[Haskell-cafe] Avoid sharing

[Tom Ellis]

I'm not going to contradict Edward's answer in practice, because he knows
far more about the internals of GHC than I do.  However, I will contradict
it in theory.

There is a straightforward operational semantics for Haskell programs as
compiled by GHC via Core to STG (without any optimisations applied).  I
described this semantics in the following talk at Haskell eXchange 2016:

    https://skillsmatter.com/skillscasts/8726-haskell-programs-how-do-they-run#video

and I wrote it up as an article here:

    http://h2.jaguarpaw.co.uk/posts/haskell-programs-how-do-they-run/

Under that semantics, yes, 'someGenerator s' is bound twice and thus is not
shared.

Now, GHC may try to apply an "optimisation" and (accidentally?) introduce
sharing.  That would be a compiler bug, in my opinion.  In fact, it would be
a terrible blow to Haskell's future as a practical language, because we
really need fine-grained control over sharing.  So I really hope that Edward
is wrong about this.

Tom


On Mon, Nov 07, 2016 at 12:56:02PM -0800, Edward Z. Yang wrote:
> It's not guaranteed.  Unfortunately there aren't really good ways
> to avoid sharing; the general advice is to convert values into
> functions, and apply them at the use site where sharing is OK.
> 
> Unrelatedly, in your sample code, dropping 1000000000 entries
> is not a good way to build a splittable RNG.  Check out
> http://publications.lib.chalmers.se/records/fulltext/183348/local_183348.pdf
> and also its related work for some bettera pproaches.
> 
> Edward
> 
> Excerpts from Michael Roth's message of 2016-11-07 20:56:54 +0100:
> > Hello! A short question, given:
> > 
> > 
> >      data Seed = ...
> >      data Value = ...
> > 
> >      someGenerator :: Seed -> [Value]
> > 
> >      createTwo :: Seed -> ([Value], [Value])
> >      createTwo s = (as, bs) where
> >        as = someGenerator s
> >        bs = drop 1000000000 (someGenerator s)
> > 
> > 
> > Is it guaranteed that 'someGenerator s' is created twice and not shared 
> > between 'as' and 'bs'? Is this by language design? Are there any GHC 
> > options that change the behaviour?