| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | experimental |
| Portability | portable |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Data.IORef
Contents
Description
Mutable references in the IO monad.
Synopsis
- data IORef a
- newIORef :: a -> IO (IORef a)
- readIORef :: IORef a -> IO a
- writeIORef :: IORef a -> a -> IO ()
- modifyIORef :: IORef a -> (a -> a) -> IO ()
- modifyIORef' :: IORef a -> (a -> a) -> IO ()
- atomicModifyIORef :: IORef a -> (a -> (a, b)) -> IO b
- atomicModifyIORef' :: IORef a -> (a -> (a, b)) -> IO b
- atomicWriteIORef :: IORef a -> a -> IO ()
- mkWeakIORef :: IORef a -> IO () -> IO (Weak (IORef a))
IORefs
A mutable variable in the IO monad
modifyIORef :: IORef a -> (a -> a) -> IO () Source #
Mutate the contents of an IORef.
Be warned that modifyIORef does not apply the function strictly. This
means if the program calls modifyIORef many times, but seldomly uses the
value, thunks will pile up in memory resulting in a space leak. This is a
common mistake made when using an IORef as a counter. For example, the
following will likely produce a stack overflow:
ref <- newIORef 0 replicateM_ 1000000 $ modifyIORef ref (+1) readIORef ref >>= print
To avoid this problem, use modifyIORef' instead.
modifyIORef' :: IORef a -> (a -> a) -> IO () Source #
Strict version of modifyIORef
Since: base-4.6.0.0
atomicModifyIORef :: IORef a -> (a -> (a, b)) -> IO b Source #
Atomically modifies the contents of an IORef.
This function is useful for using IORef in a safe way in a multithreaded
program. If you only have one IORef, then using atomicModifyIORef to
access and modify it will prevent race conditions.
Extending the atomicity to multiple IORefs is problematic, so it
is recommended that if you need to do anything more complicated
then using MVar instead is a good idea.
atomicModifyIORef does not apply the function strictly. This is important
to know even if all you are doing is replacing the value. For example, this
will leak memory:
ref <- newIORef '1'
forever $ atomicModifyIORef ref (\_ -> ('2', ()))Use atomicModifyIORef' or atomicWriteIORef to avoid this problem.
atomicModifyIORef' :: IORef a -> (a -> (a, b)) -> IO b Source #
Strict version of atomicModifyIORef. This forces both
the value stored in the IORef and the value returned. The new value
is installed in the IORef before the returned value is forced.
So
atomicModifyIORef' ref (x -> (x+1, undefined))
will increment the IORef and then throw an exception in the calling
thread.
Since: base-4.6.0.0
atomicWriteIORef :: IORef a -> a -> IO () Source #
Variant of writeIORef with the "barrier to reordering" property that
atomicModifyIORef has.
Since: base-4.6.0.0
Memory Model
In a concurrent program, IORef operations may appear out-of-order
to another thread, depending on the memory model of the underlying
processor architecture. For example, on x86, loads can move ahead
of stores, so in the following example:
import Data.IORef import Control.Monad (unless) import Control.Concurrent (forkIO, threadDelay) maybePrint :: IORef Bool -> IORef Bool -> IO () maybePrint myRef yourRef = do writeIORef myRef True yourVal <- readIORef yourRef unless yourVal $ putStrLn "critical section" main :: IO () main = do r1 <- newIORef False r2 <- newIORef False forkIO $ maybePrint r1 r2 forkIO $ maybePrint r2 r1 threadDelay 1000000
it is possible that the string "critical section" is printed
twice, even though there is no interleaving of the operations of the
two threads that allows that outcome. The memory model of x86
allows readIORef to happen before the earlier writeIORef.
The implementation is required to ensure that reordering of memory
operations cannot cause type-correct code to go wrong. In
particular, when inspecting the value read from an IORef, the
memory writes that created that value must have occurred from the
point of view of the current thread.
atomicModifyIORef acts as a barrier to reordering. Multiple
atomicModifyIORef operations occur in strict program order. An
atomicModifyIORef is never observed to take place ahead of any
earlier (in program order) IORef operations, or after any later
IORef operations.