-% ------------------------------------------------------------------------------
-% $Id: Exception.lhs,v 1.2 2001/07/03 14:13:32 simonmar Exp $
-%
-% (c) The University of Glasgow, 1998-2000
-%
-
-Exceptions and exception-handling functions.
-
\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
-#ifndef __HUGS__
-module GHC.Exception
- ( module GHC.Exception,
- Exception(..), AsyncException(..),
- IOException(..), ArithException(..), ArrayException(..),
- throw, ioError )
- where
-
-import Data.Either
-
+{-# LANGUAGE NoImplicitPrelude
+ , ExistentialQuantification
+ , MagicHash
+ , DeriveDataTypeable
+ #-}
+{-# OPTIONS_HADDOCK hide #-}
+-----------------------------------------------------------------------------
+-- |
+-- Module : GHC.Exception
+-- Copyright : (c) The University of Glasgow, 1998-2002
+-- License : see libraries/base/LICENSE
+--
+-- Maintainer : cvs-ghc@haskell.org
+-- Stability : internal
+-- Portability : non-portable (GHC extensions)
+--
+-- Exceptions and exception-handling functions.
+--
+-----------------------------------------------------------------------------
+
+-- #hide
+module GHC.Exception where
+
+import Data.Maybe
+import {-# SOURCE #-} Data.Typeable (Typeable, cast)
import GHC.Base
-import GHC.IOBase
-
-#endif
+import GHC.Show
\end{code}
%*********************************************************
-%* *
-\subsection{Primitive catch}
-%* *
+%* *
+\subsection{Exceptions}
+%* *
%*********************************************************
-catchException used to handle the passing around of the state to the
-action and the handler. This turned out to be a bad idea - it meant
-that we had to wrap both arguments in thunks so they could be entered
-as normal (remember IO returns an unboxed pair...).
-
-Now catch# has type
-
- catch# :: IO a -> (b -> IO a) -> IO a
-
-(well almost; the compiler doesn't know about the IO newtype so we
-have to work around that in the definition of catchException below).
-
\begin{code}
-catchException :: IO a -> (Exception -> IO a) -> IO a
-#ifdef __HUGS__
-catchException m k = ST (\s -> unST m s `primCatch'` \ err -> unST (k err) s)
-#else
-catchException (IO m) k = IO $ \s -> catch# m (\ex -> unIO (k ex)) s
-#endif
-
-catch :: IO a -> (Exception -> IO a) -> IO a
-catch m k = catchException m handler
- where handler err@(IOException _) = k err
- handler err@(UserError _) = k err
- handler other = throw other
+{- |
+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 = forall e . Exception e => SomeException e
+ deriving Typeable
+
+instance Show SomeException where
+ showsPrec p (SomeException e) = showsPrec p e
+
+{- |
+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, Typeable)
+>
+> 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
+> deriving Typeable
+>
+> 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
+> deriving Typeable
+>
+> 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 (Typeable, 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 where
+ toException :: e -> SomeException
+ fromException :: SomeException -> Maybe e
+
+ toException = SomeException
+ fromException (SomeException e) = cast e
+
+instance Exception SomeException where
+ toException se = se
+ fromException = Just
\end{code}
-
%*********************************************************
-%* *
-\subsection{Try and bracket}
-%* *
+%* *
+\subsection{Primitive throw}
+%* *
%*********************************************************
-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
+-- | Throw an exception. Exceptions may be thrown from purely
+-- functional code, but may only be caught within the 'IO' monad.
+throw :: Exception e => e -> a
+throw e = raise# (toException e)
\end{code}
+\begin{code}
+-- |This is thrown when the user calls 'error'. The @String@ is the
+-- argument given to 'error'.
+data ErrorCall = ErrorCall String
+ deriving Typeable
-%*********************************************************
-%* *
-\subsection{Controlling asynchronous exception delivery}
-%* *
-%*********************************************************
+instance Exception ErrorCall
-\begin{code}
-#ifndef __HUGS__
-block :: IO a -> IO a
-block (IO io) = IO $ blockAsyncExceptions# io
-
-unblock :: IO a -> IO a
-unblock (IO io) = IO $ unblockAsyncExceptions# io
-#else
--- Not implemented yet in Hugs.
-block :: IO a -> IO a
-block (IO io) = IO io
-
-unblock :: IO a -> IO a
-unblock (IO io) = IO io
-#endif
-\end{code}
+instance Show ErrorCall where
+ showsPrec _ (ErrorCall err) = showString err
+
+-----
+
+-- |Arithmetic exceptions.
+data ArithException
+ = Overflow
+ | Underflow
+ | LossOfPrecision
+ | DivideByZero
+ | Denormal
+ deriving (Eq, Ord, Typeable)
+instance Exception ArithException
+instance Show ArithException where
+ showsPrec _ Overflow = showString "arithmetic overflow"
+ showsPrec _ Underflow = showString "arithmetic underflow"
+ showsPrec _ LossOfPrecision = showString "loss of precision"
+ showsPrec _ DivideByZero = showString "divide by zero"
+ showsPrec _ Denormal = showString "denormal"
+
+\end{code}
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