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|
-----------------------------------------------------------------------------
--
-- Module : Control.Exception
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/core/LICENSE)
--
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable
--
-- $Id: Exception.hs,v 1.1 2001/06/28 14:15:01 simonmar Exp $
--
-- The External API for exceptions. The functions provided in this
-- module allow catching of exceptions in the IO monad.
--
-----------------------------------------------------------------------------
module Control.Exception (
Exception(..), -- instance Eq, Ord, Show, Typeable
IOException, -- instance Eq, Ord, Show, Typeable
ArithException(..), -- instance Eq, Ord, Show, Typeable
ArrayException(..), -- instance Eq, Ord, Show, Typeable
AsyncException(..), -- instance Eq, Ord, Show, Typeable
try, -- :: IO a -> IO (Either Exception a)
tryJust, -- :: (Exception -> Maybe b) -> a -> IO (Either b a)
catch, -- :: IO a -> (Exception -> IO a) -> IO a
catchJust, -- :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
evaluate, -- :: a -> IO a
-- Exception predicates (for catchJust, tryJust)
ioErrors, -- :: Exception -> Maybe IOError
arithExceptions, -- :: Exception -> Maybe ArithException
errorCalls, -- :: Exception -> Maybe String
dynExceptions, -- :: Exception -> Maybe Dynamic
assertions, -- :: Exception -> Maybe String
asyncExceptions, -- :: Exception -> Maybe AsyncException
userErrors, -- :: Exception -> Maybe String
-- Throwing exceptions
throw, -- :: Exception -> a
#ifndef __STGHUGS__
-- for now
throwTo, -- :: ThreadId -> Exception -> a
#endif
-- Dynamic exceptions
throwDyn, -- :: Typeable ex => ex -> b
throwDynTo, -- :: Typeable ex => ThreadId -> ex -> b
catchDyn, -- :: Typeable ex => IO a -> (ex -> IO a) -> IO a
-- Async exception control
block, -- :: IO a -> IO a
unblock, -- :: IO a -> IO a
-- Assertions
-- for now
assert, -- :: Bool -> a -> a
-- Utilities
finally, -- :: IO a -> IO b -> IO b
bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO ()
bracket_, -- :: IO a -> IO b -> IO c -> IO ()
) where
#ifdef __GLASGOW_HASKELL__
import Prelude hiding (catch)
import GHC.Prim ( assert )
import GHC.Exception hiding (try, catch, bracket, bracket_)
import GHC.Conc ( throwTo, ThreadId )
import GHC.IOBase ( IO(..) )
#endif
#ifdef __HUGS__
import Prelude hiding ( catch )
import PrelPrim ( catchException
, Exception(..)
, throw
, ArithException(..)
, AsyncException(..)
, assert
)
#endif
import Data.Dynamic
#include "Dynamic.h"
INSTANCE_TYPEABLE0(Exception,exceptionTc,"Exception")
INSTANCE_TYPEABLE0(IOException,ioExceptionTc,"IOException")
INSTANCE_TYPEABLE0(ArithException,arithExceptionTc,"ArithException")
INSTANCE_TYPEABLE0(ArrayException,arrayExceptionTc,"ArrayException")
INSTANCE_TYPEABLE0(AsyncException,asyncExceptionTc,"AsyncException")
-----------------------------------------------------------------------------
-- Catching exceptions
-- PrelException defines 'catchException' for us.
catch :: IO a -> (Exception -> IO a) -> IO a
catch = catchException
catchJust :: (Exception -> Maybe b) -> IO a -> (b -> IO a) -> IO a
catchJust p a handler = catch a handler'
where handler' e = case p e of
Nothing -> throw e
Just b -> handler b
-----------------------------------------------------------------------------
-- evaluate
evaluate :: a -> IO a
evaluate a = a `seq` return a
-----------------------------------------------------------------------------
-- 'try' and variations.
try :: IO a -> IO (Either Exception a)
try a = catch (a >>= \ v -> return (Right v)) (\e -> return (Left e))
tryJust :: (Exception -> 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)
-----------------------------------------------------------------------------
-- Dynamic exception types. Since one of the possible kinds of exception
-- is a dynamically typed value, we can effectively have polymorphic
-- exceptions.
-- throwDyn will raise any value as an exception, provided it is in the
-- Typeable class (see Dynamic.lhs).
-- catchDyn will catch any exception of a given type (determined by the
-- handler function). Any raised exceptions that don't match are
-- re-raised.
throwDyn :: Typeable exception => exception -> b
throwDyn exception = throw (DynException (toDyn exception))
throwDynTo :: Typeable exception => ThreadId -> exception -> IO ()
throwDynTo t exception = throwTo t (DynException (toDyn exception))
catchDyn :: Typeable exception => IO a -> (exception -> IO a) -> IO a
catchDyn m k = catchException m handle
where handle ex = case ex of
(DynException dyn) ->
case fromDynamic dyn of
Just exception -> k exception
Nothing -> throw ex
_ -> throw ex
-----------------------------------------------------------------------------
-- Exception Predicates
ioErrors :: Exception -> Maybe IOError
arithExceptions :: Exception -> Maybe ArithException
errorCalls :: Exception -> Maybe String
dynExceptions :: Exception -> Maybe Dynamic
assertions :: Exception -> Maybe String
asyncExceptions :: Exception -> Maybe AsyncException
userErrors :: Exception -> Maybe String
ioErrors e@(IOException _) = Just e
ioErrors _ = Nothing
arithExceptions (ArithException e) = Just e
arithExceptions _ = Nothing
errorCalls (ErrorCall e) = Just e
errorCalls _ = Nothing
assertions (AssertionFailed e) = Just e
assertions _ = Nothing
dynExceptions (DynException e) = Just e
dynExceptions _ = Nothing
asyncExceptions (AsyncException e) = Just e
asyncExceptions _ = Nothing
userErrors (UserError e) = Just e
userErrors _ = Nothing
-----------------------------------------------------------------------------
-- Some Useful Functions
bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
bracket before after thing =
block (do
a <- before
r <- catch
(unblock (thing a))
(\e -> do { after a; throw e })
after a
return r
)
-- finally is an instance of bracket, but it's quite common
-- so we give the specialised version for efficiency.
finally :: IO a -> IO b -> IO a
a `finally` sequel =
block (do
r <- catch
(unblock a)
(\e -> do { sequel; throw e })
sequel
return r
)
bracket_ :: IO a -> IO b -> IO c -> IO c
bracket_ before after thing = bracket before (const after) (const thing)
|
