base-4.7.0.0: Basic libraries

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

Data.STRef

Contents

- STRefs

Description

Mutable references in the (strict) ST monad.

Synopsis

- data STRef s a
- newSTRef :: a -> ST s (STRef s a)
- readSTRef :: STRef s a -> ST s a
- writeSTRef :: STRef s a -> a -> ST s ()
- modifySTRef :: STRef s a -> (a -> a) -> ST s ()
- modifySTRef' :: STRef s a -> (a -> a) -> ST s ()

# STRefs

data STRef s a

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

Instances

Typeable2 STRef |
Eq (STRef s a) |
Typeable (* -> * -> *) STRef |

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

Build a new `STRef` in the current state thread

readSTRef :: STRef s a -> ST s a

Read the value of an `STRef`

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

Write a new value into an `STRef`

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

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 ()

Strict version of `modifySTRef`

Since: 4.6.0.0