+
+%*********************************************************
+%* *
+\subsection{Exception datatype and operations}
+%* *
+%*********************************************************
+
+\begin{code}
+data Exception
+ = IOException IOException -- IO exceptions
+ | ArithException ArithException -- Arithmetic exceptions
+ | ArrayException ArrayException -- Array-related exceptions
+ | ErrorCall String -- Calls to 'error'
+ | NoMethodError String -- A non-existent method was invoked
+ | PatternMatchFail String -- A pattern match / guard failure
+ | RecSelError String -- Selecting a non-existent field
+ | RecConError String -- Field missing in record construction
+ | RecUpdError String -- Record doesn't contain updated field
+ | AssertionFailed String -- Assertions
+ | DynException Dynamic -- Dynamic exceptions
+ | AsyncException AsyncException -- Externally generated errors
+ | PutFullMVar -- Put on a full MVar
+ | BlockedOnDeadMVar -- Blocking on a dead MVar
+ | NonTermination
+ | UserError String
+
+data ArithException
+ = Overflow
+ | Underflow
+ | LossOfPrecision
+ | DivideByZero
+ | Denormal
+ deriving (Eq, Ord)
+
+data AsyncException
+ = StackOverflow
+ | HeapOverflow
+ | ThreadKilled
+ deriving (Eq, Ord)
+
+data ArrayException
+ = IndexOutOfBounds String -- out-of-range array access
+ | UndefinedElement String -- evaluating an undefined element
+ deriving (Eq, Ord)
+
+stackOverflow, heapOverflow :: Exception -- for the RTS
+stackOverflow = AsyncException StackOverflow
+heapOverflow = AsyncException HeapOverflow
+
+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"
+
+instance Show AsyncException where
+ showsPrec _ StackOverflow = showString "stack overflow"
+ showsPrec _ HeapOverflow = showString "heap overflow"
+ showsPrec _ ThreadKilled = showString "thread killed"
+
+instance Show ArrayException where
+ showsPrec _ (IndexOutOfBounds s)
+ = showString "array index out of range"
+ . (if not (null s) then showString ": " . showString s
+ else id)
+ showsPrec _ (UndefinedElement s)
+ = showString "undefined array element"
+ . (if not (null s) then showString ": " . showString s
+ else id)
+
+instance Show Exception where
+ showsPrec _ (IOException err) = shows err
+ showsPrec _ (ArithException err) = shows err
+ showsPrec _ (ArrayException err) = shows err
+ showsPrec _ (ErrorCall err) = showString err
+ showsPrec _ (NoMethodError err) = showString err
+ showsPrec _ (PatternMatchFail err) = showString err
+ showsPrec _ (RecSelError err) = showString err
+ showsPrec _ (RecConError err) = showString err
+ showsPrec _ (RecUpdError err) = showString err
+ showsPrec _ (AssertionFailed err) = showString err
+ showsPrec _ (AsyncException e) = shows e
+ showsPrec _ (DynException _err) = showString "unknown exception"
+ showsPrec _ (PutFullMVar) = showString "putMVar: full MVar"
+ showsPrec _ (BlockedOnDeadMVar) = showString "thread blocked indefinitely"
+ showsPrec _ (NonTermination) = showString "<<loop>>"
+\end{code}
+
+%*********************************************************
+%* *
+\subsection{Primitive throw}
+%* *
+%*********************************************************
+
+\begin{code}
+throw :: Exception -> a
+throw exception = raise# exception
+
+ioError :: Exception -> IO a
+ioError err = IO $ \s -> throw err s
+
+ioException :: IOException -> IO a
+ioException err = IO $ \s -> throw (IOException err) s
+\end{code}
+
+%*********************************************************
+%* *
+\subsection{Type @IOError@}
+%* *
+%*********************************************************
+
+A value @IOError@ encode errors occurred in the @IO@ monad.
+An @IOError@ records a more specific error type, a descriptive
+string and maybe the handle that was used when the error was
+flagged.
+
+\begin{code}
+type IOError = Exception
+
+data IOException
+ = IOError
+ (Maybe Handle) -- the handle used by the action flagging the
+ -- the error.
+ IOErrorType -- what it was.
+ String -- location
+ String -- error type specific information.
+
+instance Eq IOException where
+ (IOError h1 e1 loc1 str1) == (IOError h2 e2 loc2 str2) =
+ e1==e2 && str1==str2 && h1==h2 && loc1 == loc2
+
+data IOErrorType
+ = AlreadyExists | HardwareFault
+ | IllegalOperation | InappropriateType
+ | Interrupted | InvalidArgument
+ | NoSuchThing | OtherError
+ | PermissionDenied | ProtocolError
+ | ResourceBusy | ResourceExhausted
+ | ResourceVanished | SystemError
+ | TimeExpired | UnsatisfiedConstraints
+ | UnsupportedOperation
+ | EOF
+#if defined(cygwin32_TARGET_OS) || defined(mingw32_TARGET_OS)
+ | ComError Int -- HRESULT
+#endif
+ deriving (Eq)
+
+instance Show IOErrorType where
+ showsPrec _ e =
+ showString $
+ case e of
+ AlreadyExists -> "already exists"
+ HardwareFault -> "hardware fault"
+ IllegalOperation -> "illegal operation"
+ InappropriateType -> "inappropriate type"
+ Interrupted -> "interrupted"
+ InvalidArgument -> "invalid argument"
+ NoSuchThing -> "does not exist"
+ OtherError -> "failed"
+ PermissionDenied -> "permission denied"
+ ProtocolError -> "protocol error"
+ ResourceBusy -> "resource busy"
+ ResourceExhausted -> "resource exhausted"
+ ResourceVanished -> "resource vanished"
+ SystemError -> "system error"
+ TimeExpired -> "timeout"
+ UnsatisfiedConstraints -> "unsatisified constraints" -- ultra-precise!
+ UnsupportedOperation -> "unsupported operation"
+ EOF -> "end of file"
+#if defined(cygwin32_TARGET_OS) || defined(mingw32_TARGET_OS)
+ ComError _ -> "COM error"
+#endif
+
+
+
+userError :: String -> IOError
+userError str = UserError str
+\end{code}
+
+Predicates on IOError; little effort made on these so far...
+
+\begin{code}
+
+isAlreadyExistsError :: IOError -> Bool
+isAlreadyExistsError (IOException (IOError _ AlreadyExists _ _)) = True
+isAlreadyExistsError _ = False
+
+isAlreadyInUseError :: IOError -> Bool
+isAlreadyInUseError (IOException (IOError _ ResourceBusy _ _)) = True
+isAlreadyInUseError _ = False
+
+isFullError :: IOError -> Bool
+isFullError (IOException (IOError _ ResourceExhausted _ _)) = True
+isFullError _ = False
+
+isEOFError :: IOError -> Bool
+isEOFError (IOException (IOError _ EOF _ _)) = True
+isEOFError _ = False
+
+isIllegalOperation :: IOError -> Bool
+isIllegalOperation (IOException (IOError _ IllegalOperation _ _)) = True
+isIllegalOperation _ = False
+
+isPermissionError :: IOError -> Bool
+isPermissionError (IOException (IOError _ PermissionDenied _ _)) = True
+isPermissionError _ = False
+
+isDoesNotExistError :: IOError -> Bool
+isDoesNotExistError (IOException (IOError _ NoSuchThing _ _)) = True
+isDoesNotExistError _ = False
+
+isUserError :: IOError -> Bool
+isUserError (UserError _) = True
+isUserError _ = False
+\end{code}
+
+Showing @IOError@s
+
+\begin{code}
+#ifdef __HUGS__
+-- For now we give a fairly uninformative error message which just happens to
+-- be like the ones that Hugs used to give.
+instance Show IOException where
+ showsPrec p (IOError hdl iot loc s) = showString s . showChar '\n'
+#else
+instance Show IOException where
+ showsPrec p (IOError hdl iot loc s) =
+ showsPrec p iot .
+ showChar '\n' .
+ (case loc of
+ "" -> id
+ _ -> showString "Action: " . showString loc . showChar '\n') .
+ showHdl .
+ (case s of
+ "" -> id
+ _ -> showString "Reason: " . showString s)
+ where
+ showHdl =
+ case hdl of
+ Nothing -> id
+ Just h -> showString "Handle: " . showsPrec p h
+
+#endif
+\end{code}
+
+The @String@ part of an @IOError@ is platform-dependent. However, to
+provide a uniform mechanism for distinguishing among errors within
+these broad categories, each platform-specific standard shall specify
+the exact strings to be used for particular errors. For errors not
+explicitly mentioned in the standard, any descriptive string may be
+used.
+
+\begin{code}
+constructErrorAndFail :: String -> IO a
+constructErrorAndFail call_site
+ = constructError call_site >>= \ io_error ->
+ ioError (IOException io_error)
+
+constructErrorAndFailWithInfo :: String -> String -> IO a
+constructErrorAndFailWithInfo call_site reason
+ = constructErrorMsg call_site (Just reason) >>= \ io_error ->
+ ioError (IOException io_error)
+
+\end{code}
+
+This doesn't seem to be documented/spelled out anywhere,
+so here goes: (SOF)
+
+The implementation of the IO prelude uses various C stubs
+to do the actual interaction with the OS. The bandwidth
+\tr{C<->Haskell} is somewhat limited, so the general strategy
+for flaggging any errors (apart from possibly using the
+return code of the external call), is to set the @ghc_errtype@
+to a value that is one of the \tr{#define}s in @includes/error.h@.
+@ghc_errstr@ holds a character string providing error-specific
+information. Error constructing functions will then reach out
+and grab these values when generating
+
+\begin{code}
+constructError :: String -> IO IOException
+constructError call_site = constructErrorMsg call_site Nothing
+
+constructErrorMsg :: String -> Maybe String -> IO IOException
+constructErrorMsg call_site reason =
+ getErrType__ >>= \ errtype ->
+ getErrStr__ >>= \ str ->
+ let
+ iot =
+ case (errtype::Int) of
+ ERR_ALREADYEXISTS -> AlreadyExists
+ ERR_HARDWAREFAULT -> HardwareFault
+ ERR_ILLEGALOPERATION -> IllegalOperation
+ ERR_INAPPROPRIATETYPE -> InappropriateType
+ ERR_INTERRUPTED -> Interrupted
+ ERR_INVALIDARGUMENT -> InvalidArgument
+ ERR_NOSUCHTHING -> NoSuchThing
+ ERR_OTHERERROR -> OtherError
+ ERR_PERMISSIONDENIED -> PermissionDenied
+ ERR_PROTOCOLERROR -> ProtocolError
+ ERR_RESOURCEBUSY -> ResourceBusy
+ ERR_RESOURCEEXHAUSTED -> ResourceExhausted
+ ERR_RESOURCEVANISHED -> ResourceVanished
+ ERR_SYSTEMERROR -> SystemError
+ ERR_TIMEEXPIRED -> TimeExpired
+ ERR_UNSATISFIEDCONSTRAINTS -> UnsatisfiedConstraints
+ ERR_UNSUPPORTEDOPERATION -> UnsupportedOperation
+ ERR_EOF -> EOF
+ _ -> OtherError
+
+ msg =
+ unpackCString str ++
+ (case iot of
+ OtherError -> "(error code: " ++ show errtype ++ ")"
+ _ -> "") ++
+ (case reason of
+ Nothing -> ""
+ Just m -> ' ':m)
+ in
+ return (IOError Nothing iot call_site msg)
+\end{code}
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