[Haskell-cafe] ST not strict enough?

Tristan Ravitch travitch at cs.wisc.edu
Wed Nov 16 21:14:25 CET 2011

On Wed, Nov 16, 2011 at 07:58:51PM +0000, Jason Dusek wrote:
> 2011/11/15 Johan Tibell <johan.tibell at gmail.com>:
> > On Tue, Nov 15, 2011 at 12:08 PM, Jason Dusek <jason.dusek at gmail.com> wrote:
> >> Should I be annotating my functions with strictness, for the
> >> vector reference, for example? Should I be using STUArrays,
> >> instead?
> >
> > From
> > http://www.haskell.org/ghc/docs/latest/html/libraries/base-
> >
> >     "The >>= and >> operations are strict in the state (though not in values
> > stored in the state)."
> >
> > which implies that
> >
> >      modifySTRef counter (+1)
> >
> > is too lazy.
> As a first cut at strictifying the ST operations, I introduced a
> strict plus and strict vector write operation, strictifying
> every parameter that admitted it.
>   (+!) a b                   =  ((+) $!! a) $!! b
>   w v n b = (Vector.unsafeWrite v $!! n) $!! b
> This did not alter memory usage in any noticeable way. (Tried it
> with strict and lazy ByteStrings and both had the same memory
> usage as they did without the extra strictness.)
> It does seem off odd that building a vector byte by byte is so
> hard to do performantly. Maybe the memory usage ends up being
> okay when working with larger structures, though.

I think your use of (+1) in the STRef was safe here because
unsafeWrite was forcing it relatively quickly.  The laziness in ST
values just bit me a few days ago because I wasn't forcing the values
soon enough and was getting stack overflows when evaluation was
finally triggered.

Have you tried building the vector using things besides write/ST?  It
might be a bit faster to use something like Vector.unfoldr or
Vector.generateM and ByteString.index to build up a pure Vector.
After that you could use Vector.unsafeThaw to convert that pure Vector
into an MVector.
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