Copyright	(c) The University of Glasgow 2001
License	BSD-style (see the file libraries/base/LICENSE)
Maintainer	[email protected]
Stability	experimental
Portability	non-portable (uses Control.Monad.ST)
Safe Haskell	Trustworthy
Language	Haskell2010

STRefs

data STRef s a Source

a value of type STRef s a is a mutable variable in state thread s, containing a value of type a

newSTRef :: a -> ST s (STRef s a) Source

Build a new STRef in the current state thread

readSTRef :: STRef s a -> ST s a Source

Read the value of an STRef

writeSTRef :: STRef s a -> a -> ST s () Source

Write a new value into an STRef

modifySTRef :: STRef s a -> (a -> a) -> ST s () Source

Mutate the contents of an STRef.

Be warned that modifySTRef does not apply the function strictly. This means if the program calls modifySTRef 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 STRef as a counter. For example, the following will leak memory and likely produce a stack overflow:

print $ runST $ do
    ref <- newSTRef 0
    replicateM_ 1000000 $ modifySTRef ref (+1)
    readSTRef ref

To avoid this problem, use modifySTRef' instead.

modifySTRef' :: STRef s a -> (a -> a) -> ST s () Source

Strict version of modifySTRef

Since: 4.6.0.0