Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | libraries@haskell.org |
Stability | experimental |
Portability | non-portable (extended exceptions) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
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.
Synopsis
- data SomeException = forall e.Exception e => SomeException e
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- data IOException
- data ArithException
- data ArrayException
- newtype AssertionFailed = AssertionFailed String
- data SomeAsyncException = forall e.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 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
- throw :: forall (r :: RuntimeRep). forall (a :: TYPE r). forall e. Exception e => e -> a
- throwIO :: Exception e => e -> IO a
- ioError :: IOError -> IO a
- throwTo :: Exception e => ThreadId -> e -> IO ()
- catch :: Exception e => IO a -> (e -> IO a) -> IO a
- catches :: IO a -> [Handler a] -> IO a
- data Handler a = forall e.Exception e => Handler (e -> 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)
- 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
- interruptible :: IO a -> IO a
- allowInterrupt :: IO ()
- 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
- onException :: IO a -> IO b -> IO a
The Exception type
data SomeException #
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 |
Instances
Show SomeException # | Since: base-3.0 |
Defined in GHC.Exception.Type showsPrec :: Int -> SomeException -> ShowS # show :: SomeException -> String # showList :: [SomeException] -> ShowS # | |
Exception SomeException # | Since: base-3.0 |
Defined in GHC.Exception.Type |
class (Typeable e, Show e) => Exception e where #
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
Nothing
toException :: e -> SomeException #
fromException :: SomeException -> Maybe e #
displayException :: e -> String #
Render this exception value in a human-friendly manner.
Default implementation:
.show
Since: base-4.8.0.0
Instances
data IOException #
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
Eq IOException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception (==) :: IOException -> IOException -> Bool Source # (/=) :: IOException -> IOException -> Bool Source # | |
Show IOException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> IOException -> ShowS # show :: IOException -> String # showList :: [IOException] -> ShowS # | |
Exception IOException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception |
data ArithException #
Arithmetic exceptions.
Overflow | |
Underflow | |
LossOfPrecision | |
DivideByZero | |
Denormal | |
RatioZeroDenominator | Since: base-4.6.0.0 |
Instances
Eq ArithException # | Since: base-3.0 |
Defined in GHC.Exception.Type (==) :: ArithException -> ArithException -> Bool Source # (/=) :: ArithException -> ArithException -> Bool Source # | |
Ord ArithException # | Since: base-3.0 |
Defined in GHC.Exception.Type compare :: ArithException -> ArithException -> Ordering Source # (<) :: ArithException -> ArithException -> Bool Source # (<=) :: ArithException -> ArithException -> Bool Source # (>) :: ArithException -> ArithException -> Bool Source # (>=) :: ArithException -> ArithException -> Bool Source # max :: ArithException -> ArithException -> ArithException Source # min :: ArithException -> ArithException -> ArithException Source # | |
Show ArithException # | Since: base-4.0.0.0 |
Defined in GHC.Exception.Type showsPrec :: Int -> ArithException -> ShowS # show :: ArithException -> String # showList :: [ArithException] -> ShowS # | |
Exception ArithException # | Since: base-4.0.0.0 |
Defined in GHC.Exception.Type |
data ArrayException #
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. |
Instances
Eq ArrayException # | Since: base-4.2.0.0 |
Defined in GHC.IO.Exception (==) :: ArrayException -> ArrayException -> Bool Source # (/=) :: ArrayException -> ArrayException -> Bool Source # | |
Ord ArrayException # | Since: base-4.2.0.0 |
Defined in GHC.IO.Exception compare :: ArrayException -> ArrayException -> Ordering Source # (<) :: ArrayException -> ArrayException -> Bool Source # (<=) :: ArrayException -> ArrayException -> Bool Source # (>) :: ArrayException -> ArrayException -> Bool Source # (>=) :: ArrayException -> ArrayException -> Bool Source # max :: ArrayException -> ArrayException -> ArrayException Source # min :: ArrayException -> ArrayException -> ArrayException Source # | |
Show ArrayException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> ArrayException -> ShowS # show :: ArrayException -> String # showList :: [ArrayException] -> ShowS # | |
Exception ArrayException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception |
newtype AssertionFailed #
Instances
Show AssertionFailed # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> AssertionFailed -> ShowS # show :: AssertionFailed -> String # showList :: [AssertionFailed] -> ShowS # | |
Exception AssertionFailed # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception |
data SomeAsyncException #
Superclass for asynchronous exceptions.
Since: base-4.7.0.0
forall e.Exception e => SomeAsyncException e |
Instances
Show SomeAsyncException # | Since: base-4.7.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> SomeAsyncException -> ShowS # show :: SomeAsyncException -> String # showList :: [SomeAsyncException] -> ShowS # | |
Exception SomeAsyncException # | Since: base-4.7.0.0 |
Defined in GHC.IO.Exception |
data AsyncException #
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). |
Instances
Eq AsyncException # | Since: base-4.2.0.0 |
Defined in GHC.IO.Exception (==) :: AsyncException -> AsyncException -> Bool Source # (/=) :: AsyncException -> AsyncException -> Bool Source # | |
Ord AsyncException # | Since: base-4.2.0.0 |
Defined in GHC.IO.Exception compare :: AsyncException -> AsyncException -> Ordering Source # (<) :: AsyncException -> AsyncException -> Bool Source # (<=) :: AsyncException -> AsyncException -> Bool Source # (>) :: AsyncException -> AsyncException -> Bool Source # (>=) :: AsyncException -> AsyncException -> Bool Source # max :: AsyncException -> AsyncException -> AsyncException Source # min :: AsyncException -> AsyncException -> AsyncException Source # | |
Show AsyncException # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> AsyncException -> ShowS # show :: AsyncException -> String # showList :: [AsyncException] -> ShowS # | |
Exception AsyncException # | Since: base-4.7.0.0 |
Defined in GHC.IO.Exception |
asyncExceptionToException :: Exception e => e -> SomeException #
Since: base-4.7.0.0
asyncExceptionFromException :: Exception e => SomeException -> Maybe e #
Since: base-4.7.0.0
data NonTermination #
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.
Instances
Show NonTermination # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> NonTermination -> ShowS # show :: NonTermination -> String # showList :: [NonTermination] -> ShowS # | |
Exception NonTermination # | Since: base-4.0 |
Defined in Control.Exception.Base |
data NestedAtomically #
Thrown when the program attempts to call atomically
, from the stm
package, inside another call to atomically
.
Instances
Show NestedAtomically # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> NestedAtomically -> ShowS # show :: NestedAtomically -> String # showList :: [NestedAtomically] -> ShowS # | |
Exception NestedAtomically # | Since: base-4.0 |
Defined in Control.Exception.Base |
data BlockedIndefinitelyOnMVar #
The thread is blocked on an MVar
, but there are no other references
to the MVar
so it can't ever continue.
Instances
Show BlockedIndefinitelyOnMVar # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> BlockedIndefinitelyOnMVar -> ShowS # show :: BlockedIndefinitelyOnMVar -> String # showList :: [BlockedIndefinitelyOnMVar] -> ShowS # | |
Exception BlockedIndefinitelyOnMVar # | Since: base-4.1.0.0 |
data BlockedIndefinitelyOnSTM #
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.
Instances
Show BlockedIndefinitelyOnSTM # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> BlockedIndefinitelyOnSTM -> ShowS # show :: BlockedIndefinitelyOnSTM -> String # showList :: [BlockedIndefinitelyOnSTM] -> ShowS # | |
Exception BlockedIndefinitelyOnSTM # | Since: base-4.1.0.0 |
data AllocationLimitExceeded #
This thread has exceeded its allocation limit. See
setAllocationCounter
and
enableAllocationLimit
.
Since: base-4.8.0.0
Instances
Show AllocationLimitExceeded # | Since: base-4.7.1.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> AllocationLimitExceeded -> ShowS # show :: AllocationLimitExceeded -> String # showList :: [AllocationLimitExceeded] -> ShowS # | |
Exception AllocationLimitExceeded # | Since: base-4.8.0.0 |
newtype CompactionFailed #
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
Instances
Show CompactionFailed # | Since: base-4.10.0.0 |
Defined in GHC.IO.Exception showsPrec :: Int -> CompactionFailed -> ShowS # show :: CompactionFailed -> String # showList :: [CompactionFailed] -> ShowS # | |
Exception CompactionFailed # | Since: base-4.10.0.0 |
Defined in GHC.IO.Exception |
There are no runnable threads, so the program is deadlocked.
The Deadlock
exception is raised in the main thread only.
Instances
Show Deadlock # | Since: base-4.1.0.0 |
Exception Deadlock # | Since: base-4.1.0.0 |
Defined in GHC.IO.Exception toException :: Deadlock -> SomeException # fromException :: SomeException -> Maybe Deadlock # displayException :: Deadlock -> String # |
newtype NoMethodError #
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.
Instances
Show NoMethodError # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> NoMethodError -> ShowS # show :: NoMethodError -> String # showList :: [NoMethodError] -> ShowS # | |
Exception NoMethodError # | Since: base-4.0 |
Defined in Control.Exception.Base |
newtype PatternMatchFail #
A pattern match failed. The String
gives information about the
source location of the pattern.
Instances
Show PatternMatchFail # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> PatternMatchFail -> ShowS # show :: PatternMatchFail -> String # showList :: [PatternMatchFail] -> ShowS # | |
Exception PatternMatchFail # | Since: base-4.0 |
Defined in Control.Exception.Base |
newtype RecConError #
An uninitialised record field was used. The String
gives
information about the source location where the record was
constructed.
Instances
Show RecConError # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> RecConError -> ShowS # show :: RecConError -> String # showList :: [RecConError] -> ShowS # | |
Exception RecConError # | Since: base-4.0 |
Defined in Control.Exception.Base |
newtype RecSelError #
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.
Instances
Show RecSelError # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> RecSelError -> ShowS # show :: RecSelError -> String # showList :: [RecSelError] -> ShowS # | |
Exception RecSelError # | Since: base-4.0 |
Defined in Control.Exception.Base |
newtype RecUpdError #
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.
Instances
Show RecUpdError # | Since: base-4.0 |
Defined in Control.Exception.Base showsPrec :: Int -> RecUpdError -> ShowS # show :: RecUpdError -> String # showList :: [RecUpdError] -> ShowS # | |
Exception RecUpdError # | Since: base-4.0 |
Defined in 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.
Instances
Eq ErrorCall # | Since: base-4.7.0.0 |
Ord ErrorCall # | Since: base-4.7.0.0 |
Defined in GHC.Exception | |
Show ErrorCall # | Since: base-4.0.0.0 |
Exception ErrorCall # | Since: base-4.0.0.0 |
Defined in GHC.Exception toException :: ErrorCall -> SomeException # fromException :: SomeException -> Maybe ErrorCall # displayException :: ErrorCall -> String # |
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
Show TypeError # | Since: base-4.9.0.0 |
Exception TypeError # | Since: base-4.9.0.0 |
Defined in Control.Exception.Base toException :: TypeError -> SomeException # fromException :: SomeException -> Maybe TypeError # displayException :: TypeError -> String # |
Throwing exceptions
throw :: forall (r :: RuntimeRep). forall (a :: TYPE r). forall e. Exception e => e -> a #
Throw an exception. Exceptions may be thrown from purely
functional code, but may only be caught within the IO
monad.
throwIO :: Exception e => e -> IO a #
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.
throwTo :: Exception e => ThreadId -> e -> IO () #
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
:: 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.
catches :: IO a -> [Handler a] -> IO a #
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
.
:: 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 #
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) #
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)
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
and evaluate
x
,
analogous to the difference between return
$!
xthrowIO
and throw
. If the lazy
value x
throws an exception,
will fail to return an
return
$!
xIO
action and will throw an exception instead.
, on the
other hand, always produces an 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 #
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 #
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
.
uninterruptibleMask :: ((forall a. IO a -> IO a) -> IO b) -> IO b #
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 #
Like uninterruptibleMask
, but does not pass a restore
action
to the argument.
data MaskingState #
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 |
Instances
Eq MaskingState # | Since: base-4.3.0.0 |
Defined in GHC.IO (==) :: MaskingState -> MaskingState -> Bool Source # (/=) :: MaskingState -> MaskingState -> Bool Source # | |
Show MaskingState # | Since: base-4.3.0.0 |
Defined in GHC.IO showsPrec :: Int -> MaskingState -> ShowS # show :: MaskingState -> String # showList :: [MaskingState] -> ShowS # |
getMaskingState :: IO MaskingState #
Returns the MaskingState
for the current thread.
interruptible :: IO a -> IO a #
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 () #
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
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 c #
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.