Documentation

A value of type IO a is a computation which, when performed, does some I/O before returning a value of type a.

There is really only one way to "perform" an I/O action: bind it to Main.main in your program. When your program is run, the I/O will be performed. It isn't possible to perform I/O from an arbitrary function, unless that function is itself in the IO monad and called at some point, directly or indirectly, from Main.main.

IO is a monad, so IO actions can be combined using either the do-notation or the >> and >>= operations from the Monad class.

This is the "back door" into the IO monad, allowing IO computation to be performed at any time. For this to be safe, the IO computation should be free of side effects and independent of its environment.

If the I/O computation wrapped in unsafePerformIO performs side effects, then the relative order in which those side effects take place (relative to the main I/O trunk, or other calls to unsafePerformIO) is indeterminate. Furthermore, when using unsafePerformIO to cause side-effects, you should take the following precautions to ensure the side effects are performed as many times as you expect them to be. Note that these precautions are necessary for GHC, but may not be sufficient, and other compilers may require different precautions:

• Use {-# NOINLINE foo #-} as a pragma on any function foo that calls unsafePerformIO. If the call is inlined, the I/O may be performed more than once.
• Use the compiler flag -fno-cse to prevent common sub-expression elimination being performed on the module, which might combine two side effects that were meant to be separate. A good example is using multiple global variables (like test in the example below).
• Make sure that the either you switch off let-floating (-fno-full-laziness), or that the call to unsafePerformIO cannot float outside a lambda. For example, if you say: f x = unsafePerformIO (newIORef []) you may get only one reference cell shared between all calls to f. Better would be f x = unsafePerformIO (newIORef [x]) because now it can't float outside the lambda.

It is less well known that unsafePerformIO is not type safe. For example:

    test :: IORef [a]
    test = unsafePerformIO $ newIORef []

    main = do
            writeIORef test [42]
            bang <- readIORef test
            print (bang :: [Char])

This program will core dump. This problem with polymorphic references is well known in the ML community, and does not arise with normal monadic use of references. There is no easy way to make it impossible once you use unsafePerformIO. Indeed, it is possible to write coerce :: a -> b with the help of unsafePerformIO. So be careful!

WARNING: If you're looking for "a way to get a String from an 'IO String'", then unsafePerformIO is not the way to go. Learn about do-notation and the <- syntax element before you proceed.

unsafeInterleaveIO allows an IO computation to be deferred lazily. When passed a value of type IO a, the IO will only be performed when the value of the a is demanded. This is used to implement lazy file reading, see hGetContents.

This version of unsafePerformIO is more efficient because it omits the check that the IO is only being performed by a single thread. Hence, when you use unsafeDupablePerformIO, there is a possibility that the IO action may be performed multiple times (on a multiprocessor), and you should therefore ensure that it gives the same results each time. It may even happen that one of the duplicated IO actions is only run partially, and then interrupted in the middle without an exception being raised. Therefore, functions like bracket cannot be used safely within unsafeDupablePerformIO.

@since base-4.4.0.0

unsafeDupableInterleaveIO allows an IO computation to be deferred lazily. When passed a value of type IO a, the IO will only be performed when the value of the a is demanded.

The computation may be performed multiple times by different threads, possibly at the same time. To ensure that the computation is performed only once, use unsafeInterleaveIO instead.

Ensures that the suspensions under evaluation by the current thread are unique; that is, the current thread is not evaluating anything that is also under evaluation by another thread that has also executed noDuplicate.

This operation is used in the definition of unsafePerformIO to prevent the IO action from being executed multiple times, which is usually undesirable.

Embed a strict state thread in an IO action. The RealWorld parameter indicates that the internal state used by the ST computation is a special one supplied by the IO monad, and thus distinct from those used by invocations of runST.

Convert an IO action into an ST action. The type of the result is constrained to use a RealWorld state thread, and therefore the result cannot be passed to runST.

Convert an IO action to an ST action. This relies on IO and ST having the same representation modulo the constraint on the state thread type parameter.

Convert an ST action to an IO action. This relies on IO and ST having the same representation modulo the constraint on the state thread type parameter.

For an example demonstrating why this is unsafe, see https://mail.haskell.org/pipermail/haskell-cafe/2009-April/060719.html

File and directory names are values of type String, whose precise meaning is operating system dependent. Files can be opened, yielding a handle which can then be used to operate on the contents of that file.

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 section "Catching all exceptions" (in Control.Exception) 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 propagated further up. If you call it again, you might get the opposite behaviour. This is ok, because catch is an IO computation.

Catch an exception in the IO monad.

Note that this function is strict in the action. That is, catchException undefined b == _|_. See for details.

Catch any Exception type in the IO monad.

Note that this function is strict in the action. That is, catchAny undefined b == _|_. See for details.

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` ()  ===> throw e
throwIO e `seq` ()  ===> ()

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 operations, whereas throw does not. We say that throwIO throws *precise* exceptions and throw, error, etc. all throw *imprecise* exceptions. For example

throw e + error "boom" ===> error "boom"
throw e + error "boom" ===> throw e

are both valid reductions and the compiler may pick any (loop, even), whereas

throwIO e >> error "boom" ===> throwIO e

will always throw e when executed.

See also the GHC wiki page on precise exceptions for a more technical introduction to how GHC optimises around precise vs. imprecise exceptions.

Executes an IO computation with asynchronous exceptions masked. That is, any thread which attempts to raise an exception in the current thread with throwTo will be blocked until asynchronous exceptions are unmasked again.

The argument passed to mask is a function that takes as its argument another function, which can be used to restore the prevailing masking state within the context of the masked computation. For example, a common way to use mask is to protect the acquisition of a resource:

mask $ \restore -> do
    x <- acquire
    restore (do_something_with x) `onException` release
    release

This code guarantees that acquire is paired with release, by masking asynchronous exceptions for the critical parts. (Rather than write this code yourself, it would be better to use bracket which abstracts the general pattern).

Note that the restore action passed to the argument to mask does not necessarily unmask asynchronous exceptions, it just restores the masking state to that of the enclosing context. Thus if asynchronous exceptions are already masked, mask cannot be used to unmask exceptions again. This is so that if you call a library function with exceptions masked, you can be sure that the library call will not be able to unmask exceptions again. If you are writing library code and need to use asynchronous exceptions, the only way is to create a new thread; see forkIOWithUnmask.

Asynchronous exceptions may still be received while in the masked state if the masked thread blocks in certain ways; see Control.Exception.

Threads created by forkIO inherit the MaskingState from the parent; that is, to start a thread in the MaskedInterruptible state, use mask_ $ forkIO .... This is particularly useful if you need to establish an exception handler in the forked thread before any asynchronous exceptions are received. To create a new thread in an unmasked state use forkIOWithUnmask.

Like mask, but does not pass a restore action to the argument.

Like mask, but the masked computation is not interruptible (see Control.Exception). THIS SHOULD BE USED WITH GREAT CARE, because if a thread executing in uninterruptibleMask blocks for any reason, then the thread (and possibly the program, if this is the main thread) will be unresponsive and unkillable. This function should only be necessary if you need to mask exceptions around an interruptible operation, and you can guarantee that the interruptible operation will only block for a short period of time.

Like uninterruptibleMask, but does not pass a restore action to the argument.

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

Returns the MaskingState for the current thread.

Allow asynchronous exceptions to be raised even inside mask, making the operation interruptible (see the discussion of "Interruptible operations" in Exception).

When called outside mask, or inside uninterruptibleMask, this function has no effect.

@since base-4.9.0.0

Evaluate the argument to weak head normal form.

evaluate is typically used to uncover any exceptions that a lazy value may contain, and possibly handle them.

evaluate only evaluates to weak head normal form. If deeper evaluation is needed, the force function from Control.DeepSeq may be handy:

evaluate $ force x

There is a subtle difference between evaluate x and return $! x, analogous to the difference between throwIO and throw. If the lazy value x throws an exception, return $! x will fail to return an IO action and will throw an exception instead. evaluate x, on the other hand, always produces an IO action; that action will throw an exception upon execution iff x throws an exception upon evaluation.

The practical implication of this difference is that due to the imprecise exceptions semantics,

(return $! error "foo") >> error "bar"

may throw either "foo" or "bar", depending on the optimizations performed by the compiler. On the other hand,

evaluate (error "foo") >> error "bar"

is guaranteed to throw "foo".

The rule of thumb is to use evaluate to force or handle exceptions in lazy values. If, on the other hand, you are forcing a lazy value for efficiency reasons only and do not care about exceptions, you may use return $! x.