Data.STRef (original) (raw)

Description

Mutable references in the (strict) ST monad.

Synopsis

• data STRef s a
• newSTRef :: a -> ST s (STRef s a)
• readSTRef :: STRef s a -> ST s a
• writeSTRef :: STRef s a -> a -> ST s ()
• modifySTRef :: STRef s a -> (a -> a) -> ST s ()
• modifySTRef' :: STRef s a -> (a -> a) -> ST s ()

STRefs

data STRef s a Source #

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](Data-STRef.html#t:STRef "Data.STRef").

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

Be warned that [modifySTRef](Data-STRef.html#v:modifySTRef "Data.STRef") does not apply the function strictly. This means if the program calls [modifySTRef](Data-STRef.html#v:modifySTRef "Data.STRef") 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'](Data-STRef.html#v:modifySTRef-39- "Data.STRef") instead.