base-4.14.0.0: Basic libraries
Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Maintainerlibraries@haskell.org
Stabilityexperimental
Portabilitynon-portable (uses Control.Monad.ST)
Safe HaskellTrustworthy
LanguageHaskell2010

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

>>> :{
runST (do
    ref <- newSTRef "hello"
    x <- readSTRef ref
    writeSTRef ref (x ++ "world")
    readSTRef ref )
:}
"helloworld"

Instances

Instances details
Eq (STRef s a) #

Pointer equality.

Since: base-2.1

Instance details

Defined in GHC.STRef

Methods

(==) :: STRef s a -> STRef s a -> Bool Source #

(/=) :: STRef s a -> STRef s a -> Bool Source #

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.

>>> :{
runST (do
    ref <- newSTRef ""
    modifySTRef ref (const "world")
    modifySTRef ref (++ "!")
    modifySTRef ref ("Hello, " ++)
    readSTRef ref )
:}
"Hello, world!"

Be warned that modifySTRef does not apply the function strictly. This means if the program calls modifySTRef many times, but seldom 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 may produce a stack overflow:

>>> import Control.Monad (replicateM_)
>>> :{
print (runST (do
    ref <- newSTRef 0
    replicateM_ 1000 $ modifySTRef ref (+1)
    readSTRef ref ))
:}
1000

To avoid this problem, use modifySTRef' instead.

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

Strict version of modifySTRef

Since: base-4.6.0.0