Control/Exception.hs

    {-# LANGUAGE Trustworthy #-}
    {-# LANGUAGE NoImplicitPrelude, ExistentialQuantification #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Control.Exception
    -- 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)
    --
    -- 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/.
    --
    -----------------------------------------------------------------------------

    module Control.Exception (

            -- * The Exception type
            SomeException(..),
            Exception(..),          -- class
            IOException,            -- instance Eq, Ord, Show, Typeable, Exception
            ArithException(..),     -- instance Eq, Ord, Show, Typeable, Exception
            ArrayException(..),     -- instance Eq, Ord, Show, Typeable, Exception
            AssertionFailed(..),
            SomeAsyncException(..),
            AsyncException(..),     -- instance Eq, Ord, Show, Typeable, Exception
            asyncExceptionToException, asyncExceptionFromException,

            NonTermination(..),
            NestedAtomically(..),
            BlockedIndefinitelyOnMVar(..),
            BlockedIndefinitelyOnSTM(..),
            AllocationLimitExceeded(..),
            CompactionFailed(..),
            Deadlock(..),
            NoMethodError(..),
            PatternMatchFail(..),
            RecConError(..),
            RecSelError(..),
            RecUpdError(..),
            ErrorCall(..),
            TypeError(..),

            -- * Throwing exceptions
            throw,
            throwIO,
            ioError,
            throwTo,

            -- * Catching Exceptions

            -- $catching

            -- ** Catching all exceptions

            -- $catchall

            -- ** The @catch@ functions
            catch,
            catches, Handler(..),
            catchJust,

            -- ** The @handle@ functions
            handle,
            handleJust,

            -- ** The @try@ functions
            try,
            tryJust,

            -- ** The @evaluate@ function
            evaluate,

            -- ** The @mapException@ function
            mapException,

            -- * Asynchronous Exceptions

            -- $async

            -- ** Asynchronous exception control

            -- |The following functions allow a thread to control delivery of
            -- asynchronous exceptions during a critical region.

            mask,
            mask_,
            uninterruptibleMask,
            uninterruptibleMask_,
            MaskingState(..),
            getMaskingState,
            interruptible,
            allowInterrupt,

            -- *** Applying @mask@ to an exception handler

            -- $block_handler

            -- *** Interruptible operations

            -- $interruptible

            -- * Assertions

            assert,

            -- * Utilities

            bracket,
            bracket_,
            bracketOnError,

            finally,
            onException,

      ) where

    import Control.Exception.Base

    import GHC.Base
    import GHC.IO (interruptible)

    -- | You need this when using 'catches'.
    data Handler a = forall e . Exception e => Handler (e -> IO a)

    -- | @since 4.6.0.0
    instance Functor Handler where
         fmap f (Handler h) = Handler (fmap f . h)

    {- |
    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)]
    -}
    catches :: IO a -> [Handler a] -> IO a
    catches io handlers = io `catch` catchesHandler handlers

    catchesHandler :: [Handler a] -> SomeException -> IO a
    catchesHandler handlers e = foldr tryHandler (throw e) handlers
        where tryHandler (Handler handler) res
                  = case fromException e of
                    Just e' -> handler e'
                    Nothing -> res

    -- -----------------------------------------------------------------------------
    -- Catching exceptions

    {- $catching

    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' or 'onException'.

     * 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' or 'catchJust'.

    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 -> ...

    -}

    -- -----------------------------------------------------------------------------
    -- Asynchronous exceptions

    -- | 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 4.4.0.0
    allowInterrupt :: IO ()
    allowInterrupt = interruptible $ return ()

    {- $async

     #AsynchronousExceptions# 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 'Control.Concurrent.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'.
    -}

    {- $block_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

     #interruptible#
    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
    'Control.Concurrent.MVar.takeMVar'
    (but not 'Control.Concurrent.MVar.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 'Control.Concurrent.MVar.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 'Control.Concurrent.MVar.takeMVar' interruptible, however, we can be
    safe in the knowledge that the thread can receive exceptions right up
    until the point when the 'Control.Concurrent.MVar.takeMVar' succeeds.
    Similar arguments apply for other interruptible operations like
    'System.IO.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 of 'uninterruptibleMask' 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 the @MVar@ is definitely full, and conversely @putMVar@ if the @MVar@ is definitely empty

     * @newEmptyMVar@, @newMVar@

     * @forkIO@, @forkIOUnmasked@, @myThreadId@

    -}

    {- $catchall

    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 occassions 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.
    -}