Description
Mutable references in the (strict) ST monad.
STRefsa value of type STRef s a is a mutable variable in state thread s, containing a value of type a
modifySTRef :: STRef s a -> (a -> a) -> ST s ()Source
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.
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