| base-4.1.0.0: Basic libraries | Source code | Contents | Index

Control.Exception.Base

Portability | non-portable (extended exceptions)
Stability | experimental
Maintainer | libraries@haskell.org

Contents

The Exception type
Throwing exceptions
Catching Exceptions
  The `catch` functions
  The `handle` functions
  The `try` functions
  The `evaluate` function
  The `mapException` function
Asynchronous Exceptions
  Asynchronous exception control
Assertions
Utilities
Calls for GHC runtime

Description
Extensible exceptions, except for multiple handlers.
Synopsis

data SomeException = forall e . Exception e => SomeException e
class (Typeable e, Show e) => Exception e where

toException :: e -> SomeException
fromException :: SomeException -> Maybe e

data IOException

data ArithException

= Overflow
| Underflow
| LossOfPrecision
| DivideByZero
| Denormal

data ArrayException

= IndexOutOfBounds String
| UndefinedElement String

data AssertionFailed = AssertionFailed String

data AsyncException

= StackOverflow
| HeapOverflow
| ThreadKilled
| UserInterrupt

data NonTermination = NonTermination
data NestedAtomically = NestedAtomically
data BlockedOnDeadMVar = BlockedOnDeadMVar
data BlockedIndefinitely = BlockedIndefinitely
data Deadlock = Deadlock
data NoMethodError = NoMethodError String
data PatternMatchFail = PatternMatchFail String
data RecConError = RecConError String
data RecSelError = RecSelError String
data RecUpdError = RecUpdError String
data ErrorCall = ErrorCall String
throwIO :: Exception e => e -> IO a
throw :: Exception e => e -> a
ioError :: IOError -> IO a
throwTo :: Exception e => ThreadId -> e -> IO ()
catch :: Exception e => IO a -> (e -> IO a) -> IO a
catchJust :: Exception e => (e -> Maybe b) -> IO a -> (b -> IO a) -> IO a
handle :: Exception e => (e -> IO a) -> IO a -> IO a
handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
try :: Exception e => IO a -> IO (Either e a)
tryJust :: Exception e => (e -> Maybe b) -> IO a -> IO (Either b a)
onException :: IO a -> IO b -> IO a
evaluate :: a -> IO a
mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a
block :: IO a -> IO a
unblock :: IO a -> IO a
blocked :: IO Bool
assert :: Bool -> a -> a
bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
bracket_ :: IO a -> IO b -> IO c -> IO c
bracketOnError :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
finally :: IO a -> IO b -> IO a
recSelError :: Addr# -> a
recConError :: Addr# -> a
irrefutPatError :: Addr# -> a
runtimeError :: Addr# -> a
nonExhaustiveGuardsError :: Addr# -> a
patError :: Addr# -> a
noMethodBindingError :: Addr# -> a
nonTermination :: SomeException
nestedAtomically :: SomeException

The Exception type

data SomeException | Source

The `SomeException` type is the root of the exception type hierarchy. When an exception of type `e` is thrown, behind the scenes it is encapsulated in a `SomeException`.
Constructors

forall e . Exception e => SomeException e |

Instances

Show SomeException
Typeable SomeException
Exception SomeException

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, Typeable)

     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
         deriving Typeable

     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
         deriving Typeable

     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 (Typeable, 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

Methods

toException :: e -> SomeException | Source

fromException :: SomeException -> Maybe e | Source

Instances

Exception ArithException
Exception ErrorCall
Exception SomeException
Exception IOException
Exception ExitCode
Exception ArrayException
Exception AsyncException
Exception AssertionFailed
Exception Deadlock
Exception BlockedIndefinitely
Exception BlockedOnDeadMVar
Exception Dynamic
Exception NestedAtomically
Exception NonTermination
Exception NoMethodError
Exception RecUpdError
Exception RecConError
Exception RecSelError
Exception PatternMatchFail
Exception Exception
Exception Timeout

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

Eq IOException
Show IOException
Typeable IOException
Exception IOException

data ArithException | Source

Arithmetic exceptions.
Constructors

Overflow |
Underflow |
LossOfPrecision |
DivideByZero |
Denormal |

Instances

Eq ArithException
Ord ArithException
Show ArithException
Typeable ArithException
Exception ArithException

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

Eq ArrayException
Ord ArrayException
Show ArrayException
Typeable ArrayException
Exception ArrayException

data AssertionFailed | Source

Exceptions generated by `assert`. The `String` gives information about the source location of the assertion.
Constructors

AssertionFailed String |

Instances

Show AssertionFailed
Typeable AssertionFailed
Exception AssertionFailed

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:

- It is undefined which thread receives this exception.
- GHC currently does not throw `HeapOverflow` exceptions.

ThreadKilled | This exception is raised by another thread calling Control.Concurrent.killThread, or by the system if it needs to terminate the thread for some reason.
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
Ord AsyncException
Show AsyncException
Typeable AsyncException
Exception AsyncException

data NonTermination | Source

Thrown when the runtime system detects that the computation is guaranteed not to terminate. Note that there is no guarantee that the runtime system will notice whether any given computation is guaranteed to terminate or not.
Constructors

NonTermination |

Instances

Show NonTermination
Typeable NonTermination
Exception NonTermination

data NestedAtomically | Source

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

NestedAtomically |

Instances

Show NestedAtomically
Typeable NestedAtomically
Exception NestedAtomically

data BlockedOnDeadMVar | Source

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

BlockedOnDeadMVar |

Instances

Show BlockedOnDeadMVar
Typeable BlockedOnDeadMVar
Exception BlockedOnDeadMVar

data BlockedIndefinitely | Source

The thread is awiting to retry an STM transaction, but there are no other references to any `TVar`s involved, so it can't ever continue.
Constructors

BlockedIndefinitely |

Instances

Show BlockedIndefinitely
Typeable BlockedIndefinitely
Exception BlockedIndefinitely

data Deadlock | Source

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

Deadlock |

Instances

Show Deadlock
Typeable Deadlock
Exception Deadlock

data NoMethodError | Source

A class method without a definition (neither a default definition, nor a definition in the appropriate instance) was called. The `String` gives information about which method it was.
Constructors

NoMethodError String |

Instances

Show NoMethodError
Typeable NoMethodError
Exception NoMethodError

data PatternMatchFail | Source

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

PatternMatchFail String |

Instances

Show PatternMatchFail
Typeable PatternMatchFail
Exception PatternMatchFail

data RecConError | Source

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

RecConError String |

Instances

Show RecConError
Typeable RecConError
Exception RecConError

data RecSelError | Source

A record selector was applied to a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another. The `String` gives information about the source location of the record selector.
Constructors

RecSelError String |

Instances

Show RecSelError
Typeable RecSelError
Exception RecSelError

data RecUpdError | Source

A record update was performed on a constructor without the appropriate field. This can only happen with a datatype with multiple constructors, where some fields are in one constructor but not another. The `String` gives information about the source location of the record update.
Constructors

RecUpdError String |

Instances

Show RecUpdError
Typeable RecUpdError
Exception RecUpdError

data ErrorCall | Source

This is thrown when the user calls `error`. The `String` is the argument given to `error`.
Constructors

ErrorCall String |

Instances

Show ErrorCall
Typeable ErrorCall
Exception ErrorCall

Throwing exceptions

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` 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.

throw :: 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.

ioError :: IOError -> IO a | Source

Raise an `IOError` in the `IO` monad.

throwTo :: Exception e => ThreadId -> e -> IO () | Source

`throwTo` raises an arbitrary exception in the target thread (GHC only).

`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. This is a useful property to know when dealing with race conditions: eg. 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.

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 `block` or not.

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).

There is currently no guarantee that the exception delivered by `throwTo` will be delivered at the first possible opportunity. In particular, a thread may unblock and then re-block exceptions (using `unblock` and `block`) without receiving a pending `throwTo`. This is arguably undesirable behaviour.

Catching Exceptions
The `catch` functions

catch | Source

:: 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 $catchall 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.

Note that the Prelude also exports a function called Prelude.catch with a similar type to Control.Exception.catch, except that the Prelude version only catches the IO and user families of exceptions (as required by Haskell 98).

We recommend either hiding the Prelude version of Prelude.catch when importing Control.Exception:

     import Prelude hiding (catch)

or importing Control.Exception qualified, to avoid name-clashes:

     import qualified Control.Exception as C

and then using `C.catch`

catchJust | Source

:: Exception e |
=> e -> Maybe b | Predicate to select exceptions
-> IO a | Computation to run
-> b -> IO a | Handler
-> IO a |

The function `catchJust` is like `catch`, but it takes an extra argument which is an exception predicate, a function which selects which type of exceptions we're interested in.

     catchJust (\e -> if isDoesNotExistErrorType (ioeGetErrorType e) then Just () else Nothing)
               (readFile f)
               (\_ -> do hPutStrLn stderr ("No such file: " ++ show f)
                         return "")

Any other exceptions which are not matched by the predicate are re-raised, and may be caught by an enclosing `catch`, `catchJust`, etc.

The `handle` functions

handle :: Exception e => (e -> IO a) -> IO a -> IO a | Source

A version of `catch` with the arguments swapped around; useful in situations where the code for the handler is shorter. For example:

       do handle (\NonTermination -> exitWith (ExitFailure 1)) $
          ...

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 than it will be propogated up to the next enclosing exception handler.

      try a = catch (Right `liftM` a) (return . Left)

Note that System.IO.Error also exports a function called System.IO.Error.try with a similar type to Control.Exception.try, except that it catches only the IO and user families of exceptions (as required by the Haskell 98 `IO` module).

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.

onException :: IO a -> IO b -> IO a | Source

Like `finally`, but only performs the final action if there was an exception raised by the computation.
The `evaluate` function

evaluate :: a -> IO a | Source

Forces its argument to be evaluated to weak head normal form when the resultant `IO` action is executed. It can be used to order evaluation with respect to other `IO` operations; its semantics are given by

       evaluate x `seq` y    ==>  y
       evaluate x `catch` f  ==>  (return $! x) `catch` f
       evaluate x >>= f      ==>  (return $! x) >>= f

Note: the first equation implies that `(evaluate x)` is not the same as `(return $! x)`. A correct definition is

       evaluate x = (return $! x) >>= return

The `mapException` function

mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a | Source

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

block :: IO a -> IO a | Source

Applying `block` to a computation will execute that computation with asynchronous exceptions blocked. That is, any thread which attempts to raise an exception in the current thread with Control.Exception.throwTo will be blocked until asynchronous exceptions are enabled again. There's no need to worry about re-enabling asynchronous exceptions; that is done automatically on exiting the scope of `block`.

Threads created by Control.Concurrent.forkIO inherit the blocked state from the parent; that is, to start a thread in blocked mode, use `block $ forkIO ...`. This is particularly useful if you need to establish an exception handler in the forked thread before any asynchronous exceptions are received.

unblock :: IO a -> IO a | Source

To re-enable asynchronous exceptions inside the scope of `block`, `unblock` can be used. It scopes in exactly the same way, so on exit from `unblock` asynchronous exception delivery will be disabled again.

blocked :: IO Bool | Source

returns True if asynchronous exceptions are blocked in the current thread.
Assertions

assert :: Bool -> a -> a | Source

If the first argument evaluates to `True`, then the result is the second argument. Otherwise an AssertionFailed exception is raised, containing a `String` with the source file and line number of the call to `assert`.

Assertions can normally be turned on or off with a compiler flag (for GHC, assertions are normally on unless optimisation is turned on with `-O` or the `-fignore-asserts` option is given). When assertions are turned off, the first argument to `assert` is ignored, and the second argument is returned as the result.

Utilities

bracket | Source

:: |
=> 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 | Source

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

bracketOnError | Source

:: |
=> 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.

finally | Source

:: |
=> IO a | computation to run first
-> IO b | computation to run afterward (even if an exception was raised)
-> IO a |
A specialised variant of `bracket` with just a computation to run afterward.

Calls for GHC runtime

recSelError :: Addr# -> a | Source

recConError :: Addr# -> a | Source

irrefutPatError :: Addr# -> a | Source

runtimeError :: Addr# -> a | Source

nonExhaustiveGuardsError :: Addr# -> a | Source

patError :: Addr# -> a | Source

noMethodBindingError :: Addr# -> a | Source

nonTermination :: SomeException | Source

nestedAtomically :: SomeException | Source

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