Facts, Rules, and Queries

Prolog programs are knowledge bases (KBs) which are a collection of facts and rules.
Using the prolog program is then performing queries on that KB.

Querying

If a fact is not written in the KB, or it cannot be inferred from the rules, then it is assumed to not be true.

Example: Does Ed play guitar?

person(Ed).
person(Emily).
playsGuitar(Emily).

Asking:

?-playsGuitar(Ed). % should be 'no'

Since it is not written in the KB that ‘Ed’ plays guitar, it is assumed to be false.

Rules

F

happy(ed):- hasVimto(ed).

‘ed’ is happy given ed has vimto, or ‘ed’ is happy if ed has vimto

modus ponens

AAAAAAAAAAAAA

Conjunction, Disjunction

You can express conjunction for a rule by having predicates next to each other, joined by a comma.

Example 1:

happy(ed):- noCoursework(ed), hasVimto(ed).

The above means ed is happy if ed has no coursework AND ed has vimto. So the comma serves as a conjunction operator.

You can express disjunction for a rule by having multiple rules, or by using a semicolon.
Let’s have a look at multiple rules first.

Example 2:

happy(ed):- noCoursework(ed), hasVimto(ed).
happy(ed):- bigBagOfMoney(ed).

The above means that ed is happy if either of the following is true:

• ed has no coursework AND ed has vimto
• ed has a big bag of money

This could also be expressed in a single rule using ’;’ which is the disjunction operator.

Example 3:

happy(ed) :- 
	noCoursework(ed), hasVimto(ed);
	bigBagOfMoney(ed).

Multiple Rules or Semicolon?

Multiple Rules can be more readable than having many semicolons.

Programs with semicolons may have a single rule where you would otherwise write many; therefore, it’s a generally more efficient program since having a single rule will generally be faster.

Tutorial

KB5

Definition lets people be jealous of themselves; could be better to add condition that $X,Y$ be distinct.