[Haskell-cafe] ST not strict enough?

[Jason Dusek]

2011/11/16 Johan Tibell <johan.tibell at gmail.com>:
> On Wed, Nov 16, 2011 at 12:07 PM, Johan Tibell <johan.tibell at gmail.com> wrote:
>> +! doesn't work unless modifySTRef is already strict in the result of the
>> function application. You need to write modifySTRef' that seq:s the result
>> of the function application before calling writeSTRef.
>
> 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.

--
Jason Dusek
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