Nanitous wrote:
> Hi,
>
> I accidentally stumbled over a neat programming trick which I want to
> share with other Oz users.
>
> I'm not sure whether my explaination of /why/ the trick works is
> correct, if its not, I surely would
> like to hear what's wrong with it.
>
> Inspired by the CTM Wiki page "99 problems in other languages:Oz"
> (http://codepoetics.com/wiki/index.php?title=Topics:99_Problems_in_other_languages:Oz)
>
> I've been converting a number of prolog sample programs (originating
> from http://kti.mff.cuni.cz/~bartak/prolog/combinatorics.html) into Oz
> as an
> exercise using the guidelines described in CTM section 9.7.3 "Translating
> Prolog into a relational program".
>
> Because these sample programs are written in pure prolog, the
> guidelines make that
> conversion effortless, but the resulting translation less elegantly
> expresses the
> program than the original prolog clauses.
>
> Let me illustrate it by means of a contrived example, using relational
> style:
>
> (* In prolog: *)
> fact(0,1).
> fact(1,1).
> fact(N,F) :- N>1, N1 is N-1, fact(N1, FN), F is N*FN.
>
> Using the CTM guidelines, these clauses translate into Oz:
> (note the use of the deterministic choice by the if-statement)
> proc {Fact N F}
> if N==0 then F=1
> elseif N==1 then F=1
> elseif N>1 then
> FN in
> {Fact N-1 FN}
> F = N*FN
> else
> fail
> end
> end
> % The next line just demonstrates the computation of 6! :
> {ExploreOne proc{$ F} {Fact 6 F} end}
>
> It is obvious that the lay-out and relatively large amount of text
> of the Oz translation leaves something to be desired. It would have
> been nice to be able to write:
>
> proc {Fact N F}
> choice
> N==0 F=1
> [] N==1 then F=1
> [] FN in
> N > 1
> {Fact N-1 FN}
> F = N*FN
> end
> end
you can write:
proc {Fact N F}
choice
N=0 F=1
[] N=1 F=1
[] N>1=true F=N*{Fact N-1} end end
{Browse
{Search.base.one
fun {$} {Fact 10} end}.1}
this is a trivial rewrite of the obvious functional version using choice
and statements instead of conditionals and expressions.
vQ
>
> The textual structure of this procedure is less cluttered and follows
> the structure or the original prolog clauses closely. Moreover the use of
> the choice-statement keeps its semantics close the prolog-semantics of
> its original.
>
> But 'N==0', 'N==1' and 'N>1' are expressions and the Oz compiler will
> complain and tell you it doesn't accept expression where it expects
> statements.
>
> Don't ask me why I tried it, but I did. I used the FD constraint
> propagators instead
> of the expressions; that is, "N>:1" instead of "N>1" and so on:
>
> proc {Fact N F}
> choice
> N=:0 F=1
> [] N=:1 F=1
> [] FN in
> N >: 1
> {Fact N-1 FN}
> F = N*FN
> end
> end
>
> % And find the answer by using, e.g.
> {Browse {Seach.base.one proc{$ F} {Fact 6 F} end}.1 }
>
> At first I was suprised that this even would work, it seemed something
> like magic.
> One would expect constraint propagators to need a constraint store and
> all other required CSP trappings.
>
> But if you think a little bit further, I think, it is an obvious
> (side) effect.
> All constraint propagators must either return 'entailed', 'sleep', or
> 'failed'
> (See the "constraint extension tutorial" ) to the current computation
> space.
> In case of a fail, the computation space failes and the search engine
> explores the next choice points (See also Christian Schulte's
> excellent "Programming
> Constraint Services"). Since we use the same search engine for CSP as
> well
> as well to explore the choice points in {Fact}, the comparison
> constraint propagators, we can expect the same behaviour.
>
> I'm sure that the trick costs some performance, but who can resist
> such elegance, moreover it
> is a beautiful example of Oz's orthogonality.
>
> Best regards,
>
>
> /Twan
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