a value of typeSTRef s a is a mutable variable in state threads, containing a value of typea

>>>:{runST (do    ref <- newSTRef "hello"    x <- readSTRef ref    writeSTRef ref (x ++ "world")    readSTRef ref ):}"helloworld"

Build a newSTRef in the current state thread

Read the value of anSTRef

Write a new value into anSTRef

Mutate the contents of anSTRef.

>>>:{runST (do    ref <- newSTRef ""    modifySTRef ref (const "world")    modifySTRef ref (++ "!")    modifySTRef ref ("Hello, " ++)    readSTRef ref ):}"Hello, world!"

Be warned thatmodifySTRef does not apply the function strictly. This means if the program callsmodifySTRef 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, usemodifySTRef' instead.

Strict version ofmodifySTRef

Since: 4.6.0.0