[m-rev.] For review: change the way we handle inst any non-locals in negated contexts (again)

[Mark Brown]

On 15-Dec-2005, Julien Fischer <juliensf at cs.mu.OZ.AU> wrote:
> On Thu, 15 Dec 2005, Mark Brown wrote:
> > On 15-Dec-2005, Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> > > Just as a matter of style, it would be bad to have `promise_pure' on
> > > an exported predicate; in that case I'd use the pragma instead.
> >
> > That's a good point, and one I didn't consider.  The intent was actually to
> > abolish that pragma entirely -- it never seemed right to me that the promise
> > was made via a pragma, even though such pragmas are currently considered
> > a standard part of the language (see the start of chapter 16).  The rules
> > I've set out imply that this append definition will be inferred impure, and
> > therefore that an annotation on the declaration would be necessary -- either
> > impure, promise_pure or promise_semipure.
> >
> > However, the principle that implementation details shouldn't have to be
> > exposed in the interface is a good one, so perhaps we should keep the
> > pragmas after all, or find some other way to make the promise in the
> > implementation.  Anyone got any suggestions?
> >
> 
> I would still like to get rid of the pragma - I don't think promise_pure
> should be a pragma at all.  Since the proposal adds promise_(semi)pure
> annotations maybe we don't need to annotate the predicate declarations
> at all.  If you want promise something is (semi)pure then just put a
> promise_(semi)pure annotation on the top-level goal.

That won't be able to express what we want.  A predicate or function
definition (as opposed to a goal) can be inferred impure for a number
of reasons:

	- because the body is impure,
	- because it has a foreign_proc,
	- because it has different code for different modes.

Placing a promise on the top-level goal (that is, on the body) will deal with
the first of these, and there are foreign_proc attributes that can deal with
the second.  But the only way to deal with the third is to have a promise
in the declaration, or in some other item like a pragma.

Cheers,
Mark.

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