Lifted throwIO.

Lifted catch.

A variant of catch that fully forces evaluation of the result value to find all impure exceptions.

Lifted catchJust.

Catch an exception only if it satisfies a specific predicate.

catch specialized to catch IOException.

catch specialized to catch all exceptions considered to be synchronous.

catchSync ≡ catchIf @SomeException isSyncException

See the check exception type section for more information.

A variant of catchSync that fully forces evaluation of the result value to find all impure exceptions.

Flipped version of catch.

Flipped version of catchDeep.

Flipped version of catchJust.

Flipped version of catchIf.

Flipped version of catchIO.

Flipped version of catchSync.

Flipped version of catchSyncDeep.

Lifted try.

A variant of try that fully forces evaluation of the result value to find all impure exceptions.

Lifted tryJust.

Catch an exception only if it satisfies a specific predicate.

try specialized to catch IOException.

try specialized to catch all exceptions considered to be synchronous.

trySync ≡ tryIf @SomeException isSyncException

See the check exception type section for more information.

A variant of trySync that fully forces evaluation of the result value to find all impure exceptions.

Generalized version of Handler

Lifted catches.

A variant of catches that fully forces evaluation of the result value to find all impure exceptions.

Lifted bracket.

Lifted bracket_.

Lifted bracketOnError.

Generalization of bracket.

See generalBracket for more information.

A MonadMask computation may either succeed with a value, abort with an exception, or abort for some other reason. For example, in ExceptT e IO you can use throwM to abort with an exception (ExitCaseException) or throwE to abort with a value of type e (ExitCaseAbort).

Lifted finally.

Lifted onException.

Lifted evaluate.

Deeply evaluate a value using evaluate and NFData.

Check if the given exception is considered synchronous.

Check if the given exception is considered asynchronous.

Lifted mask.

Lifted mask_.

Lifted uninterruptibleMask.

Lifted uninterruptibleMask_.

Describes the behaviour of a thread when an asynchronous exception is received.

Lifted getMaskingState.

Lifted interruptible.

Lifted allowInterrupt.

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.

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

This function maps one exception into another as proposed in the paper "A semantics for imprecise exceptions".

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.

Arithmetic exceptions.

Exceptions generated by array operations

assert was applied to False.

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.

A pattern match failed. The String gives information about the source location of the pattern.

An uninitialised record field was used. The String gives information about the source location where the record was constructed.

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.

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.

This is thrown when the user calls error. The first String is the argument given to error, second String is the location.

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

Superclass for asynchronous exceptions.

Since: base-4.7.0.0

Asynchronous exceptions.

Since: base-4.7.0.0

Since: base-4.7.0.0

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.

Thrown when the program attempts to call atomically, from the stm package, inside another call to atomically.

The thread is blocked on an MVar, but there are no other references to the MVar so it can't ever continue.

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.

This thread has exceeded its allocation limit. See setAllocationCounter and enableAllocationLimit.

Since: base-4.8.0.0

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

There are no runnable threads, so the program is deadlocked. The Deadlock exception is raised in the main thread only.

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.