[Haskell-cafe] ST not strict enough?

[Johan Tibell]

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.

-- Johan
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