Portability | non-portable (extended exceptions) |
---|---|
Stability | experimental |
Maintainer | libraries@haskell.org |
Safe Haskell | Trustworthy |
Extensible exceptions, except for multiple handlers.
- data SomeException = forall e . Exception e => SomeException e
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- data IOException
- data ArithException
- data ArrayException
- data AssertionFailed = AssertionFailed String
- data AsyncException
- data NonTermination = NonTermination
- data NestedAtomically = NestedAtomically
- data BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
- data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
- data Deadlock = Deadlock
- data NoMethodError = NoMethodError String
- data PatternMatchFail = PatternMatchFail String
- data RecConError = RecConError String
- data RecSelError = RecSelError String
- data RecUpdError = RecUpdError String
- data ErrorCall = ErrorCall String
- throwIO :: Exception e => e -> IO a
- throw :: Exception e => e -> a
- ioError :: IOError -> IO a
- throwTo :: Exception e => ThreadId -> e -> IO ()
- catch :: Exception e => IO a -> (e -> IO a) -> IO a
- catchJust :: Exception e => (e -> Maybe b) -> IO a -> (b -> IO a) -> IO a
- handle :: Exception e => (e -> IO a) -> IO a -> IO a
- handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
- try :: Exception e => IO a -> IO (Either e a)
- tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
- onException :: IO a -> IO b -> IO a
- evaluate :: a -> IO a
- mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
- mask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- mask_ :: IO a -> IO a
- uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b
- uninterruptibleMask_ :: IO a -> IO a
- data MaskingState
- getMaskingState :: IO MaskingState
- block :: IO a -> IO a
- unblock :: IO a -> IO a
- blocked :: IO Bool
- assert :: Bool -> a -> a
- bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- bracket_ :: IO a -> IO b -> IO c -> IO c
- bracketOnError :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
- finally :: IO a -> IO b -> IO a
- recSelError :: Addr# -> a
- recConError :: Addr# -> a
- irrefutPatError :: Addr# -> a
- runtimeError :: Addr# -> a
- nonExhaustiveGuardsError :: Addr# -> a
- patError :: Addr# -> a
- noMethodBindingError :: Addr# -> a
- absentError :: Addr# -> a
- nonTermination :: SomeException
- nestedAtomically :: SomeException
The Exception type
data SomeException Source
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
.
forall e . Exception e => SomeException e |
class (Typeable e, Show e) => Exception e whereSource
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 MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException)) Caught MismatchedParentheses *Main> throw MismatchedParenthesescatch
e -> putStrLn ("Caught " ++ show (e :: IOException)) *** Exception: MismatchedParentheses
toException :: e -> SomeExceptionSource
fromException :: SomeException -> Maybe eSource
data IOException Source
Exceptions that occur in the IO
monad.
An IOException
records a more specific error type, a descriptive
string and maybe the handle that was used when the error was
flagged.
data ArithException Source
Arithmetic exceptions.
data ArrayException Source
Exceptions generated by array operations
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. |
data AssertionFailed Source
data AsyncException Source
Asynchronous exceptions.
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:
|
ThreadKilled | This exception is raised by another thread
calling |
UserInterrupt | This exception is raised by default in the main thread of the program when the user requests to terminate the program via the usual mechanism(s) (e.g. Control-C in the console). |
data NonTermination Source
Thrown when the runtime system detects that the computation is guaranteed not to terminate. Note that there is no guarantee that the runtime system will notice whether any given computation is guaranteed to terminate or not.
data NestedAtomically Source
Thrown when the program attempts to call atomically
, from the stm
package, inside another call to atomically
.
data BlockedIndefinitelyOnMVar Source
The thread is blocked on an MVar
, but there are no other references
to the MVar
so it can't ever continue.
data BlockedIndefinitelyOnSTM Source
The thread is waiting to retry an STM transaction, but there are no
other references to any TVar
s involved, so it can't ever continue.
There are no runnable threads, so the program is deadlocked.
The Deadlock
exception is raised in the main thread only.
data NoMethodError Source
A class method without a definition (neither a default definition,
nor a definition in the appropriate instance) was called. The
String
gives information about which method it was.
data PatternMatchFail Source
A pattern match failed. The String
gives information about the
source location of the pattern.
data RecConError Source
An uninitialised record field was used. The String
gives
information about the source location where the record was
constructed.
data RecSelError Source
A record selector was applied to a constructor without the
appropriate field. This can only happen with a datatype with
multiple constructors, where some fields are in one constructor
but not another. The String
gives information about the source
location of the record selector.
data RecUpdError Source
A record update was performed on a constructor without the
appropriate field. This can only happen with a datatype with
multiple constructors, where some fields are in one constructor
but not another. The String
gives information about the source
location of the record update.
Throwing exceptions
throwIO :: Exception e => e -> IO aSource
A variant of throw
that can only 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.
throw :: Exception e => e -> aSource
Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO
monad.
throwTo :: Exception e => ThreadId -> e -> IO ()Source
throwTo
raises an arbitrary exception in the target thread (GHC only).
throwTo
does not return until the exception has been raised in the
target thread.
The calling thread can thus be certain that the target
thread has received the exception. This is a useful property to know
when dealing with race conditions: eg. if there are two threads that
can kill each other, it is guaranteed that only one of the threads
will get to kill the other.
Whatever work the target thread was doing when the exception was raised is not lost: the computation is suspended until required by another thread.
If the target thread is currently making a foreign call, then the
exception will not be raised (and hence throwTo
will not return)
until the call has completed. This is the case regardless of whether
the call is inside a mask
or not. However, in GHC a foreign call
can be annotated as interruptible
, in which case a throwTo
will
cause the RTS to attempt to cause the call to return; see the GHC
documentation for more details.
Important note: the behaviour of throwTo
differs from that described in
the paper "Asynchronous exceptions in Haskell"
(http://research.microsoft.com/~simonpj/Papers/asynch-exns.htm).
In the paper, throwTo
is non-blocking; but the library implementation adopts
a more synchronous design in which throwTo
does not return until the exception
is received by the target thread. The trade-off is discussed in Section 9 of the paper.
Like any blocking operation, throwTo
is therefore interruptible (see Section 5.3 of
the paper). Unlike other interruptible operations, however, throwTo
is always interruptible, even if it does not actually block.
There is no guarantee that the exception will be delivered promptly,
although the runtime will endeavour to ensure that arbitrary
delays don't occur. In GHC, an exception can only be raised when a
thread reaches a safe point, where a safe point is where memory
allocation occurs. Some loops do not perform any memory allocation
inside the loop and therefore cannot be interrupted by a throwTo
.
If the target of throwTo
is the calling thread, then the behaviour
is the same as throwIO
, except that the exception
is thrown as an asynchronous exception. This means that if there is
an enclosing pure computation, which would be the case if the current
IO operation is inside unsafePerformIO
or unsafeInterleaveIO
, that
computation is not permanently replaced by the exception, but is
suspended as if it had received an asynchronous exception.
Note that if throwTo
is called with the current thread as the
target, the exception will be thrown even if the thread is currently
inside mask
or uninterruptibleMask
.
Catching Exceptions
The catch
functions
:: Exception e | |
=> IO a | The computation to run |
-> (e -> IO a) | Handler to invoke if an exception is raised |
-> IO a |
This is the simplest of the exception-catching functions. It takes a single argument, runs it, and if an exception is raised the "handler" is executed, with the value of the exception passed as an argument. Otherwise, the result is returned as normal. For example:
catch (readFile f) (\e -> do let err = show (e :: IOException) hPutStr stderr ("Warning: Couldn't open " ++ f ++ ": " ++ err) return "")
Note that we have to give a type signature to e
, or the program
will not typecheck as the type is ambiguous. While it is possible
to catch exceptions of any type, see the section "Catching all
exceptions" (in Control.Exception) for an explanation of the problems with doing so.
For catching exceptions in pure (non-IO
) expressions, see the
function evaluate
.
Note that due to Haskell's unspecified evaluation order, an
expression may throw one of several possible exceptions: consider
the expression (error "urk") + (1 `div` 0)
. Does
the expression throw
ErrorCall "urk"
, or DivideByZero
?
The answer is "it might throw either"; the choice is
non-deterministic. If you are catching any type of exception then you
might catch either. If you are calling catch
with type
IO Int -> (ArithException -> IO Int) -> IO Int
then the handler may
get run with DivideByZero
as an argument, or an ErrorCall "urk"
exception may be propogated further up. If you call it again, you
might get a the opposite behaviour. This is ok, because catch
is an
IO
computation.
Note that the Prelude also exports a function called
catch
with a similar type to catch
,
except that the Prelude version only catches the IO and user
families of exceptions (as required by Haskell 98).
We recommend either hiding the Prelude version of catch
when importing Control.Exception:
import Prelude hiding (catch)
or importing Control.Exception qualified, to avoid name-clashes:
import qualified Control.Exception as C
and then using C.catch
:: Exception e | |
=> (e -> Maybe b) | Predicate to select exceptions |
-> IO a | Computation to run |
-> (b -> IO a) | Handler |
-> IO a |
The function catchJust
is like catch
, but it takes an extra
argument which is an exception predicate, a function which
selects which type of exceptions we're interested in.
catchJust (\e -> if isDoesNotExistErrorType (ioeGetErrorType e) then Just () else Nothing) (readFile f) (\_ -> do hPutStrLn stderr ("No such file: " ++ show f) return "")
Any other exceptions which are not matched by the predicate
are re-raised, and may be caught by an enclosing
catch
, catchJust
, etc.
The handle
functions
handle :: Exception e => (e -> IO a) -> IO a -> IO aSource
A version of catch
with the arguments swapped around; useful in
situations where the code for the handler is shorter. For example:
do handle (\NonTermination -> exitWith (ExitFailure 1)) $ ...
The try
functions
try :: Exception e => IO a -> IO (Either e a)Source
Similar to catch
, but returns an Either
result which is
(
if no exception of type Right
a)e
was raised, or (
if an exception of type Left
ex)e
was raised and its value is ex
.
If any other type of exception is raised than it will be propogated
up to the next enclosing exception handler.
try a = catch (Right `liftM` a) (return . Left)
Note that System.IO.Error also exports a function called
try
with a similar type to try
,
except that it catches only the IO and user families of exceptions
(as required by the Haskell 98 IO
module).
onException :: IO a -> IO b -> IO aSource
Like finally
, but only performs the final action if there was an
exception raised by the computation.
The evaluate
function
Forces its argument to be evaluated to weak head normal form 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
The mapException
function
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> aSource
This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".
Asynchronous Exceptions
Asynchronous exception control
mask :: ((forall a. IO a -> IO a) -> IO b) -> IO bSource
Executes an IO computation with asynchronous
exceptions masked. That is, any thread which attempts to raise
an exception in the current thread with throwTo
will be blocked until asynchronous exceptions are unmasked again.
The argument passed to mask
is a function that takes as its
argument another function, which can be used to restore the
prevailing masking state within the context of the masked
computation. For example, a common way to use mask
is to protect
the acquisition of a resource:
mask $ \restore -> do x <- acquire restore (do_something_with x) `onException` release release
This code guarantees that acquire
is paired with release
, by masking
asynchronous exceptions for the critical parts. (Rather than write
this code yourself, it would be better to use
bracket
which abstracts the general pattern).
Note that the restore
action passed to the argument to mask
does not necessarily unmask asynchronous exceptions, it just
restores the masking state to that of the enclosing context. Thus
if asynchronous exceptions are already masked, mask
cannot be used
to unmask exceptions again. This is so that if you call a library function
with exceptions masked, you can be sure that the library call will not be
able to unmask exceptions again. If you are writing library code and need
to use asynchronous exceptions, the only way is to create a new thread;
see forkIOWithUnmask
.
Asynchronous exceptions may still be received while in the masked state if the masked thread blocks in certain ways; see Control.Exception.
Threads created by forkIO
inherit the masked
state from the parent; that is, to start a thread in blocked mode,
use mask_ $ forkIO ...
. This is particularly useful if you need
to establish an exception handler in the forked thread before any
asynchronous exceptions are received. To create a a new thread in
an unmasked state use forkIOUnmasked
.
uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO bSource
Like mask
, but the masked computation is not interruptible (see
Control.Exception). THIS SHOULD BE USED WITH
GREAT CARE, because if a thread executing in uninterruptibleMask
blocks for any reason, then the thread (and possibly the program,
if this is the main thread) will be unresponsive and unkillable.
This function should only be necessary if you need to mask
exceptions around an interruptible operation, and you can guarantee
that the interruptible operation will only block for a short period
of time.
uninterruptibleMask_ :: IO a -> IO aSource
Like uninterruptibleMask
, but does not pass a restore
action
to the argument.
data MaskingState Source
Describes the behaviour of a thread when an asynchronous exception is received.
Unmasked | asynchronous exceptions are unmasked (the normal state) |
MaskedInterruptible | the state during |
MaskedUninterruptible | the state during |
getMaskingState :: IO MaskingStateSource
Returns the MaskingState
for the current thread.
(deprecated) Asynchronous exception control
Note: this function is deprecated, please use mask
instead.
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 throwTo
will be
blocked until asynchronous exceptions are unblocked 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 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.
Assertions
assert :: Bool -> a -> aSource
If the first argument evaluates to True
, then the result is the
second argument. Otherwise an AssertionFailed
exception is raised,
containing a String
with the source file and line number of the
call to assert
.
Assertions can normally be turned on or off with a compiler flag
(for GHC, assertions are normally on unless optimisation is turned on
with -O
or the -fignore-asserts
option is given). When assertions are turned off, the first
argument to assert
is ignored, and the second argument is
returned as the result.
Utilities
:: IO a | computation to run first ("acquire resource") |
-> (a -> IO b) | computation to run last ("release resource") |
-> (a -> IO c) | computation to run in-between |
-> IO c |
When you want to acquire a resource, do some work with it, and
then release the resource, it is a good idea to use bracket
,
because bracket
will install the necessary exception handler to
release the resource in the event that an exception is raised
during the computation. If an exception is raised, then bracket
will
re-raise the exception (after performing the release).
A common example is opening a file:
bracket (openFile "filename" ReadMode) (hClose) (\fileHandle -> do { ... })
The arguments to bracket
are in this order so that we can partially apply
it, e.g.:
withFile name mode = bracket (openFile name mode) hClose
bracket_ :: IO a -> IO b -> IO c -> IO cSource
A variant of bracket
where the return value from the first computation
is not required.
:: IO a | computation to run first ("acquire resource") |
-> (a -> IO b) | computation to run last ("release resource") |
-> (a -> IO c) | computation to run in-between |
-> IO c |
Like bracket
, but only performs the final action if there was an
exception raised by the in-between computation.
:: IO a | computation to run first |
-> IO b | computation to run afterward (even if an exception was raised) |
-> IO a |
A specialised variant of bracket
with just a computation to run
afterward.
Calls for GHC runtime
recSelError :: Addr# -> aSource
recConError :: Addr# -> aSource
irrefutPatError :: Addr# -> aSource
runtimeError :: Addr# -> aSource
nonExhaustiveGuardsError :: Addr# -> aSource
noMethodBindingError :: Addr# -> aSource
absentError :: Addr# -> aSource