| base-4.2.0.2: Basic libraries | Contents | Index

Control.Exception

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

Contents

The Exception type
Throwing exceptions
Catching Exceptions
  Catching all exceptions
  The `catch` functions
  The `handle` functions
  The `try` functions
  The `evaluate` function
  The `mapException` function
Asynchronous Exceptions
  Asynchronous exception control
    Applying `block` to an exception handler
    Interruptible operations
Assertions
Utilities

Description

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

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 BlockedIndefinitelyOnMVar = BlockedIndefinitelyOnMVar
data BlockedIndefinitelyOnSTM = BlockedIndefinitelyOnSTM
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
throw :: 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
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
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`.
Constructors

forall e . Exception e => SomeException e |

Instances

Show SomeException
Typeable SomeException
Exception SomeException

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, 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
fromException :: SomeException -> Maybe e

Instances

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

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
Show IOException
Typeable IOException
Exception IOException

data ArithException

Arithmetic exceptions.
Constructors

Overflow |
Underflow |
LossOfPrecision |
DivideByZero |
Denormal |

Instances

Eq ArithException
Ord ArithException
Show ArithException
Typeable ArithException
Exception ArithException

data ArrayException

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

`assert` was applied to `False`.
Constructors

AssertionFailed String |

Instances

Show AssertionFailed
Typeable AssertionFailed
Exception AssertionFailed

data AsyncException

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

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

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 BlockedIndefinitelyOnMVar

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

BlockedIndefinitelyOnMVar |

Instances

Show BlockedIndefinitelyOnMVar
Typeable BlockedIndefinitelyOnMVar
Exception BlockedIndefinitelyOnMVar

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.
Constructors

BlockedIndefinitelyOnSTM |

Instances

Show BlockedIndefinitelyOnSTM
Typeable BlockedIndefinitelyOnSTM
Exception BlockedIndefinitelyOnSTM

data Deadlock

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

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

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

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

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

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

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
throw :: 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.

ioError :: IOError -> IO a
Raise an `IOError` in the `IO` monad.
throwTo :: Exception e => ThreadId -> e -> IO ()

`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.

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 `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 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`.

Blocked `throwTo` is fair: if multiple threads are trying to throw an exception to the same target thread, they will succeed in FIFO order.

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` 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 `block` (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 `block` 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 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.

The `catch` functions
catch

:: 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 previous section "Catching all exceptions" 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`

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)]

data Handler a

You need this when using `catches`.
Constructors

forall e . Exception e => Handler (e -> IO a) |

catchJust

:: 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)) $
          ...

handleJust :: Exception e => (e -> Maybe b) -> (b -> IO a) -> IO a -> IO a
A version of `catchJust` with the arguments swapped around (see `handle`).
The `try` functions
try :: Exception e => IO a -> IO (Either e a)

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)
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.
The `evaluate` function
evaluate :: a -> IO a

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
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 throwDynTo and 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`.

Asynchronous exception control
The following two functions allow a thread to control delivery of asynchronous exceptions during a critical region.
block :: IO a -> IO a

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
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
returns True if asynchronous exceptions are blocked in the current thread.
Applying `block` to an exception handler

There's an implied `block` 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 blocked 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

          block (
               catch (unblock (...))
                          (\e -> handler)
          )

If you need to unblock asynchronous exceptions again in the exception handler, just use `unblock` as normal.

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 `block`. 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

          block (
             a <- takeMVar m
             catch (unblock (...))
                   (\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.

Assertions
assert :: Bool -> a -> a

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

:: 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.
bracketOnError

:: 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

:: 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.

onException :: IO a -> IO b -> IO a
Like `finally`, but only performs the final action if there was an exception raised by the computation.
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