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"

Build a new STRef in the current state thread

Read the value of an STRef

Write a new value into an 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 seldom 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 GHC.Internal.Control.Monad (replicateM_)
>>> :{
print (runST (do
    ref <- newSTRef 0
    replicateM_ 1000 $ modifySTRef ref (+1)
    readSTRef ref ))
:}
1000

To avoid this problem, use modifySTRef' instead.

Strict version of modifySTRef

Since: base-4.6.0.0