-- ------------------------------------------------------------------------
-- Exception datatype and operations
+-- |The type of exceptions. Every kind of system-generated exception
+-- 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).
+--
+-- For backwards compatibility with Haskell 98, 'IOError' is a type synonym
+-- for 'Exception'.
data Exception
- = IOException IOException -- IO exceptions
- | ArithException ArithException -- Arithmetic exceptions
- | ArrayException ArrayException -- Array-related exceptions
- | ErrorCall String -- Calls to 'error'
- | ExitException ExitCode -- Call to System.exitWith
- | 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
- | BlockedOnDeadMVar -- Blocking on a dead MVar
- | Deadlock -- no threads can run (raised in main thread)
+ = ArithException ArithException
+ -- ^Exceptions raised by arithmetic
+ -- operations. (NOTE: GHC currently does not throw
+ -- 'ArithException's).
+ | ArrayException ArrayException
+ -- ^Exceptions raised by array-related
+ -- operations. (NOTE: GHC currently does not throw
+ -- 'ArrayException's).
+ | AssertionFailed String
+ -- ^This exception is thrown by the
+ -- 'assert' operation when the condition
+ -- fails. The 'String' argument contains the
+ -- location of the assertion in the source program.
+ | AsyncException AsyncException
+ -- ^Asynchronous exceptions (see section on Asynchronous Exceptions).
+ | BlockedOnDeadMVar
+ -- ^The current thread was executing a call to
+ -- 'takeMVar' that could never return, because there are no other
+ -- references to this 'MVar'.
+ | Deadlock
+ -- ^There are no runnable threads, so the program is
+ -- deadlocked. The 'Deadlock' exception is
+ -- raised in the main thread only (see also: "Control.Concurrent").
+ | DynException Dynamic
+ -- ^Dynamically typed exceptions (see section on Dynamic Exceptions).
+ | ErrorCall String
+ -- ^The 'ErrorCall' exception is thrown by 'error'. The 'String'
+ -- argument of 'ErrorCall' is the string passed to 'error' when it was
+ -- called.
+ | ExitException ExitCode
+ -- ^The 'ExitException' exception is thrown by 'System.exitWith' (and
+ -- 'System.exitFailure'). The 'ExitCode' argument is the value passed
+ -- to 'System.exitWith'. An unhandled 'ExitException' exception in the
+ -- main thread will cause the program to be terminated with the given
+ -- exit code.
+ | IOException IOException
+ -- ^These are the standard IO exceptions generated by
+ -- Haskell\'s @IO@ operations. See also "System.IO.Error".
+ | NoMethodError String
+ -- ^An attempt was made to invoke a class method which has
+ -- no definition in this instance, and there was no default
+ -- definition given in the class declaration. GHC issues a
+ -- warning when you compile an instance which has missing
+ -- methods.
| NonTermination
-
+ -- ^The current thread is stuck in an infinite loop. This
+ -- exception may or may not be thrown when the program is
+ -- non-terminating.
+ | PatternMatchFail String
+ -- ^A pattern matching failure. The 'String' argument should contain a
+ -- descriptive message including the function name, source file
+ -- and line number.
+ | RecConError String
+ -- ^An attempt was made to evaluate a field of a record
+ -- for which no value was given at construction time. The
+ -- 'String' argument gives the location of the
+ -- record construction in the source program.
+ | RecSelError String
+ -- ^A field selection was attempted on a constructor that
+ -- doesn\'t have the requested field. This can happen with
+ -- multi-constructor records when one or more fields are
+ -- missing from some of the constructors. The
+ -- 'String' argument gives the location of the
+ -- record selection in the source program.
+ | RecUpdError String
+ -- ^An attempt was made to update a field in a record,
+ -- where the record doesn\'t have the requested field. This can
+ -- only occur with multi-constructor records, when one or more
+ -- fields are missing from some of the constructors. The
+ -- 'String' argument gives the location of the
+ -- record update in the source program.
+
+-- |The type of arithmetic exceptions
data ArithException
= Overflow
| Underflow
| Denormal
deriving (Eq, Ord)
+
+-- |Asynchronous exceptions
data AsyncException
= StackOverflow
+ -- ^The current thread\'s stack exceeded its limit.
+ -- Since an exception has been raised, the thread\'s stack
+ -- will certainly be below its limit again, but the
+ -- programmer should take remedial action
+ -- immediately.
| HeapOverflow
+ -- ^The program\'s heap is reaching its limit, and
+ -- the program should take action to reduce the amount of
+ -- live data it has. Notes:
+ --
+ -- * It is undefined which thread receives this exception.
+ --
+ -- * GHC currently does not throw 'HeapOverflow' exceptions.
| ThreadKilled
+ -- ^This exception is raised by another thread
+ -- calling 'killThread', or by the system
+ -- if it needs to terminate the thread for some
+ -- reason.
deriving (Eq, Ord)
+-- | Exceptions generated by array operations
data ArrayException
- = IndexOutOfBounds String -- out-of-range array access
- | UndefinedElement String -- evaluating an undefined element
+ = IndexOutOfBounds String
+ -- ^An attempt was made to index an array outside
+ -- its declared bounds.
+ | UndefinedElement String
+ -- ^An attempt was made to evaluate an element of an
+ -- array that had not been initialized.
deriving (Eq, Ord)
stackOverflow, heapOverflow :: Exception -- for the RTS
-- --------------------------------------------------------------------------
-- Primitive throw
+-- | Throw an exception. Exceptions may be thrown from purely
+-- functional code, but may only be caught within the 'IO' monad.
throw :: Exception -> a
throw exception = raise# exception
+-- | 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
+-- two functions are subtly different:
+--
+-- > throw e `seq` return () ===> throw e
+-- > ioError 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
+-- an exception to be raised when it is used within the 'IO' monad.
+-- The 'ioError' 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
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