Data/STRef.hs

    {-# LANGUAGE Trustworthy #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.STRef
    -- 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)
    --
    -- Mutable references in the (strict) ST monad.
    --
    -----------------------------------------------------------------------------

    module Data.STRef (
            -- * STRefs
            STRef,          -- abstract
            newSTRef,
            readSTRef,
            writeSTRef,
            modifySTRef,
            modifySTRef'
     ) where

    import GHC.ST
    import GHC.STRef

    -- | 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 ()
    modifySTRef ref f = writeSTRef ref . f =<< readSTRef ref

    -- | Strict version of 'modifySTRef'
    --
    -- @since 4.6.0.0
    modifySTRef' :: STRef s a -> (a -> a) -> ST s ()
    modifySTRef' ref f = do
        x <- readSTRef ref
        let x' = f x
        x' `seq` writeSTRef ref x'