar' was not interruptible,
    then this particular
    combination could lead to deadlock, because the thread itself would be
    blocked in a state where it can\'t receive any asynchronous exceptions.
    With 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
    safe in the knowledge that the thread can receive exceptions right up
    until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
    Similar arguments apply for other interruptible operations like
    'System.IO.openFile'.
    -}

    ----------------------------------------------------------------------
    -- Exception instance for the legacy Exception type

    instance Exception E.Exception

    ----------------------------------------------------------------------
    -- The new Exception types. These need to map to/from E.Exception so
    -- that uses of legacy catch/throw functions work.

    ----

    instance Exception E.ArithException where
        toException ae = toException (E.ArithException ae)
        fromException (SomeException e) = case cast e of
                                          Just (E.ArithException ae) ->
                                             Just ae
                                          _ -> Nothing
    ----

    instance Exception E.ArrayException where
        toException ae = toException (E.ArrayException ae)
        fromException (SomeException e) = case cast e of
                                          Just (E.ArrayException ae) ->
                                              Just ae
                                          _ -> Nothing

    ----

    data AssertionFailed = AssertionFailed String
        deriving Typeable

    instance Exception AssertionFailed where
        toException (AssertionFailed str) = toException (E.AssertionFailed str)
        fromException (SomeException e) = case cast e of
                                          Just (E.AssertionFailed str) ->
                                              Just (AssertionFailed str)
                                          _ -> Nothing

    instance Show AssertionFailed where
        showsPrec _ (AssertionFailed err) = showString err

    -----

    instance Exception E.AsyncException where
        toException ae = toException (E.AsyncException ae)
        fromException (SomeException e) = case cast e of
                                          Just (E.AsyncException ae) ->
                                              Just ae
                                          _ -> Nothing

    ----

    data BlockedOnDeadMVar = BlockedOnDeadMVar
        deriving Typeable

    instance Exception BlockedOnDeadMVar where
        toException BlockedOnDeadMVar = toException (E.BlockedOnDeadMVar)
        fromException (SomeException e) = case cast e of
                                          Just E.BlockedOnDeadMVar ->
                                              Just BlockedOnDeadMVar
                                          _ -> Nothing
    instance Show BlockedOnDeadMVar where
        showsPrec n BlockedOnDeadMVar = showsPrec n E.BlockedOnDeadMVar

    ----

    data BlockedIndefinitely = BlockedIndefinitely
        deriving Typeable

    instance Exception BlockedIndefinitely where
        toException BlockedIndefinitely = toException E.BlockedIndefinitely
        fromException (SomeException e) = case cast e of
                                          Just E.BlockedIndefinitely ->
                                              Just BlockedIndefinitely
                                          _ -> Nothing

    instance Show BlockedIndefinitely where
        showsPrec n BlockedIndefinitely = showsPrec n E.BlockedIndefinitely

    ----

    data NestedAtomically = NestedAtomically
        deriving Typeable

    instance Exception NestedAtomically where
        toException NestedAtomically = toException E.NestedAtomically
        fromException (SomeException e) = case cast e of
                                        Just E.NestedAtomically ->
                                            Just NestedAtomically
                                        _ -> Nothing

    instance Show NestedAtomically where
        showsPrec n NestedAtomically = showsPrec n E.NestedAtomically

    ----

    data Deadlock = Deadlock
        deriving Typeable

    instance Exception Deadlock where
        toException Deadlock = toException E.Deadlock
        fromException (SomeException e) = case cast e of
                                          Just E.Deadlock ->
                                              Just Deadlock
                                          _ -> Nothing

    instance Show Deadlock where
        showsPrec n Deadlock = showsPrec n E.Deadlock

    -----

    data ErrorCall = ErrorCall String
        deriving Typeable

    instance Exception ErrorCall where
        toException (ErrorCall str) = toException (E.ErrorCall str)
        fromException (SomeException e) = case cast e of
                                          Just (E.ErrorCall str) ->
                                              Just (ErrorCall str)
                                          _ -> Nothing

    instance Show ErrorCall where
        showsPrec _ (ErrorCall err) = showString err

    -----

    instance Typeable ExitCode where
        typeOf _ = mkTyConApp (mkTyCon "ExitCode") []

    instance Exception ExitCode where
        toException ee = toException (E.ExitException ee)
        fromException (SomeException e) = case cast e of
                                          Just (E.ExitException ee) ->
                                              Just ee
                                          _ -> Nothing
    -----

    instance Exception E.IOException where
        toException ioe = toException (E.IOException ioe)
        fromException (SomeException e) = case cast e of
                                          Just (E.IOException ioe) ->
                                              Just ioe
                                          _ -> Nothing

    ----

    data NoMethodError = NoMethodError String
        deriving Typeable

    instance Exception NoMethodError where
        toException (NoMethodError str) = toException (E.NoMethodError str)
        fromException (SomeException e) = case cast e of
                                          Just (E.NoMethodError str) ->
                                              Just (NoMethodError str)
                                          _ -> Nothing

    instance Show NoMethodError where
        showsPrec _ (NoMethodError str) = showString str

    ----

    data NonTermination = NonTermination
        deriving Typeable

    instance Exception NonTermination where
        toException NonTermination = toException E.NonTermination
        fromException (SomeException e) = case cast e of
                                          Just E.NonTermination ->
                                              Just NonTermination
                                          _ -> Nothing

    instance Show NonTermination where
        showsPrec n NonTermination = showsPrec n E.NonTermination

    ----

    data PatternMatchFail = PatternMatchFail String
        deriving Typeable

    instance Exception PatternMatchFail where
        toException (PatternMatchFail str) = toException (E.PatternMatchFail str)
        fromException (SomeException e) = case cast e of
                                          Just (E.PatternMatchFail str) ->
                                              Just (PatternMatchFail str)
                                          _ -> Nothing

    instance Show PatternMatchFail where
        showsPrec _ (PatternMatchFail str) = showString str

    ----

    data RecConError = RecConError String
        deriving Typeable

    instance Exception RecConError where
        toException (RecConError str) = toException (E.RecConError str)
        fromException (SomeException e) = case cast e of
                                          Just (E.RecConError str) ->
                                              Just (RecConError str)
                                          _ -> Nothing

    instance Show RecConError where
        showsPrec _ (RecConError str) = showString str

    ----

    data RecSelError = RecSelError String
        deriving Typeable

    instance Exception RecSelError where
        toException (RecSelError str) = toException (E.RecSelError str)
        fromException (SomeException e) = case cast e of
                                          Just (E.RecSelError str) ->
                                              Just (RecSelError str)
                                          _ -> Nothing

    instance Show RecSelError where
        showsPrec _ (RecSelError str) = showString str

    ----

    data RecUpdError = RecUpdError String
        deriving Typeable

    instance Exception RecUpdError where
        toException (RecUpdError str) = toException (E.RecUpdError str)
        fromException (SomeException e) = case cast e of
                                          Just (E.RecUpdError str) ->
                                              Just (RecUpdError str)
                                          _ -> Nothing

    instance Show RecUpdError where
        showsPrec _ (RecUpdError str) = showString str

    #endif