Re: [Haskell-cafe] ST not strict enough?

I ran into a similar problem with modifySTRef causing allocation and GC. Creating my own strict version of modifySTRef got rid of all that and my program ran without any allocation at all. On Nov 16, 3:16 pm, Johan Tibell wrote:

On Wed, Nov 16, 2011 at 12:07 PM, Johan Tibell wrote:

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On Wed, Nov 16, 2011 at 11:58 AM, Jason Dusek wrote:

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diff --git a/Rebuild.hs b/Rebuild.hs @@ -15,6 +15,7 @@ import Data.STRef  import Data.String  import Data.Word

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+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

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+! 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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