[Haskell-cafe] ST not strict enough?

[Johan Tibell]

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
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