Page 206 - DCAP506_ARTIFICIAL_INTELLIGENCE

Artificial Intelligence

Notes The incorporated predicate true will always do well (without any further side-effects), while fail
will at all times fail. At times there is more than one method of satisfying the existing goal.
Prolog selects the first option (as determined by the order of clauses in a program), but the fact
that there are choices is recorded. If at some point Prolog fails to confirm a certain subgoal, the
system can go back and attempt a substitute way of executing the earlier goal. This process is
called backtracking. We shall demonstrate the process of goal execution by using the following
well-known argument:
All men are mortal.
Socrates is a man.
Hence, Socrates is mortal.

In Prolog terms, the first statement shows a rule: X is mortal, if X is a man (for all X). The second
one comprises a fact: Socrates is a man. This can be executed in Prolog as below:
mortal(X) :- man(X).

man(socrates).
Note that X is a variable, while socrates is an atom. The conclusion of the argument, “Socrates is
mortal”, can be displayed via the predicate mortal(socrates). After having conferred with the
above program we can propose this predicate to Prolog as a query, which will cause the following
reaction:

?- mortal(socrates).
Yes
Prolog consents with our own rational reasoning, which is nice. But how did it reach to its
conclusion? Let’s follow the goal execution step by step.
1. The query mortal(socrates) is made the initial goal.
2. Scanning via the clauses of our program, Prolog attempts to match mortal(socrates) with
the first probable fact or head of rule. It finds mortal(X), the head of the first (and only)
rule. When matching the two terms the instantiation X = socrates requires to be made.
3. The variable instantiation is extended to the body of the rule, i.e. man(X) turns out to be
man(socrates).
4. The newly instantiated body turns out to be our new goal: man(socrates).
5. Prolog implements the new goal by again attempting to match it with a rule-head or a
fact. Clearly, the goal man(socrates) matches the fact man(socrates), since they are identical.
This means the current goal succeeds.

6. This, again, means that also the initial goal succeeds.
Self Assessment

Fill in the blank:
5. Proposing a ......................... means demanding Prolog to attempt to prove that the
statement(s) implied by the query can be prepared true given the right variable
instantiations are prepared.

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