[Oz] Help: choice statement ...

[Denys Duchier]

ravi at konark.ncst.ernet.in (P Ravi Prakash) writes:

> I have a question regarding `choice' statement in distributers. The
> documentation says that the 2 choices (Var = Val and Var \= Val) created
> by this statement could be executed in any order.  Is there any way to
> force the order?

Let me add my 2 cents to Christian's reply.

A frequent technique for exercising control over the distribution
strategy is to introduce a "control" or "choice" variable I:

        I::1#3 
        thread
          or I=1 {P1}
          [] I=2 {P2}
          [] I=3 {P3}
          end
        end


Now all you need to do is decide what distribution strategy to apply
to I.  For example {FD.distribute naive [I]} gives you back what you
got by just using choice directly.  However, much more is possible.

E.g. using this technique, you can easily obtain constructive
disjunction for I by splitting the above 3 way disjunction into 3
disjunctions, one for each possible value of I:

        thread
          or I=1 {P1} [] I\=:1 end
        end
        thread
          or I=2 {P2} [] I\=:2 end
        end
        thread
          or I=3 {P3} [] I\=:3 end
        end

Thus, if {P2} is inconsistent, 2 is removed from the domain of I.

Cheers,

-- 
Dr. Denys Duchier			Denys.Duchier at ps.uni-sb.de
Forschungsbereich Programmiersysteme	(Programming Systems Lab)
Universitaet des Saarlandes, Geb. 45	http://www.ps.uni-sb.de/~duchier
Postfach 15 11 50			Phone: +49 681 302 5618
66041 Saarbruecken, Germany		Fax:   +49 681 302 5615
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