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 |
Mutable references in the (strict) ST monad.
STRefs
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"
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 )) :} 1000
To avoid this problem, use modifySTRef'
instead.
modifySTRef' :: STRef s a -> (a -> a) -> ST s () Source #
Strict version of modifySTRef
Since: 4.6.0.0