Changes to the exception interface, as discussed on the libraries list.
1) Move bracket and bracket_ from GHC.Exception (and hence System.IO)
to haskell98/IO.hs. These two should now never be used (except in
all-H98 programs), and this will save users of the new libraries from
having to hide them. Use the ones in Control.Exception instead.
2) Define
type IOError = IOException -- was Exception
leaving the type of Prelude.ioError as IOError -> IO a,
but adding to Control.Exception
throwIO :: Exception -> IO a
The result is a type distinction between the variants of catch and try:
Prelude.catch :: IO a -> (IOError -> IO a) -> IO a
Control.Exception.catch :: IO a -> (Exception -> IO a) -> IO a
System.IO.Error.try :: IO a -> IO (Either IOError a)
Control.Exception.try :: IO a -> IO (Either Exception a)
These are breaking changes: the first one affects only import lists,
but the second will bite in the following situations:
- using ioError on general Exceptions: use throwIO instead.
- using throw on IOErrors: if in the IO monad, use ioError instead.
Otherwise, use throw (IOException e), but why are you throwing
IO exceptions outside of the IO monad?
Minor changes:
- System.IO.Error now exports catch and try
- moved try from GHC.Exception to System.IO.Error, because it's
portable and can be shared by Hugs.
#endif
-- * Throwing exceptions
-#ifdef __HUGS__
throwIO, -- :: Exception -> IO a
-#else
+#ifndef __HUGS__
throw, -- :: Exception -> a
- ioError, -- :: Exception -> IO a
+#endif
+ ioError, -- :: IOError -> IO a
+#ifndef __HUGS__
throwTo, -- :: ThreadId -> Exception -> a
#endif
#ifdef __GLASGOW_HASKELL__
import GHC.Base ( assert )
-import GHC.Exception as ExceptionBase hiding (try, catch, bracket, bracket_)
+import GHC.Exception as ExceptionBase hiding (catch)
import GHC.Conc ( throwTo, ThreadId )
import GHC.IOBase ( IO(..) )
#endif
#endif
import Prelude hiding ( catch )
-import System.IO.Error
+import System.IO.Error hiding ( catch, try )
import System.IO.Unsafe (unsafePerformIO)
import Data.Dynamic
--
-- Note that 'catch' catches all types of exceptions, and is generally
-- used for \"cleaning up\" before passing on the exception using
--- 'ioError'. It is not good practice to discard the exception and
+-- '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.
#endif /* __GLASGOW_HASKELL__ */
#ifdef __GLASGOW_HASKELL__
-ioErrors e@(IOException _) = Just e
+ioErrors (IOException e) = Just e
ioErrors _ = Nothing
arithExceptions (ArithException e) = Just e
asyncExceptions (AsyncException e) = Just e
asyncExceptions _ = Nothing
-userErrors e@IOException{} | isUserError e = Just (ioeGetErrorString e)
+userErrors (IOException e) | isUserError e = Just (ioeGetErrorString e)
userErrors _ = Nothing
#endif /* __GLASGOW_HASKELL__ */
-- GHC allows us to get at the guts inside IO errors/exceptions
--
#if __GLASGOW_HASKELL__
-import GHC.IOBase (Exception(..), IOException(..), IOErrorType(..))
+import GHC.IOBase (IOException(..), IOErrorType(..))
#endif /* __GLASGOW_HASKELL__ */
errnoToIOError loc errno maybeHdl maybeName = unsafePerformIO $ do
str <- strerror errno >>= peekCString
#if __GLASGOW_HASKELL__
- return (IOException (IOError maybeHdl errType loc str maybeName))
+ return (IOError maybeHdl errType loc str maybeName)
where
errType
| errno == eOK = OtherError
import Foreign.Storable ( Storable(sizeOf) )
#ifdef __GLASGOW_HASKELL__
-import GHC.Exception ( bracket )
import GHC.IOBase
import GHC.Real
import GHC.Ptr
( module GHC.Exception,
Exception(..), AsyncException(..),
IOException(..), ArithException(..), ArrayException(..),
- throw, ioError )
+ throw, throwIO, ioError )
where
-import Data.Either
-
import GHC.Base
import GHC.IOBase
catchException (IO m) k = IO $ \s -> catch# m (\ex -> unIO (k ex)) s
#endif
-catch :: IO a -> (Exception -> IO a) -> IO a
+catch :: IO a -> (IOError -> IO a) -> IO a
catch m k = catchException m handler
- where handler err@(IOException _) = k err
+ where handler (IOException err) = k err
handler other = throw other
\end{code}
%*********************************************************
%* *
-\subsection{Try and bracket}
-%* *
-%*********************************************************
-
-The construct @try comp@ exposes errors which occur within a
-computation, and which are not fully handled. It always succeeds.
-
-These are the IO-only try/bracket. For the full exception try/bracket
-see hslibs/lang/Exception.lhs.
-
-\begin{code}
-try :: IO a -> IO (Either Exception a)
-try f = catch (do r <- f
- return (Right r))
- (return . Left)
-
-bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
-bracket before after m = do
- x <- before
- rs <- try (m x)
- after x
- case rs of
- Right r -> return r
- Left e -> ioError e
-
--- variant of the above where middle computation doesn't want x
-bracket_ :: IO a -> (a -> IO b) -> IO c -> IO c
-bracket_ before after m = do
- x <- before
- rs <- try m
- after x
- case rs of
- Right r -> return r
- Left e -> ioError e
-\end{code}
-
-
-%*********************************************************
-%* *
\subsection{Controlling asynchronous exception delivery}
%* *
%*********************************************************
block $ do
h_ <- takeMVar m
checkBufferInvariants h_
- (h',v) <- catchException (act h_)
- (\ ex -> putMVar m h_ >> throw (augmentIOError ex fun h h_))
+ (h',v) <- catch (act h_)
+ (\ ex -> putMVar m h_ >> ioError (augmentIOError ex fun h h_))
checkBufferInvariants h'
putMVar m h'
return v
block $ do
h_ <- takeMVar m
checkBufferInvariants h_
- v <- catchException (act h_)
- (\ ex -> putMVar m h_ >> throw (augmentIOError ex fun h h_))
+ v <- catch (act h_)
+ (\ ex -> putMVar m h_ >> ioError (augmentIOError ex fun h h_))
checkBufferInvariants h_
putMVar m h_
return v
block $ do
h_ <- takeMVar m
checkBufferInvariants h_
- h' <- catchException (act h_)
- (\ ex -> putMVar m h_ >> throw (augmentIOError ex fun h h_))
+ h' <- catch (act h_)
+ (\ ex -> putMVar m h_ >> ioError (augmentIOError ex fun h h_))
checkBufferInvariants h'
putMVar m h'
return ()
-augmentIOError (IOException (IOError _ iot _ str fp)) fun h h_
- = IOException (IOError (Just h) iot fun str filepath)
+augmentIOError (IOError _ iot _ str fp) fun h h_
+ = IOError (Just h) iot fun str filepath
where filepath | Just _ <- fp = fp
| otherwise = Just (haFilePath h_)
-augmentIOError other_exception _ _ _
- = other_exception
-- ---------------------------------------------------------------------------
-- Wrapper for write operations.
| TextMode IOMode
deriving (Eq, Read, Show)
-addFilePathToIOError fun fp (IOException (IOError h iot _ str _))
- = IOException (IOError h iot fun str (Just fp))
-addFilePathToIOError _ _ other_exception
- = other_exception
+addFilePathToIOError fun fp (IOError h iot _ str _)
+ = IOError h iot fun str (Just fp)
openFile :: FilePath -> IOMode -> IO Handle
openFile fp im =
(openFile' fp (if dEFAULT_OPEN_IN_BINARY_MODE
then BinaryMode im
else TextMode im))
- (\e -> throw (addFilePathToIOError "openFile" fp e))
+ (\e -> ioError (addFilePathToIOError "openFile" fp e))
openFileEx :: FilePath -> IOModeEx -> IO Handle
openFileEx fp m =
catch
(openFile' fp m)
- (\e -> throw (addFilePathToIOError "openFileEx" fp e))
+ (\e -> ioError (addFilePathToIOError "openFileEx" fp e))
openFile' filepath ex_mode =
hIsEOF handle =
catch
(do hLookAhead handle; return False)
- (\e -> if isEOFError e then return True else throw e)
+ (\e -> if isEOFError e then return True else ioError e)
isEOF :: IO Bool
isEOF = hIsEOF stdin
import GHC.Num
import GHC.Show
import GHC.List
-import GHC.Exception ( ioError, catch, throw )
+import GHC.Exception ( ioError, catch )
import GHC.Conc
-- ---------------------------------------------------------------------------
)
(\e -> do if isEOFError e
then return Nothing
- else throw e)
+ else ioError e)
unpack :: RawBuffer -> Int -> Int -> IO [Char]
-- has a constructor in the 'Exception' type, and values of other
-- types may be injected into 'Exception' by coercing them to
-- 'Dynamic' (see the section on Dynamic Exceptions: "Control.Exception\#DynamicExceptions").
---
--- For backwards compatibility with Haskell 98, 'IOError' is a type synonym
--- for 'Exception'.
data Exception
= ArithException ArithException
-- ^Exceptions raised by arithmetic
-- | A variant of 'throw' that can be used within the 'IO' monad.
--
--- Although 'ioError' has a type that is an instance of the type of 'throw', the
+-- Although 'throwIO' has a type that is an instance of the type of 'throw', the
-- two functions are subtly different:
--
-- > throw e `seq` return () ===> throw e
--- > ioError e `seq` return () ===> return ()
+-- > throwIO e `seq` return () ===> return ()
--
-- The first example will cause the exception @e@ to be raised,
--- whereas the second one won\'t. In fact, 'ioError' will only cause
+-- whereas the second one won\'t. In fact, 'throwIO' will only cause
-- an exception to be raised when it is used within the 'IO' monad.
--- The 'ioError' variant should be used in preference to 'throw' to
+-- The 'throwIO' variant should be used in preference to 'throw' to
-- raise an exception within the 'IO' monad because it guarantees
-- ordering with respect to other 'IO' operations, whereas 'throw'
-- does not.
-ioError :: Exception -> IO a
-ioError err = IO $ \s -> throw err s
+throwIO :: Exception -> IO a
+throwIO err = IO $ \s -> throw err s
ioException :: IOException -> IO a
ioException err = IO $ \s -> throw (IOException err) s
+ioError :: IOError -> IO a
+ioError = ioException
+
-- ---------------------------------------------------------------------------
-- IOError type
--- A value @IOError@ encode errors occurred in the @IO@ monad.
--- An @IOError@ records a more specific error type, a descriptive
+-- | The Haskell 98 type for exceptions in the @IO@ monad.
+-- In Haskell 98, this is an opaque type.
+type IOError = IOException
+
+-- |Exceptions that occur in the @IO@ monad.
+-- An @IOException@ records a more specific error type, a descriptive
-- string and maybe the handle that was used when the error was
-- flagged.
-
-type IOError = Exception
-
data IOException
= IOError {
ioe_handle :: Maybe Handle, -- the handle used by the action flagging
DynIOError{} -> "unknown IO error"
userError :: String -> IOError
-userError str = IOException (IOError Nothing UserError "" str Nothing)
+userError str = IOError Nothing UserError "" str Nothing
-- ---------------------------------------------------------------------------
-- Showing IOErrors
import Prelude
+import Control.Exception ( bracket )
import System.Posix.Types
import System.Time ( ClockTime(..) )
import System.IO
) where
import Prelude
-import System.IO ( bracket )
+import Control.Exception ( bracket )
#ifdef __GLASGOW_HASKELL__
import Foreign
ioeGetFileName, -- :: IOError -> Maybe FilePath
try, -- :: IO a -> IO (Either IOError a)
- bracket, -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- bracket_, -- :: IO a -> (a -> IO b) -> IO c -> IO c
- -- Non-standard extension (but will hopefully become standard with 1.5) is
- -- to export the Prelude io functions via IO (in addition to exporting them
- -- from the prelude...for now.)
+ -- re-exports of Prelude names
IO, -- instance MonadFix
FilePath, -- :: String
IOError,
, ioeGetErrorString -- :: IOError -> String
, ioeGetHandle -- :: IOError -> Maybe Handle
, ioeGetFileName -- :: IOError -> Maybe FilePath
- , try -- :: IO a -> IO (Either IOError a)
- , bracket -- :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- , bracket_ -- :: IO a -> (a -> IO b) -> IO c -> IO c
, IO ()
, FilePath -- :: String
IOErrorType, -- abstract
#endif
+ catch, -- :: IO a -> (IOError -> IO a) -> IO a
+ try, -- :: IO a -> IO (Either IOError a)
+
ioError, -- :: IOError -> IO a
userError, -- :: String -> IOError
) where
+import Data.Either
#ifdef __GLASGOW_HASKELL__
import GHC.Base
import Data.Maybe
import GHC.IOBase
+import GHC.Exception
import Text.Show
#endif
#ifdef __NHC__
import IO
( IOError ()
+ , try
, ioError
, userError
, isAlreadyExistsError -- :: IOError -> Bool
--import Control.Monad (MonadPlus(mplus))
#endif
+-- | The construct @try comp@ exposes IO errors which occur within a
+-- computation, and which are not fully handled.
+-- Other exceptions are not caught by this variant;
+-- to catch all exceptions, use @try@ from "Control.Exception".
+
+#ifndef __NHC__
+try :: IO a -> IO (Either IOError a)
+try f = catch (do r <- f
+ return (Right r))
+ (return . Left)
+#endif
+
#ifdef __GLASGOW_HASKELL__
-- -----------------------------------------------------------------------------
-- Constructing an IOError
mkIOError :: IOErrorType -> String -> Maybe Handle -> Maybe FilePath -> IOError
mkIOError t location maybe_hdl maybe_filename =
- IOException IOError{ ioe_type = t,
+ IOError{ ioe_type = t,
ioe_location = location,
ioe_descr = "",
ioe_handle = maybe_hdl,
ioeGetErrorString :: IOError -> String
ioeGetFileName :: IOError -> Maybe FilePath
-ioeGetErrorType (IOException ioe) = ioe_type ioe
-ioeGetErrorType _ = error "System.IO.Error.ioeGetErrorType: not an IO error"
+ioeGetErrorType ioe = ioe_type ioe
-ioeGetHandle (IOException ioe) = ioe_handle ioe
-ioeGetHandle _ = error "System.IO.Error.ioeGetHandle: not an IO error"
+ioeGetHandle ioe = ioe_handle ioe
-ioeGetErrorString (IOException ioe)
+ioeGetErrorString ioe
| isUserErrorType (ioe_type ioe) = ioe_descr ioe
| otherwise = show (ioe_type ioe)
-ioeGetErrorString _ = error "System.IO.Error.ioeGetErrorString: not an IO error"
-ioeGetFileName (IOException ioe) = ioe_filename ioe
-ioeGetFileName _ = error "System.IO.Error.ioeGetFileName: not an IO error"
+ioeGetFileName ioe = ioe_filename ioe
#endif
-- -----------------------------------------------------------------------------
-> Maybe FilePath
-> Maybe Handle
-> IOError
-annotateIOError (IOException (IOError hdl errTy _ str path)) loc opath ohdl =
- IOException (IOError (hdl `mplus` ohdl) errTy loc str (path `mplus` opath))
+annotateIOError (IOError hdl errTy _ str path) loc opath ohdl =
+ IOError (hdl `mplus` ohdl) errTy loc str (path `mplus` opath)
where
Nothing `mplus` ys = ys
xs `mplus` _ = xs
-annotateIOError exc _ _ _ =
- exc
#endif
#ifdef 0 /*__NHC__*/
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