\begin{code}
-{-# OPTIONS_GHC -XNoImplicitPrelude #-}
+{-# LANGUAGE NoImplicitPrelude
+ , ExistentialQuantification
+ , MagicHash
+ , DeriveDataTypeable
+ #-}
{-# OPTIONS_HADDOCK hide #-}
-----------------------------------------------------------------------------
-- |
-----------------------------------------------------------------------------
-- #hide
-module GHC.Exception
- ( module GHC.Exception,
- throwIO, ioError )
- where
+module GHC.Exception where
import Data.Maybe
-import {-# SOURCE #-} Data.Typeable
+import {-# SOURCE #-} Data.Typeable (Typeable, cast)
import GHC.Base
-import GHC.IOBase hiding (Exception)
-import qualified GHC.IOBase
import GHC.Show
\end{code}
%*********************************************************
\begin{code}
+{- |
+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
fromException = Just
\end{code}
-For now at least, make the monolithic Exception type an instance.
-
-\begin{code}
-instance Exception GHC.IOBase.Exception
-\end{code}
-
%*********************************************************
%* *
-\subsection{Primitive catch and throw}
+\subsection{Primitive throw}
%* *
%*********************************************************
-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 :: Exception e => IO a -> (e -> IO a) -> IO a
-catchException (IO io) handler = IO $ catch# io handler'
- where handler' e = case fromException e of
- Just e' -> unIO (handler e')
- Nothing -> raise# e
-
-catchAny :: IO a -> (forall e . Exception e => e -> IO a) -> IO a
-catchAny (IO io) handler = IO $ catch# io handler'
- where handler' (SomeException e) = unIO (handler e)
-
--- | The 'catch' function establishes a handler that receives any 'IOError'
--- raised in the action protected by 'catch'. An 'IOError' is caught by
--- the most recent handler established by 'catch'. These handlers are
--- not selective: all 'IOError's are caught. Exception propagation
--- must be explicitly provided in a handler by re-raising any unwanted
--- exceptions. For example, in
---
--- > f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)
---
--- the function @f@ returns @[]@ when an end-of-file exception
--- (cf. 'System.IO.Error.isEOFError') occurs in @g@; otherwise, the
--- exception is propagated to the next outer handler.
---
--- When an exception propagates outside the main program, the Haskell
--- system prints the associated 'IOError' value and exits the program.
---
--- Non-I\/O exceptions are not caught by this variant; to catch all
--- exceptions, use 'Control.Exception.catch' from "Control.Exception".
-catch :: IO a -> (IOError -> IO a) -> IO a
-catch m k = catchException m handler
- where handler (IOException err) = k err
- handler other = throw other
-
-- | 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)
-
--- | A variant of 'throw' that can be used within the 'IO' monad.
---
--- Although 'throwIO' has a type that is an instance of the type of 'throw', the
--- two functions are subtly different:
---
--- > throw e `seq` x ===> throw e
--- > throwIO e `seq` x ===> x
---
--- The first example will cause the exception @e@ to be raised,
--- 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 '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.
-throwIO :: Exception e => e -> IO a
-throwIO e = IO (raiseIO# (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}
--- | 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 with 'Control.Exception.throwTo' 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'.
---
--- Threads created by 'Control.Concurrent.forkIO' inherit the blocked
--- state from the parent; that is, to start a thread in blocked mode,
--- use @block $ forkIO ...@. This is particularly useful if you need to
--- establish an exception handler in the forked thread before any
--- asynchronous exceptions are received.
-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
-
-block (IO io) = IO $ blockAsyncExceptions# io
-unblock (IO io) = IO $ unblockAsyncExceptions# io
-
--- | returns True if asynchronous exceptions are blocked in the
--- current thread.
-blocked :: IO Bool
-blocked = IO $ \s -> case asyncExceptionsBlocked# s of
- (# s', i #) -> (# s', i /=# 0# #)
-\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"
-\begin{code}
--- | Forces its argument to be evaluated when the resultant 'IO' action
--- is executed. It can be used to order evaluation with respect to
--- other 'IO' operations; its semantics are given by
---
--- > evaluate x `seq` y ==> y
--- > evaluate x `catch` f ==> (return $! x) `catch` f
--- > evaluate x >>= f ==> (return $! x) >>= f
---
--- /Note:/ the first equation implies that @(evaluate x)@ is /not/ the
--- same as @(return $! x)@. A correct definition is
---
--- > evaluate x = (return $! x) >>= return
---
-evaluate :: a -> IO a
-evaluate a = IO $ \s -> case a `seq` () of () -> (# s, a #)
- -- NB. can't write
- -- a `seq` (# s, a #)
- -- because we can't have an unboxed tuple as a function argument
\end{code}
projects / ghc-base.git / blobdiff