| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | stable |
| Portability | non-portable (extended exceptions) |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
GHC.Internal.Control.Exception.Base
Description
Extensible exceptions, except for multiple handlers.
Synopsis
- data SomeException = (Exception e, HasExceptionContext) => SomeException e
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- backtraceDesired :: e -> Bool
- data IOException
- data ArithException
- data ArrayException
- newtype AssertionFailed = AssertionFailed String
- data SomeAsyncException = Exception e => SomeAsyncException e
- data AsyncException
- asyncExceptionToException :: Exception e => e -> SomeException
- asyncExceptionFromException :: Exception e => SomeException -> Maybe e
- data NonTermination = NonTermination
- data NestedAtomically = NestedAtomically
- data BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
- data FixIOException = FixIOException
- data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
- data AllocationLimitExceeded = AllocationLimitExceeded
- newtype CompactionFailed = CompactionFailed String
- data Deadlock = Deadlock
- newtype NoMethodError = NoMethodError String
- newtype PatternMatchFail = PatternMatchFail String
- newtype RecConError = RecConError String
- newtype RecSelError = RecSelError String
- newtype RecUpdError = RecUpdError String
- data ErrorCall where
- newtype TypeError = TypeError String
- data NoMatchingContinuationPrompt = NoMatchingContinuationPrompt
- throwIO :: (HasCallStack, Exception e) => e -> IO a
- throw :: forall a e. (?callStack :: CallStack, 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
- 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
- impossibleError :: Addr# -> a
- impossibleConstraintError :: Addr# -> a
- nonExhaustiveGuardsError :: Addr# -> a
- patError :: Addr# -> a
- noMethodBindingError :: Addr# -> a
- typeError :: Addr# -> a
- nonTermination :: SomeException
- nestedAtomically :: SomeException
- noMatchingContinuationPrompt :: 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.
Constructors
| (Exception e, HasExceptionContext) => SomeException e |
Instances
| Exception SomeException Source # | This drops any attached Since: base-3.0 |
Defined in GHC.Internal.Exception.Type Methods toException :: SomeException -> SomeException Source # fromException :: SomeException -> Maybe SomeException Source # displayException :: SomeException -> String Source # backtraceDesired :: SomeException -> Bool Source # | |
| Show SomeException Source # | Since: ghc-internal-3.0 |
Defined in GHC.Internal.Exception.Type | |
class (Typeable e, Show e) => Exception e where Source #
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
instance Exception MyExceptionThe 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
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
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 Show
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromExceptionWe 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
Minimal complete definition
Nothing
Methods
toException :: e -> SomeException Source #
toException should produce a SomeException with no attached ExceptionContext.
fromException :: SomeException -> Maybe e Source #
displayException :: e -> String Source #
Render this exception value in a human-friendly manner.
Default implementation: show
Since: base-4.8.0.0
backtraceDesired :: e -> Bool Source #
Instances
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.
Instances
| Exception IOException Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception Methods toException :: IOException -> SomeException Source # fromException :: SomeException -> Maybe IOException Source # displayException :: IOException -> String Source # backtraceDesired :: IOException -> Bool Source # | |
| Show IOException Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
| Eq IOException Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception Methods (==) :: IOException -> IOException -> Bool Source # (/=) :: IOException -> IOException -> Bool Source # | |
data ArithException Source #
Arithmetic exceptions.
Constructors
| Overflow | |
| Underflow | |
| LossOfPrecision | |
| DivideByZero | |
| Denormal | |
| RatioZeroDenominator | Since: base-4.6.0.0 |
Instances
data ArrayException Source #
Exceptions generated by array operations
Constructors
| 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. |
Instances
newtype AssertionFailed Source #
Constructors
| AssertionFailed String |
Instances
| Exception AssertionFailed Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception Methods toException :: AssertionFailed -> SomeException Source # fromException :: SomeException -> Maybe AssertionFailed Source # | |
| Show AssertionFailed Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
data SomeAsyncException Source #
Superclass for asynchronous exceptions.
Since: base-4.7.0.0
Constructors
| Exception e => SomeAsyncException e |
Instances
| Exception SomeAsyncException Source # | Since: base-4.7.0.0 |
Defined in GHC.Internal.IO.Exception | |
| Show SomeAsyncException Source # | Since: base-4.7.0.0 |
Defined in GHC.Internal.IO.Exception | |
data AsyncException Source #
Asynchronous exceptions.
Constructors
| 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). |
Instances
asyncExceptionToException :: Exception e => e -> SomeException Source #
Since: base-4.7.0.0
asyncExceptionFromException :: Exception e => SomeException -> Maybe e Source #
Since: base-4.7.0.0
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.
Constructors
| NonTermination |
Instances
| Exception NonTermination Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: NonTermination -> SomeException Source # fromException :: SomeException -> Maybe NonTermination Source # | |
| Show NonTermination Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
data NestedAtomically Source #
Thrown when the program attempts to call atomically, from the stm
package, inside another call to atomically.
Constructors
| NestedAtomically |
Instances
| Exception NestedAtomically Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: NestedAtomically -> SomeException Source # fromException :: SomeException -> Maybe NestedAtomically Source # | |
| Show NestedAtomically Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
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.
Constructors
| BlockedIndefinitelyOnMVar |
Instances
| Exception BlockedIndefinitelyOnMVar Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
| Show BlockedIndefinitelyOnMVar Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
data FixIOException Source #
The exception thrown when an infinite cycle is detected in
fixIO.
Since: base-4.11.0.0
Constructors
| FixIOException |
Instances
| Exception FixIOException Source # | Since: base-4.11.0.0 |
Defined in GHC.Internal.IO.Exception Methods toException :: FixIOException -> SomeException Source # fromException :: SomeException -> Maybe FixIOException Source # | |
| Show FixIOException Source # | Since: base-4.11.0.0 |
Defined in GHC.Internal.IO.Exception | |
data BlockedIndefinitelyOnSTM Source #
The thread is waiting to retry an STM transaction, but there are no
other references to any TVars involved, so it can't ever continue.
Constructors
| BlockedIndefinitelyOnSTM |
Instances
| Exception BlockedIndefinitelyOnSTM Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
| Show BlockedIndefinitelyOnSTM Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception | |
data AllocationLimitExceeded Source #
This thread has exceeded its allocation limit. See
setAllocationCounter and
enableAllocationLimit.
Since: base-4.8.0.0
Constructors
| AllocationLimitExceeded |
Instances
| Exception AllocationLimitExceeded Source # | Since: base-4.8.0.0 |
Defined in GHC.Internal.IO.Exception | |
| Show AllocationLimitExceeded Source # | Since: base-4.7.1.0 |
Defined in GHC.Internal.IO.Exception | |
newtype CompactionFailed Source #
Compaction found an object that cannot be compacted. Functions
cannot be compacted, nor can mutable objects or pinned objects.
See compact.
Since: base-4.10.0.0
Constructors
| CompactionFailed String |
Instances
| Exception CompactionFailed Source # | Since: base-4.10.0.0 |
Defined in GHC.Internal.IO.Exception Methods toException :: CompactionFailed -> SomeException Source # fromException :: SomeException -> Maybe CompactionFailed Source # | |
| Show CompactionFailed Source # | Since: base-4.10.0.0 |
Defined in GHC.Internal.IO.Exception | |
There are no runnable threads, so the program is deadlocked.
The Deadlock exception is raised in the main thread only.
Constructors
| Deadlock |
Instances
| Exception Deadlock Source # | Since: base-4.1.0.0 |
Defined in GHC.Internal.IO.Exception Methods toException :: Deadlock -> SomeException Source # fromException :: SomeException -> Maybe Deadlock Source # displayException :: Deadlock -> String Source # backtraceDesired :: Deadlock -> Bool Source # | |
| Show Deadlock Source # | Since: base-4.1.0.0 |
newtype 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.
Constructors
| NoMethodError String |
Instances
| Exception NoMethodError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: NoMethodError -> SomeException Source # fromException :: SomeException -> Maybe NoMethodError Source # displayException :: NoMethodError -> String Source # backtraceDesired :: NoMethodError -> Bool Source # | |
| Show NoMethodError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
newtype PatternMatchFail Source #
A pattern match failed. The String gives information about the
source location of the pattern.
Constructors
| PatternMatchFail String |
Instances
| Exception PatternMatchFail Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: PatternMatchFail -> SomeException Source # fromException :: SomeException -> Maybe PatternMatchFail Source # | |
| Show PatternMatchFail Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
newtype RecConError Source #
An uninitialised record field was used. The String gives
information about the source location where the record was
constructed.
Constructors
| RecConError String |
Instances
| Exception RecConError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecConError -> SomeException Source # fromException :: SomeException -> Maybe RecConError Source # displayException :: RecConError -> String Source # backtraceDesired :: RecConError -> Bool Source # | |
| Show RecConError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
newtype 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.
Constructors
| RecSelError String |
Instances
| Exception RecSelError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecSelError -> SomeException Source # fromException :: SomeException -> Maybe RecSelError Source # displayException :: RecSelError -> String Source # backtraceDesired :: RecSelError -> Bool Source # | |
| Show RecSelError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
newtype 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.
Constructors
| RecUpdError String |
Instances
| Exception RecUpdError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: RecUpdError -> SomeException Source # fromException :: SomeException -> Maybe RecUpdError Source # displayException :: RecUpdError -> String Source # backtraceDesired :: RecUpdError -> Bool Source # | |
| Show RecUpdError Source # | Since: base-4.0 |
Defined in GHC.Internal.Control.Exception.Base | |
This is thrown when the user calls error. The first String is the
argument given to error, second String is the location.
Constructors
| ErrorCallWithLocation String String |
Instances
| Exception ErrorCall Source # | Since: base-4.0.0.0 |
Defined in GHC.Internal.Exception Methods toException :: ErrorCall -> SomeException Source # fromException :: SomeException -> Maybe ErrorCall Source # displayException :: ErrorCall -> String Source # backtraceDesired :: ErrorCall -> Bool Source # | |
| Show ErrorCall Source # | Since: base-4.0.0.0 |
| Eq ErrorCall Source # | Since: base-4.7.0.0 |
| Ord ErrorCall Source # | Since: base-4.7.0.0 |
Defined in GHC.Internal.Exception | |
An expression that didn't typecheck during compile time was called.
This is only possible with -fdefer-type-errors. The String gives
details about the failed type check.
Since: base-4.9.0.0
Instances
| Exception TypeError Source # | Since: base-4.9.0.0 |
Defined in GHC.Internal.Control.Exception.Base Methods toException :: TypeError -> SomeException Source # fromException :: SomeException -> Maybe TypeError Source # displayException :: TypeError -> String Source # backtraceDesired :: TypeError -> Bool Source # | |
| Show TypeError Source # | Since: base-4.9.0.0 |
data NoMatchingContinuationPrompt Source #
Thrown when the program attempts a continuation capture, but no prompt with the given prompt tag exists in the current continuation.
Since: base-4.18
Constructors
| NoMatchingContinuationPrompt |
Instances
| Exception NoMatchingContinuationPrompt Source # | Since: base-4.18 |
Defined in GHC.Internal.Control.Exception.Base | |
| Show NoMatchingContinuationPrompt Source # | Since: base-4.18 |
Defined in GHC.Internal.Control.Exception.Base | |
Throwing exceptions
throwIO :: (HasCallStack, Exception e) => e -> IO a Source #
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` () ===> throw e throwIO e `seq` () ===> ()
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 operations, whereas throw
does not. We say that throwIO throws *precise* exceptions and
throw, error, etc. all throw *imprecise* exceptions.
For example
throw e + error "boom" ===> error "boom" throw e + error "boom" ===> throw e
are both valid reductions and the compiler may pick any (loop, even), whereas
throwIO e >> error "boom" ===> throwIO e
will always throw e when executed.
See also the GHC wiki page on precise exceptions for a more technical introduction to how GHC optimises around precise vs. imprecise exceptions.
throw :: forall a e. (?callStack :: CallStack, Exception e) => e -> a Source #
Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO monad.
WARNING: You may want to use throwIO instead so that your pure code
stays exception-free.
throwTo :: Exception e => ThreadId -> e -> IO () Source #
throwTo raises an arbitrary exception in the target thread (GHC only).
Exception delivery synchronizes between the source and target thread:
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. Exception delivery is also atomic
with respect to other exceptions. Atomicity is a useful property to have
when dealing with race conditions: e.g. 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
Arguments
| :: 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 propagated further up. If you call it again, you
might get the opposite behaviour. This is ok, because catch is an
IO computation.
Arguments
| :: 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 a Source #
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 then it will be propagated
up to the next enclosing exception handler.
try a = catch (Right `liftM` a) (return . Left)
onException :: IO a -> IO b -> IO a Source #
Like finally, but only performs the final action if there was an
exception raised by the computation.
The evaluate function
evaluate :: a -> IO a Source #
Evaluate the argument to weak head normal form.
evaluate is typically used to uncover any exceptions that a lazy value
may contain, and possibly handle them.
evaluate only evaluates to weak head normal form. If deeper
evaluation is needed, the force function from Control.DeepSeq
may be handy:
evaluate $ force x
There is a subtle difference between evaluate xreturn $! xthrowIO and throw. If the lazy
value x throws an exception, return $! xIO action and will throw an exception instead. evaluate xIO action; that action will throw an
exception upon execution iff x throws an exception upon evaluation.
The practical implication of this difference is that due to the imprecise exceptions semantics,
(return $! error "foo") >> error "bar"
may throw either "foo" or "bar", depending on the optimizations
performed by the compiler. On the other hand,
evaluate (error "foo") >> error "bar"
is guaranteed to throw "foo".
The rule of thumb is to use evaluate to force or handle exceptions in
lazy values. If, on the other hand, you are forcing a lazy value for
efficiency reasons only and do not care about exceptions, you may
use return $! x
The mapException function
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a Source #
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 b Source #
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
releaseThis 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
MaskingState from the parent; that is, to start a thread in the
MaskedInterruptible state,
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 new thread in
an unmasked state use forkIOWithUnmask.
uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b Source #
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 a Source #
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.
Constructors
| Unmasked | asynchronous exceptions are unmasked (the normal state) |
| MaskedInterruptible | the state during |
| MaskedUninterruptible | the state during |
Instances
| Show MaskingState Source # | Since: base-4.3.0.0 |
Defined in GHC.Internal.IO | |
| Eq MaskingState Source # | Since: base-4.3.0.0 |
Defined in GHC.Internal.IO Methods (==) :: MaskingState -> MaskingState -> Bool Source # (/=) :: MaskingState -> MaskingState -> Bool Source # | |
getMaskingState :: IO MaskingState Source #
Returns the MaskingState for the current thread.
Assertions
assert :: Bool -> a -> a Source #
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
Arguments
| :: 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 wraps the release action with mask, which is sufficient to ensure
that the release action executes to completion when it does not invoke any
interruptible actions, even in the presence of asynchronous exceptions. For
example, hClose is uninterruptible when it is not racing other uses of the
handle. Similarly, closing a socket (from "network" package) is also
uninterruptible under similar conditions. An example of an interruptible
action is killThread. Completion of interruptible release actions can be
ensured by wrapping them in uninterruptibleMask_, but this risks making
the program non-responsive to Control-C, or timeouts. Another option is to
run the release action asynchronously in its own thread:
void $ uninterruptibleMask_ $ forkIO $ do { ... }The resource will be released as soon as possible, but the thread that invoked bracket will not block in an uninterruptible state.
bracket_ :: IO a -> IO b -> IO c -> IO c Source #
A variant of bracket where the return value from the first computation
is not required.
Arguments
| :: 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.
Arguments
| :: 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# -> a Source #
recConError :: Addr# -> a Source #
impossibleError :: Addr# -> a Source #
impossibleConstraintError :: Addr# -> a Source #
nonExhaustiveGuardsError :: Addr# -> a Source #
noMethodBindingError :: Addr# -> a Source #