[m-rev.] diff: reset contains_trace_goal if necessary

[Mark Brown]

On 09-Nov-2006, Ian MacLarty <maclarty at csse.unimelb.edu.au> wrote:
> On Thu, Nov 09, 2006 at 03:02:07PM +1100, Mark Brown wrote:
> > On 06-Nov-2006, Zoltan Somogyi <zs at csse.unimelb.edu.au> wrote:
> > > @@ -2657,6 +2670,84 @@
> > >  
> > >  %-----------------------------------------------------------------------------%
> > >  
> > > +:- pred goal_contains_trace(hlds_goal::in, hlds_goal::out,
> > > +    contains_trace_goal::out) is det.
> > > +
> > > +goal_contains_trace(GoalExpr0 - GoalInfo0, GoalExpr - GoalInfo,
> > > +        ContainsTrace) :-
> > > +    (
> > 
> > 	...
> > 
> > > +    ;
> > > +        GoalExpr0 = scope(Reason, SubGoal0),
> > > +        goal_contains_trace(SubGoal0, SubGoal, ContainsTrace),
> > > +        GoalExpr = scope(Reason, SubGoal)
> > 
> > It appears that this goal feature propagates outside promise_pure scopes.
> > I realise this behaviour follows what is in the reference manual, but is
> > this really what we want?
> > 
> > I think that a goal which is explicitly promised pure should be able to
> > be optimised away even if it contains trace goals, in the same way that
> > a pure call can be optimised away even if there are trace goals in its
> > definition.
> > 
> > Any comments?
> > 
> 
> I agree, but I don't see what the difference between this and what I said
> earlier is.

I'm only talking about optimising away goals that are explicitly promised
pure (and are det with no outputs, my pedantic friends).

For example, the goal

	( Cond ->
		trace ...
	;
		trace ...
	)

should not be replaced with "true" unless both trace goals are compiled
away.

In effect, trace goals introduce a kind of impurity which is eliminated
by promise_pure (as with other kinds of impurity), but which is also
implicitly eliminated at the boundary of each clause.  I believe this
was Zoltan's intent.

Cheers,
Mark.

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