A class for monads in which exceptions may be thrown.

Instances should obey the following law:

throwM e >> x = throwM e

In other words, throwing an exception short-circuits the rest of the monadic computation.

A convenience function for throwing a user error. This is useful for cases where it would be too high a burden to define your own exception type.

This throws an exception of type StringException. When GHC supports it (base 4.9 and GHC 8.0 and onward), it includes a call stack.

Since: safe-exceptions-0.1.5.0

Exception type thrown by throwString.

Note that the second field of the data constructor depends on GHC/base version. For base 4.9 and GHC 8.0 and later, the second field is a call stack. Previous versions of GHC and base do not support call stacks, and the field is simply unit (provided to make pattern matching across GHC versions easier).

Since: safe-exceptions-0.1.5.0

A class for monads which allow exceptions to be caught, in particular exceptions which were thrown by throwM.

Instances should obey the following law:

catch (throwM e) f = f e

Note that the ability to catch an exception does not guarantee that we can deal with all possible exit points from a computation. Some monads, such as continuation-based stacks, allow for more than just a success/failure strategy, and therefore catch cannot be used by those monads to properly implement a function such as finally. For more information, see MonadMask.

catch specialized to only catching IOExceptions

Since: safe-exceptions-0.1.3.0

Catch all IOError (eqv. IOException) exceptions. Still somewhat too general, but better than using catchAll. See catchIf for an easy way of catching specific IOErrors based on the predicates in System.IO.Error.

catch specialized to catch all synchronous exception

Since: safe-exceptions-0.1.0.0

Same as catch, but fully force evaluation of the result value to find all impure exceptions.

Since: safe-exceptions-0.1.1.0

catchDeep specialized to catch all synchronous exception

Since: safe-exceptions-0.1.1.0

catch without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: safe-exceptions-0.1.0.0

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.

Since: safe-exceptions-0.1.4.0

Flipped version of catch

Since: safe-exceptions-0.1.0.0

handle specialized to only catching IOExceptions

Since: safe-exceptions-0.1.3.0

Flipped catchIOError

Flipped version of catchAny

Since: safe-exceptions-0.1.0.0

Flipped version of catchDeep

Since: safe-exceptions-0.1.1.0

Flipped version of catchAnyDeep

Since: safe-exceptions-0.1.1.0

Flipped version of catchAsync

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: safe-exceptions-0.1.0.0

Flipped catchJust.

Since: safe-exceptions-0.1.4.0

Same as upstream try, but will not catch asynchronous exceptions

Since: safe-exceptions-0.1.0.0

try specialized to only catching IOExceptions

Since: safe-exceptions-0.1.3.0

try specialized to catch all synchronous exceptions

Since: safe-exceptions-0.1.0.0

Same as try, but fully force evaluation of the result value to find all impure exceptions.

Since: safe-exceptions-0.1.1.0

tryDeep specialized to catch all synchronous exceptions

Since: safe-exceptions-0.1.1.0

try without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: safe-exceptions-0.1.0.0

A variant of try that takes an exception predicate to select which exceptions are caught.

Since: safe-exceptions-0.1.4.0

Generalized version of Handler

Same as upstream catches, but will not catch asynchronous exceptions

Since: safe-exceptions-0.1.2.0

Same as catches, but fully force evaluation of the result value to find all impure exceptions.

Since: safe-exceptions-0.1.2.0

catches without async exception safety

Generally it's better to avoid using this function since we do not want to recover from async exceptions, see https://github.com/fpco/safe-exceptions#quickstart

Since: safe-exceptions-0.1.2.0

A class for monads which provide for the ability to account for all possible exit points from a computation, and to mask asynchronous exceptions. Continuation-based monads are invalid instances of this class.

Instances should ensure that, in the following code:

fg = f `finally` g

The action g is called regardless of what occurs within f, including async exceptions. Some monads allow f to abort the computation via other effects than throwing an exception. For simplicity, we will consider aborting and throwing an exception to be two forms of "throwing an error".

If f and g both throw an error, the error thrown by fg depends on which errors we're talking about. In a monad transformer stack, the deeper layers override the effects of the inner layers; for example, ExceptT e1 (Except e2) a represents a value of type Either e2 (Either e1 a), so throwing both an e1 and an e2 will result in Left e2. If f and g both throw an error from the same layer, instances should ensure that the error from g wins.

Effects other than throwing an error are also overridden by the deeper layers. For example, StateT s Maybe a represents a value of type s -> Maybe (a, s), so if an error thrown from f causes this function to return Nothing, any changes to the state which f also performed will be erased. As a result, g will see the state as it was before f. Once g completes, f's error will be rethrown, so g' state changes will be erased as well. This is the normal interaction between effects in a monad transformer stack.

By contrast, lifted-base's version of finally always discards all of g's non-IO effects, and g never sees any of f's non-IO effects, regardless of the layer ordering and regardless of whether f throws an error. This is not the result of interacting effects, but a consequence of MonadBaseControl's approach.

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).

Async safe version of onException

Since: safe-exceptions-0.1.0.0

Async safe version of bracket

Since: safe-exceptions-0.1.0.0

Async safe version of bracket_

Since: safe-exceptions-0.1.0.0

Async safe version of finally

Since: safe-exceptions-0.1.0.0

Like onException, but provides the handler the thrown exception.

Since: safe-exceptions-0.1.0.0

Async safe version of bracketOnError

Since: safe-exceptions-0.1.0.0

A variant of bracketOnError where the return value from the first computation is not required.

Since: safe-exceptions-0.1.0.0

Async safe version of bracket with access to the exception in the cleanup action.

Since: safe-exceptions-0.1.7.0

Wrap up an asynchronous exception to be treated as a synchronous exception

This is intended to be created via toSyncException

Since: safe-exceptions-0.1.0.0

Convert an exception into a synchronous exception

For synchronous exceptions, this is the same as toException. For asynchronous exceptions, this will wrap up the exception with SyncExceptionWrapper

Since: safe-exceptions-0.1.0.0

Wrap up a synchronous exception to be treated as an asynchronous exception

This is intended to be created via toAsyncException

Since: safe-exceptions-0.1.0.0

Convert an exception into an asynchronous exception

For asynchronous exceptions, this is the same as toException. For synchronous exceptions, this will wrap up the exception with AsyncExceptionWrapper

Since: safe-exceptions-0.1.0.0

Check if the given exception is synchronous

Since: safe-exceptions-0.1.0.0

Check if the given exception is asynchronous

Since: safe-exceptions-0.1.0.0

Lifted version of evaluate.

Deeply evaluate a value using evaluate and NFData.

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

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.