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

[Julien Fischer]

On Thu, 9 Nov 2006, 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.

I assume you meant a goal that is promised pure, has no outputs, 
terminates and cannot throw exceptions as opposed to just any old
goal that is promised pure? ;-)

> Any comments?

That's what I would have thought as well.  Trace goals should not
affect optimizations ... (as much as possible, I realise that some
interactions are probably unavoidable.)

Julien.
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