IORef package:ihaskell

A mutable variable in the IO monad.

>>> import Data.IORef

>>> r <- newIORef 0

>>> readIORef r
0

>>> writeIORef r 1

>>> readIORef r
1

>>> atomicWriteIORef r 2

>>> readIORef r
2

>>> modifyIORef' r (+ 1)

>>> readIORef r
3

>>> atomicModifyIORef' r (\a -> (a + 1, ()))

>>> readIORef r
4

See also STRef and MVar.

Strict version of modifyIORef. This is not an atomic update, consider using atomicModifyIORef' when operating in a multithreaded environment.

Build a new IORef

Read the value of an IORef. Beware that the CPU executing a thread can reorder reads or writes to independent locations. See Data.IORef#memmodel for more details.

Write a new value into an IORef. This function does not create a memory barrier and can be reordered with other independent reads and writes within a thread, which may cause issues for multithreaded execution. In these cases, consider using atomicWriteIORef instead. See Data.IORef#memmodel for more details.