Applying Logical Knowledge

  Rules

A rule is like a stored query.

head :- body.

head - functor and args, just like a fact.

:- the 'neck' symbol, often read as 'if'

body - one or more goals, just like a query.

This query finds connecting flights between FROM and TO:

?- flight(FROM, VIA, _, FL1, _), flight(VIA, TO, _, FL2, _), not FROM = TO.

If this rule was in the logicbase:

connection(FROM, TO, VIA, FL1, FL2) :-
   flight(FROM, VIA, _, FL1, _),
   flight(VIA, TO, _, FL2, _),
   not FROM = TO.

Then this query would do the same thing:

?- connection(FROM, TO, VIA, FL1, FL2).

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