Computation

We have described Logica's declarative syntax and semantics. But how can the predicates be evaluated to concrete tables?

Concrete predicates

Logica predicate is called concrete if it is being evaluated to a specific table of values. All predicates that we defined so far were concrete, they were either defined by a collection of facts, or finite multisets computable from other predicates. In particular see Fruit, Book or RecentBook above.

Injectible predicates

Injectible predicates are predicates that perform calculation of some of it's arguments based on other arguments which are provided as input.

Example: Computing adjusted employee salary using an auxiliary predicate.

Employee(name: "Alice", salary: 50000);
Employee(name: "Bob", salary: 60000);
Employee(name: "Charlie", salary: 55000);

AdjustedSalary(employee_salary, increase_percent, new_salary) :-
  new_salary == employee_salary * (1 + increase_percent / 100);

EmployeeSalaryIncrease(name:, adjusted_salary:) :-
  Employee(name:, salary:),
  AdjustedSalary(salary, 10, adjusted_salary);

In this example predicate AdjustedSalary could not be just printed out as a table. Mathematically it is a set of triples, but this is an infinite set of triples (or very large if we consider finite precision of floats) and it is not practical to represent it verbatim in memory or on disk.

However when we are evaluating concrete predicate EmployeeSalaryIncrease we can injectAdjustedSalary. Inject predicate means replace call to the predicate with its body. So in this case we replace AdjustedSalary(salary, 10, adjusted_salary) with adjusted_salary == salary * (1 + 10 / 100) and thus being able to compute adjusted_salary.

For a predicate to be injectible it must be defined with a signle non-aggregating conjunctive rule.