| Portability | non-portable (uses Control.Monad.ST) |
|---|---|
| Stability | experimental |
| Maintainer | libraries@haskell.org |
| Safe Haskell | Trustworthy |
Data.STRef
Contents
Description
Mutable references in the (strict) ST monad.
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
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