\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# LANGUAGE NoImplicitPrelude
+ , ExistentialQuantification
+ , MagicHash
+ , DeriveDataTypeable
+ #-}
+{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.Exception
--
-----------------------------------------------------------------------------
-#ifndef __HUGS__
-module GHC.Exception
- ( module GHC.Exception,
- Exception(..), AsyncException(..),
- IOException(..), ArithException(..), ArrayException(..),
- throw, throwIO, ioError )
- where
+-- #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 -> (IOError -> IO a) -> IO a
-catch m k = catchException m handler
- where handler (IOException err) = 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{Controlling asynchronous exception delivery}
-%* *
+%* *
+\subsection{Primitive throw}
+%* *
%*********************************************************
\begin{code}
--- | Applying 'block' to a computation will
--- execute that computation with asynchronous exceptions
--- /blocked/. That is, any thread which
--- attempts to raise an exception in the current thread will be
--- blocked until asynchronous exceptions are enabled again. There\'s
--- no need to worry about re-enabling asynchronous exceptions; that is
--- done automatically on exiting the scope of
--- 'block'.
-block :: IO a -> IO a
-
--- | To re-enable asynchronous exceptions inside the scope of
--- 'block', 'unblock' can be
--- used. It scopes in exactly the same way, so on exit from
--- 'unblock' asynchronous exception delivery will
--- be disabled again.
-unblock :: IO a -> IO a
-
-#ifndef __HUGS__
-block (IO io) = IO $ blockAsyncExceptions# io
-unblock (IO io) = IO $ unblockAsyncExceptions# io
-#else
-unblock :: IO a -> IO a
-unblock (IO io) = IO io
-#endif
+-- | 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
+
+instance Exception ErrorCall
+
+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}
projects / ghc-base.git / blobdiff