diff options
Diffstat (limited to 'libraries/base/Data/STRef.hs')
| -rw-r--r-- | libraries/base/Data/STRef.hs | 26 |
1 files changed, 20 insertions, 6 deletions
diff --git a/libraries/base/Data/STRef.hs b/libraries/base/Data/STRef.hs index 60bccf50cb..46ca08361b 100644 --- a/libraries/base/Data/STRef.hs +++ b/libraries/base/Data/STRef.hs @@ -5,7 +5,7 @@ -- 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) @@ -29,16 +29,30 @@ import GHC.STRef -- | 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 likely produce a stack overflow: +-- following will leak memory and may produce a stack overflow: -- --- >print $ runST $ do --- > ref <- newSTRef 0 --- > replicateM_ 1000000 $ modifySTRef ref (+1) --- > readSTRef ref +-- >>> 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 () |