Examples of Facts with Arguments (Cont.)
eats(fred,oranges). /* Fred eats oranges. */
eats(fred,t_bone_steaks). /* Fred eats T-bone steaks. */
eats(tony,apples). /* Tony eats apples. */
eats(john,apples). /* John eats apples. */
eats(john,grapefruit). /* John eats grapefruit. */
?- eats(mike,apples). /* Does mike eat apples? */
no /* No, according to the database */
?- eats(fred,apples). /* Does fred eat apples? */
no /* No, we don't know. */
Let us consider another database.
This time we will use the predicate age to indicate the ages of various individuals.
age( john, 32 ). /* John is 32 years old. */
age( agnes, 41 ). /* Agnes is 41. */
age( george, 72 ). /* George is 72. */
age( ian, 2 ). /* Ian is 2. */
age( thomas, 25 ). /* Thomas is 25. */
If we now ask some queries we would get the following interaction:
?- age( ian, 2 ). /* Is Ian 2 years old? */
yes /* Yes, the fourth clause */
?- agnes( 41 ). /* Is agnes 41? */
no /* No. In the database we only */
/* know about the relation age, */
/* not about the relation agnes, */
/* so the query fails. */
?- age(ian,two). /* Is Ian two years old? */
no /* No. two and 2 are not the same. */
/* Therefore don't match. */