[Haskell-cafe] ST not strict enough?

[Johan Tibell]

On Wed, Nov 16, 2011 at 2:23 PM, Jason Dusek <jason.dusek at gmail.com> wrote:

> > Just double checked. modifySTRef is too lazy:
> > -- |Mutate the contents of an 'STRef'
> > modifySTRef :: STRef s a -> (a -> a) -> ST s ()
> > modifySTRef ref f = writeSTRef ref . f =<< readSTRef ref
> > We need Data.STRef.Strict
>
> Tried a modifySTRef' defined this way:
>
> modifySTRef' ref f           =  do
>  val                       <-  (f $!!) <$> readSTRef ref
>  writeSTRef ref (val `seq` val)
>
> ...but there was no change in memory usage.


Why not just

    modifySTRef :: STRef s a -> (a -> a) -> ST s ()
    modifySTRef ref f = do
        x <- readSTRef ref
        writeSTRef ref $! f x

(Note that I didn't check if modifySTRef was actually a problem in this
case).

-- Johan
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