[m-rev.] for review: Correct status of partial instantiation support

[Paul Bone]

On Wed, Mar 12, 2014 at 07:53:33PM +1100, Zoltan Somogyi wrote:
> 
> 
> On Wed, 12 Mar 2014 14:49:59 +1100 (EST), Julien Fischer <jfischer at opturion.com> wrote:
> > I suspect that specific code only "works" in those grades more by luck
> > than design.  In general, it does not work.
> 
> That is correct, but I think Paul needs to know *why*.
> 
> The real reason why partially instantiated data structures do not work in general
> is the lack of alias tracking in the mode system.
> 
> If you unify a variable and an argument of a term, then when either the variable
> or the argument is bound, the other should be bound to the same thing. In the
> absence of alias tracking, this won't happen. The only situation in which this
> is ok is if "the other" does not exist anymore, which the current compiler
> can be sure about only if its scope is restricted to the unification.
> 
> Even for code in which a free variable's scope is confined to its unification
> with a free variable in a term, the code generators are not guaranteed to handle
> the resulting partially instantiated terms correctly, simply because they
> weren't *designed* to. Mantis bug 311 shows this very clearly. Though
> I can imagine the different backends misbehaving differently on such code.
> Some may even generate correct code by accident :-)

Thanks Zoltan, this was very helpful.

-- 
Paul Bone