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Control.Exception
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 |
Description
This module provides support for raising and catching both built-in and user-defined exceptions.
In addition to exceptions thrown by IO
operations, exceptions may be thrown by pure code (imprecise exceptions) or by external events (asynchronous exceptions), but may only be caught in the IO
monad. For more details, see:
- A semantics for imprecise exceptions, by Simon Peyton Jones, Alastair Reid, Tony Hoare, Simon Marlow, Fergus Henderson, in PLDI'99.
- Asynchronous exceptions in Haskell, by Simon Marlow, Simon Peyton Jones, Andy Moran and John Reppy, in PLDI'01.
- An Extensible Dynamically-Typed Hierarchy of Exceptions, by Simon Marlow, in Haskell '06.
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
forall e.Exception e => SomeException e |
Instances
Exception SomeException Source | Since: base-3.0 |
Defined in GHC.Exception.Type MethodstoException :: SomeException -> SomeException Source fromException :: SomeException -> Maybe SomeException Source |
|
Show SomeException Source | Since: base-3.0 |
Defined in GHC.Exception.Type MethodsshowsPrec :: Int -> SomeException -> ShowS Source show :: SomeException -> String Source showList :: [SomeException] -> ShowS Source |
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 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
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 = 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
Minimal complete definition
Nothing
Methods
toException :: e -> SomeException Source
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
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.IO.Exception MethodstoException :: IOException -> SomeException Source |
|
Show IOException Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> IOException -> ShowS Source show :: IOException -> String Source showList :: [IOException] -> ShowS Source |
|
Eq IOException Source | Since: base-4.1.0.0 |
Defined in GHC.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
Instances
Exception AssertionFailed Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodstoException :: AssertionFailed -> SomeException Source fromException :: SomeException -> Maybe AssertionFailed Source |
|
Show AssertionFailed Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> AssertionFailed -> ShowS Source show :: AssertionFailed -> String Source showList :: [AssertionFailed] -> ShowS Source |
data SomeAsyncException Source
Superclass for asynchronous exceptions.
Since: base-4.7.0.0
Constructors
forall e.Exception e => SomeAsyncException e |
Instances
Exception SomeAsyncException Source | Since: base-4.7.0.0 |
Defined in GHC.IO.Exception MethodstoException :: SomeAsyncException -> SomeException Source fromException :: SomeException -> Maybe SomeAsyncException Source |
|
Show SomeAsyncException Source | Since: base-4.7.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> SomeAsyncException -> ShowS Source show :: SomeAsyncException -> String Source showList :: [SomeAsyncException] -> ShowS Source |
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
Instances
Exception NonTermination Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: NonTermination -> SomeException Source fromException :: SomeException -> Maybe NonTermination Source |
|
Show NonTermination Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> NonTermination -> ShowS Source show :: NonTermination -> String Source showList :: [NonTermination] -> ShowS Source |
data NestedAtomically Source
Thrown when the program attempts to call atomically
, from the stm
package, inside another call to atomically
.
Constructors
Instances
Exception NestedAtomically Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: NestedAtomically -> SomeException Source fromException :: SomeException -> Maybe NestedAtomically Source |
|
Show NestedAtomically Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> NestedAtomically -> ShowS Source show :: NestedAtomically -> String Source showList :: [NestedAtomically] -> ShowS Source |
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
Instances
Exception BlockedIndefinitelyOnMVar Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception |
|
Show BlockedIndefinitelyOnMVar Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> BlockedIndefinitelyOnMVar -> ShowS Source show :: BlockedIndefinitelyOnMVar -> String Source showList :: [BlockedIndefinitelyOnMVar] -> ShowS Source |
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.
Constructors
Instances
Exception BlockedIndefinitelyOnSTM Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception |
|
Show BlockedIndefinitelyOnSTM Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> BlockedIndefinitelyOnSTM -> ShowS Source show :: BlockedIndefinitelyOnSTM -> String Source showList :: [BlockedIndefinitelyOnSTM] -> ShowS Source |
data AllocationLimitExceeded Source
This thread has exceeded its allocation limit. See setAllocationCounter
and enableAllocationLimit
.
Since: base-4.8.0.0
Constructors
Instances
Exception AllocationLimitExceeded Source | Since: base-4.8.0.0 |
Defined in GHC.IO.Exception |
|
Show AllocationLimitExceeded Source | Since: base-4.7.1.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> AllocationLimitExceeded -> ShowS Source show :: AllocationLimitExceeded -> String Source showList :: [AllocationLimitExceeded] -> ShowS Source |
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
Instances
Exception CompactionFailed Source | Since: base-4.10.0.0 |
Defined in GHC.IO.Exception MethodstoException :: CompactionFailed -> SomeException Source fromException :: SomeException -> Maybe CompactionFailed Source |
|
Show CompactionFailed Source | Since: base-4.10.0.0 |
Defined in GHC.IO.Exception MethodsshowsPrec :: Int -> CompactionFailed -> ShowS Source show :: CompactionFailed -> String Source showList :: [CompactionFailed] -> ShowS Source |
There are no runnable threads, so the program is deadlocked. The Deadlock
exception is raised in the main thread only.
Constructors
Instances
Exception Deadlock Source | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception MethodstoException :: Deadlock -> SomeException Source fromException :: SomeException -> Maybe Deadlock Source displayException :: Deadlock -> String 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
Instances
Exception NoMethodError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: NoMethodError -> SomeException Source fromException :: SomeException -> Maybe NoMethodError Source |
|
Show NoMethodError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> NoMethodError -> ShowS Source show :: NoMethodError -> String Source showList :: [NoMethodError] -> ShowS Source |
newtype PatternMatchFail Source
A pattern match failed. The String
gives information about the source location of the pattern.
Constructors
Instances
Exception PatternMatchFail Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: PatternMatchFail -> SomeException Source fromException :: SomeException -> Maybe PatternMatchFail Source |
|
Show PatternMatchFail Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> PatternMatchFail -> ShowS Source show :: PatternMatchFail -> String Source showList :: [PatternMatchFail] -> ShowS Source |
newtype RecConError Source
An uninitialised record field was used. The String
gives information about the source location where the record was constructed.
Constructors
Instances
Exception RecConError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: RecConError -> SomeException Source |
|
Show RecConError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> RecConError -> ShowS Source show :: RecConError -> String Source showList :: [RecConError] -> ShowS Source |
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
Instances
Exception RecSelError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: RecSelError -> SomeException Source |
|
Show RecSelError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> RecSelError -> ShowS Source show :: RecSelError -> String Source showList :: [RecSelError] -> ShowS Source |
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
Instances
Exception RecUpdError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodstoException :: RecUpdError -> SomeException Source |
|
Show RecUpdError Source | Since: base-4.0 |
Defined in Control.Exception.Base MethodsshowsPrec :: Int -> RecUpdError -> ShowS Source show :: RecUpdError -> String Source showList :: [RecUpdError] -> ShowS Source |
This is thrown when the user calls error
. The first String
is the argument given to error
, second String
is the location.
Constructors
Instances
Exception ErrorCall Source | Since: base-4.0.0.0 |
Defined in GHC.Exception MethodstoException :: ErrorCall -> SomeException 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.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 Control.Exception.Base MethodstoException :: TypeError -> SomeException Source |
|
Show TypeError Source | Since: base-4.9.0.0 |
Throwing exceptions
throw :: forall (r :: RuntimeRep). forall (a :: TYPE r). forall e. 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.
throwIO :: 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.
ioError :: IOError -> IO a Source
Raise an IOException
in the IO
monad.
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
There are several functions for catching and examining exceptions; all of them may only be used from within the IO
monad.
Here's a rule of thumb for deciding which catch-style function to use:
- If you want to do some cleanup in the event that an exception is raised, use
finally
,bracket
oronException
. - To recover after an exception and do something else, the best choice is to use one of the
try
family. - ... unless you are recovering from an asynchronous exception, in which case use
catch
orcatchJust
.
The difference between using try
and catch
for recovery is that in catch
the handler is inside an implicit mask
(see "Asynchronous Exceptions") which is important when catching asynchronous exceptions, but when catching other kinds of exception it is unnecessary. Furthermore it is possible to accidentally stay inside the implicit mask
by tail-calling rather than returning from the handler, which is why we recommend using try
rather than catch
for ordinary exception recovery.
A typical use of tryJust
for recovery looks like this:
do r <- tryJust (guard . isDoesNotExistError) $ getEnv "HOME"
case r of
Left e -> ...
Right home -> ...
Catching all exceptions
It is possible to catch all exceptions, by using the type SomeException
:
catch f (\e -> ... (e :: SomeException) ...)
HOWEVER, this is normally not what you want to do!
For example, suppose you want to read a file, but if it doesn't exist then continue as if it contained "". You might be tempted to just catch all exceptions and return "" in the handler. However, this has all sorts of undesirable consequences. For example, if the user presses control-C at just the right moment then the UserInterrupt
exception will be caught, and the program will continue running under the belief that the file contains "". Similarly, if another thread tries to kill the thread reading the file then the ThreadKilled
exception will be ignored.
Instead, you should only catch exactly the exceptions that you really want. In this case, this would likely be more specific than even "any IO exception"; a permissions error would likely also want to be handled differently. Instead, you would probably want something like:
e <- tryJust (guard . isDoesNotExistError) (readFile f)
let str = either (const "") id e
There are occasions when you really do need to catch any sort of exception. However, in most cases this is just so you can do some cleaning up; you aren't actually interested in the exception itself. For example, if you open a file then you want to close it again, whether processing the file executes normally or throws an exception. However, in these cases you can use functions like bracket
, finally
and onException
, which never actually pass you the exception, but just call the cleanup functions at the appropriate points.
But sometimes you really do need to catch any exception, and actually see what the exception is. One example is at the very top-level of a program, you may wish to catch any exception, print it to a logfile or the screen, and then exit gracefully. For these cases, you can use catch
(or one of the other exception-catching functions) with the SomeException
type.
The catch
functions
Arguments
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.
catches :: IO a -> [Handler a] -> IO a Source
Sometimes you want to catch two different sorts of exception. You could do something like
f = expr `catch` \ (ex :: ArithException) -> handleArith ex
`catch` \ (ex :: IOException) -> handleIO ex
However, there are a couple of problems with this approach. The first is that having two exception handlers is inefficient. However, the more serious issue is that the second exception handler will catch exceptions in the first, e.g. in the example above, if handleArith
throws an IOException
then the second exception handler will catch it.
Instead, we provide a function catches
, which would be used thus:
f = expr `catches` [Handler (\ (ex :: ArithException) -> handleArith ex),
Handler (\ (ex :: IOException) -> handleIO ex)]
You need this when using catches
.
Instances
Arguments
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)) $
...
handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a Source
A version of catchJust
with the arguments swapped around (see handle
).
The try
functions
try :: Exception e => IO a -> IO (Either e a) Source
Similar to catch
, but returns an Either
result which is (Right a)
if no exception of type e
was raised, or (Left ex)
if an exception of type 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)
tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a) Source
A variant of try
that takes an exception predicate to select which exceptions are caught (c.f. catchJust
). If the exception does not match the predicate, it is re-thrown.
The evaluate
function
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 x
and return $! x
, analogous to the difference between throwIO
and throw
. If the lazy value x
throws an exception, return $! x
will fail to return an IO
action and will throw an exception instead. evaluate x
, on the other hand, always produces an IO
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 exceptions are so-called because they arise due to external influences, and can be raised at any point during execution. StackOverflow
and HeapOverflow
are two examples of system-generated asynchronous exceptions.
The primary source of asynchronous exceptions, however, is throwTo
:
throwTo :: ThreadId -> Exception -> IO ()
throwTo
(also killThread
) allows one running thread to raise an arbitrary exception in another thread. The exception is therefore asynchronous with respect to the target thread, which could be doing anything at the time it receives the exception. Great care should be taken with asynchronous exceptions; it is all too easy to introduce race conditions by the over zealous use of throwTo
.
Asynchronous exception control
The following functions allow a thread to control delivery of asynchronous exceptions during a critical region.
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
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 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
.
Like mask
, but does not pass a restore
action to the argument.
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.IO MethodsshowsPrec :: Int -> MaskingState -> ShowS Source show :: MaskingState -> String Source showList :: [MaskingState] -> ShowS Source |
|
Eq MaskingState Source | Since: base-4.3.0.0 |
Defined in GHC.IO Methods(==) :: MaskingState -> MaskingState -> Bool Source (/=) :: MaskingState -> MaskingState -> Bool Source |
getMaskingState :: IO MaskingState Source
Returns the MaskingState
for the current thread.
interruptible :: IO a -> IO a Source
Allow asynchronous exceptions to be raised even inside mask
, making the operation interruptible (see the discussion of "Interruptible operations" in Exception
).
When called outside mask
, or inside uninterruptibleMask
, this function has no effect.
Since: base-4.9.0.0
allowInterrupt :: IO () Source
When invoked inside mask
, this function allows a masked asynchronous exception to be raised, if one exists. It is equivalent to performing an interruptible operation (see #interruptible), but does not involve any actual blocking.
When called outside mask
, or inside uninterruptibleMask
, this function has no effect.
Since: base-4.4.0.0
Applying mask
to an exception handler
There's an implied mask
around every exception handler in a call to one of the catch
family of functions. This is because that is what you want most of the time - it eliminates a common race condition in starting an exception handler, because there may be no exception handler on the stack to handle another exception if one arrives immediately. If asynchronous exceptions are masked on entering the handler, though, we have time to install a new exception handler before being interrupted. If this weren't the default, one would have to write something like
mask $ \restore ->
catch (restore (...))
(\e -> handler)
If you need to unmask asynchronous exceptions again in the exception handler, restore
can be used there too.
Note that try
and friends do not have a similar default, because there is no exception handler in this case. Don't use try
for recovering from an asynchronous exception.
Interruptible operations
Some operations are interruptible, which means that they can receive asynchronous exceptions even in the scope of a mask
. Any function which may itself block is defined as interruptible; this includes takeMVar
(but not tryTakeMVar
), and most operations which perform some I/O with the outside world. The reason for having interruptible operations is so that we can write things like
mask $ \restore -> do
a <- takeMVar m
catch (restore (...))
(\e -> ...)
if the takeMVar
was not interruptible, then this particular combination could lead to deadlock, because the thread itself would be blocked in a state where it can't receive any asynchronous exceptions. With takeMVar
interruptible, however, we can be safe in the knowledge that the thread can receive exceptions right up until the point when the takeMVar
succeeds. Similar arguments apply for other interruptible operations like openFile
.
It is useful to think of mask
not as a way to completely prevent asynchronous exceptions, but as a way to switch from asynchronous mode to polling mode. The main difficulty with asynchronous exceptions is that they normally can occur anywhere, but within a mask
an asynchronous exception is only raised by operations that are interruptible (or call other interruptible operations). In many cases these operations may themselves raise exceptions, such as I/O errors, so the caller will usually be prepared to handle exceptions arising from the operation anyway. To perform an explicit poll for asynchronous exceptions inside mask
, use allowInterrupt
.
Sometimes it is too onerous to handle exceptions in the middle of a critical piece of stateful code. There are three ways to handle this kind of situation:
- Use STM. Since a transaction is always either completely executed or not at all, transactions are a good way to maintain invariants over state in the presence of asynchronous (and indeed synchronous) exceptions.
- Use
mask
, and avoid interruptible operations. In order to do this, we have to know which operations are interruptible. It is impossible to know for any given library function whether it might invoke an interruptible operation internally; so instead we give a list of guaranteed-not-to-be-interruptible operations below. - Use
uninterruptibleMask
. This is generally not recommended, unless you can guarantee that any interruptible operations invoked during the scope ofuninterruptibleMask
can only ever block for a short time. Otherwise,uninterruptibleMask
is a good way to make your program deadlock and be unresponsive to user interrupts.
The following operations are guaranteed not to be interruptible:
- operations on
IORef
from Data.IORef - STM transactions that do not use
retry
- everything from the
Foreign
modules - everything from Control.Exception except for
throwTo
tryTakeMVar
,tryPutMVar
,isEmptyMVar
takeMVar
if theMVar
is definitely full, and converselyputMVar
if theMVar
is definitely emptynewEmptyMVar
,newMVar
forkIO
,myThreadId
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
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 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
Like bracket
, but only performs the final action if there was an exception raised by the in-between computation.
Arguments
A specialised variant of bracket
with just a computation to run afterward.
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 University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/9.4.2/docs/libraries/base-4.17.0.0/Control-Exception.html