[m-dev.] Re: [mercury-users] Mutual Exclusivity & Exhaustiveness
Fergus Henderson
fjh at cs.mu.OZ.AU
Fri Jan 4 15:50:47 AEDT 2002
On 04-Jan-2002, Lars Yencken <lljy at students.cs.mu.oz.au> wrote:
> > I don't think mutual exclusion is necessarily the right concept to
> > use here anyway. The property that we need for determinism analysis
> > is that the disjunction has at most one distinct solution; mutual
> > exclusion of the disjuncts is a sufficient but not necessary condition.
...
> There are also big advantages to being able to declare mutual exclusivity
> though. For example, if I could declare
>
> :- all [X, Y] promise_exactly_one_solution (
> X < Y
> ; X = Y
> ; X > Y
> ).
>
> then a switch on just two of these would be considered nondet by the compiler
> instead of semidet, unless all 3 pairs of ordered signs were also declared
> using promise_at_most_one_solution.
No -- if you declare that this disjunction has exactly one solution,
then the compiler could safely infer that any disjunction
containing a subset of the disjuncts has at most one solution.
Hence it should consider such switches to be semidet rather than nondet.
So there's no difference between mutual exclusivity and
`promise_exactly_one_solution' here.
--
Fergus Henderson <fjh at cs.mu.oz.au> | "I have always known that the pursuit
The University of Melbourne | of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh> | -- the last words of T. S. Garp.
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