Re: Some clarification

Alexandre,
Thank you for your help. I was having some trouble with the terminology, and 
your email helps me quite a bit. I'm sure I'll be calling on everyone's help 
again in the future!

Thanks a LOT,
Cliff

On 9/5/05, A-S Saidi <[email protected]> wrote:
> 
> Dear Cliff,
> 
> I'm trying to answer to your question.
> 
> Suppose the following program :
> 
> 1) natural(0).
> 2) natural(succ(N)) :- 
> 3) natural(N).
> 
> 
> and some questions submitted to the Prolog engine:
> 
> 
> 4) ? |- natural(0).
> => yes
> 
> Prolog engine compared the "goal" (the question 4) and the "natural" 
> predicate cluases (i.e., the rules 1 & 2). The clause (1) was exactly the 
> same as the goal (4).
> One says that Prolog "unified" (1) and (4). Simply speaking, the engine 
> tried to make (1) and (4) lexically the same. During the unification, 
> variables may recieve a "value" (not in this case, see below).
> 
> 
> 5)- ? |- natural(X) .
> => X=0 
> 
> Again, the engine unified (5) and (1). This time, X is "associated" 
> (bound) to the constant 0. This is done in order to make them the SAME. The 
> couple [X / 0] is called a "substitution" and X in the goal is 
> "instanciated" to 0.
> 
> 6) If you hit ';' for more answers, then you'll have :
> => X= succ(0)
> 
> Prolog tries to resatsfy the goal. Prolog engine "forgets" the first 
> answer. The effect of the step 6 is thrown away and X in the goal becomes a 
> variable as before but the engine knows that (1) was alreday tried. 
> 
> This time, (5) is unifed to left part of (2) giving the substitution [ X / 
> succ(N) ]. Then, the engine has to see if the right part of (2) can succeed. 
> Therefor, the new (sub) goal becomes "succ(N)".
> As in the above question, this N will capture the value 0 such that the 
> substitution set becomes [ X / succ(N), N / 0 ].
> Hence X = succ(0).
> 
> This time, X of the "goal" (5) is "instanciated" to succ(0).
> 
> 6) if you retry ';' again, you'll have the answer succ(succ(0)) and so on 
> (never ends, same as the Natural set which is infinite).
> 
> 
> NB : I did not bother here with the "variable renaming" since the example 
> is quite simple. Note that if the goal was "succ(N)" (N instead of X), 
> instances of N in (2) are different from the 'N' in the goal. To avoid this 
> ambiguity, Prolog replaces ("renames") all the variables in the candidate 
> rule (here the rule 2). That is, the inner version of (2) considered is 
> something like :
> (2') : natural(succ(N') :- succ(N').
> 
> which has the same signification as 2.
> 
> Hope this helped.
> 
> regards.
> 
> Alex.
> 
> 
> Cliff Bender a écrit : 
> 
> Hi,
> As I read more and try more things, I suspect I'll be asking quite a few 
> questions of everyone. I'd like to thank people in advance for their 
> patience.
> 
> Right now, I'd like some clarification on terminology. Specifically, 
> "Goal", "Unify" and "Instantiate". I'm used to programming in php and C/C++, 
> and these concepts are a bit alien to me.
> 
> Thanks in advance,
> Cliff
> 
> ------------------------------
> 
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>   
>  
> -- 
> Aleksander S. Saidi
> Ecole Centrale de Lyon
> Département Mathématiques-Informatique
> Mél : [email protected]
> Tél : 04.72.18.65.30, Fax : 04.78.33.16.15
> 
> 
>

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