/Haskell 8


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



Mutable references in the (strict) ST monad.


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 )
Instances details
Eq (STRef s a)

Pointer equality.

Since: base-2.1

Instance details

Defined in GHC.STRef


(==) :: 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 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 may produce a stack overflow:

>>> import Control.Monad (replicateM_)
>>> :{
print (runST (do
    ref <- newSTRef 0
    replicateM_ 1000 $ 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: base-

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).