[m-rev.] for review: rewrite of Modes chapter

[David Overton]

On Mon, Feb 24, 2003 at 01:13:37AM +1100, Mark Brown wrote:
> On 23-Feb-2003, Fergus Henderson <fjh at cs.mu.OZ.AU> wrote:
> > I thought the "is at least as instantiated" and "matches" orderings
> > in Mark's proposed text were corresponding to inst_matches_initial
> > and inst_matches_final respectively.  If that is not the case, then I
> > need to review the change again.
> 
> "Matches" does correspond to inst_matches_final, but "is at least as
> instantiated as" doesn't correspond to inst_matches_initial; in
> particular, the uniqueness ordering condition is reversed, whereas for
> inst_matches_initial the uniqueness ordering condition should be the same.
> 
> Looking back, it seems the following part of the change relating to
> implied modes is wrong:
> 
> On 20-Feb-2003, Mark Brown <dougl at cs.mu.OZ.AU> wrote:
> >  In Mercury it is always possible to call a procedure with an
> > -argument that is is more bound than the initial inst specified for that
> > +argument that is more instantiated than the initial inst specified for that
> >  argument in the procedure's mode declaration.  In such cases, the
> >  compiler will insert additional unifications to ensure that the
> >  argument actually passed to the procedure will have the inst
> >  specified.
> 
> According to the definition I have given, 'ground' is more instantiated
> than 'unique'.  But it shouldn't be possible to call a procedure with
> 'ground' where 'unique' is expected, so the instantiatedness order is not
> the right order to use here -- inst_matches_initial would be correct.
> (I took this definition of implied modes from the thesis draft, so I think
> the same error occurs there.)

Yes, you're right.  My thesis is wrong there.  Thanks for spotting that.
Mercury (at least the current implementation) really only supports
implied modes where the initial inst of the procedure argument is `free'
so if you restrict the definition of implied modes to require that, then
it should work.


> 
> If, on the other hand, I changed the definition of the "instantiatedness"
> order to correspond to inst_matches_initial, then the other uses of it
> would be incorrect (e.g., the definition of "compatible" relies on it
> staying as it is).  So I'm going to need to think again about what
> partial order(s) would be most useful to present the material that we
> want to present.  I'll post a full diff after doing that, and addressing
> the above issue.



David
-- 
David Overton                  Uni of Melbourne     +61 3 8344 1354
dmo at cs.mu.oz.au                Monash Uni (Clayton) +61 3 9905 5779
http://www.cs.mu.oz.au/~dmo    Mobile Phone         +61 4 0337 4393
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