[Haskell-cafe] ST not strict enough?

[Antoine Latter]

On Wed, Nov 16, 2011 at 2:16 PM, Johan Tibell <johan.tibell at gmail.com> wrote:
> On Wed, Nov 16, 2011 at 12:07 PM, Johan Tibell <johan.tibell at gmail.com>
> wrote:
>>
>> On Wed, Nov 16, 2011 at 11:58 AM, Jason Dusek <jason.dusek at gmail.com>
>> wrote:
>>>
>>> diff --git a/Rebuild.hs b/Rebuild.hs
>>> @@ -15,6 +15,7 @@ import Data.STRef
>>>  import Data.String
>>>  import Data.Word
>>>
>>> +import Control.DeepSeq
>>>  import Data.Vector.Unboxed (Vector)
>>>  import qualified Data.Vector.Unboxed as Vector (create, length)
>>>  import qualified Data.Vector.Unboxed.Mutable as Vector hiding (length)
>>> @@ -46,8 +47,8 @@ rebuildAsVector bytes        =  byteVector
>>>     n                       <-  readSTRef counter
>>>     return (Vector.unsafeSlice 0 n v)
>>>   writeOneByte v counter b   =  do n <- readSTRef counter
>>> -                                   Vector.unsafeWrite v n b
>>> +                                   w v n b
>>>                                    modifySTRef counter (+!1)
>>> +  (+!) a b                   =  ((+) $!! a) $!! b
>>> +  w v n b = (Vector.unsafeWrite v $!! n) $!! b
>>
>> +! 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

We already have one in base - it re-exports Data.STRef in whole :-)

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-STRef-Strict.html

Antoine