[m-dev.] Re: [mercury-users] Mutual Exclusivity & Exhaustiveness

Ralph Becket rafe at cs.mu.OZ.AU
Fri Jan 4 15:43:13 AEDT 2002

Lars Yencken, Friday,  4 January 2002:
> 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. 
> If we don't use exclusivity declarations, then every time we do a partial 
> switch instead of a full one our promised solutions are useless to us.

No - a disjunction cannot acquire more solutions by removing disjuncts!

Removing any two (or even one) of the above disjuncts may lead to
semideterminism or failure, but not nondeterminism.

- Ralph
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