The connects Link
The Automated Tourist Guide might attempt to answer a tourist's question such as "What is near the Planetarium?" by a call on the query
?- near('the Planetarium', What).
which would, if the corresponding fact is in the
database, return What = `Madame Tussauds'. The
reverse query, "What is near Madame Tussauds?" would simply
return no, however, unless the statement
near('Madame Tussauds', `the
Planetarium').
was also explicitly given in the database. Simple Prolog queries, in other words, cannot imitate our human intuition that, if X is near to Y, then Y is also near to X. We therefore need to write a predicate that will make the correct inference, whichever way around the query is called. This can be done as follows:
near(From, To):-link(From, To).near(From, To):-link(To, From).>
?- near('Madame Tussauds', `the
Planetarium').
will then return the correct answer. [@to be completed]