base-4.6.0.0: Basic libraries

Portabilitynon-portable (uses Control.Monad.ST)
Stabilityexperimental
Maintainer[email protected]
Safe HaskellTrustworthy

Data.STRef

Contents

Description

Mutable references in the (strict) ST monad.

Synopsis

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

Instances

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

Build a new STRef in the current state thread

readSTRef :: STRef s a -> ST s aSource

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