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Rules

Linear rules​

Linear rules are rules with one single atom in the body. They define facts of the head predicate, given facts of the single body predicate.

employee(1).
employee(2).
department(Y,X) :- employee(X).
@output("department").

For each employee X there exists a department Y. The expected result is:

department(y1, 1).
department(y2, 2).

Facts​

The simplest form of linear rule is a fact: a ground head-less linear rule.

employee("Jack").
employee("Ruth").

Join rules​

Join rules are rules with multiple atoms in the body. They basically define facts of the head predicate, given facts of the body.

project(Z,X) :- employee(X), department(Y,X).

For each employee X in a department Y, there exists a project Z in which they participate.

If the atoms in the body do not have variables in common, the Cartesian product is assumed.

employee("Jack").
employee("Ruth").
department("science").
department("finance").
canWork(X, Y, Z) :- employee(X), department(Y).
@output("canWork").

Any employee X can work in any department Y on some unknown project Z. The expected result is:

canWork("Jack", "science", z1).
canWork("Jack", "finance", z2).
canWork("Ruth", "science", z3).
canWork("Ruth", "finance", z4).

Constants within rules​

Constants can appear in the atoms of the rules.

When they appear in the head, they denote specific constant values to be generated in the head facts.

When they appear in the body, they denote specific filters, or selection criteria, to be applied to the facts considered in the rule.

employee("Mark").
junior("Mark").
contract(X, "basic", 20) :- employee(X), junior(X).
@output("contract").

A junior employee will have a "basic" contract, with stipend 20. The expected result is:

contract("Mark", "basic", 20).

For this next example:

employee("Mark","junior").
employee("Ruth","senior").
contract(X,"basic", 20) :- employee(X, "junior").
contract(X,"advanced", 40) :- employee(X, "senior").
@output("contract").

Any junior employee X will have a "basic" contract with stipend 20. Any senior employee X will have an "advanced" contract with stipend 40. Basically, the constants filter the facts to which the rules apply, in such a way that the one for basic contracts applies only to Mark, and the one for advanced contracts applies only to Ruth.

The expected result is:

contract("Mark", "basic", 20).
contract("Ruth", "advanced", 40).