Language Features

By default, the language supported by ASDI is sometimes termed Pure Datalog. This language allows for positive literals only, and allow for recursion. It does support additional features with feature sets applied to programs. Without a feature specified certain types will fail well-formedness rules, the parser will report errors, and some tools will not execute. However, the enabling of features is relatively simple in both the text representation using the .feature pragma, and using FeatureSets in the API.

Negation

The feature negation enables the negation of individual literals in the body of a rule. This language is often described as $\text{\small{Datalog}}^{\lnot}$.

.features(negation).
alive(X) :- person(X) AND NOT dead(X).
alive(X) :- person(X) ∧ ￢dead(X).

The text representation allows for "!", "￢", and "NOT" to be used to denote negation.

Constraints

The feature constraints enables the specification of rules that have no head, which in turn specifies a rule that may never be true. This language is described herein as $\text{\small{Datalog}}^{\bot}$.

.features(constraints).
⊥ :- dead(X) AND alive(X).
:- dead(X) AND alive(X).

The text allows for either an entirely missing head, or the value "⊥" as the head to denote a constraint. The latter signifies that the rule implies falsity (or absurdity).

Comparisons

The feature comparisons enables the inclusion of literal terms that use standard comparison operators. This language is described herein as $\text{\small{Datalog}}^{\theta}$.

.features(comparisons).
old(X) :- age(X, Y) ∧ Y > 75.

The text representation supports the operators equality ("="), inequality ("!=", "/=", or "≠"), less than ("<"), less than or equal-to ("<=", or "≤"), greater than (">"), and greater than or equal-to (">=", or "≥").

Disjunction

The feature disjunction enables the negation of individual literals in the body of a rule. This language is often described as $\text{\small{Datalog}}^{\lor}$.

.features(disjunction).
mother(X, Y) OR father(X, Y) :- parent(X, Y).
mother(X, Y) ∨ father(X, Y) :- parent(X, Y).

The text representation allows for ";", “|”, “∨”, and “OR”` to be used to denote disjunction.

Example

The following demonstrates the text representation support for enabling features.

.features(negation, comparisons, disjunction).

Similarly, the following API example shows how to create a feature set that may be added to a program during creation.

#![allow(unused)]
fn main() {
use asdi::features::{
    FEATURE_COMPARISONS, FEATURE_DISJUNCTION, FEATURE_NEGATION, FeatureSet
};

let features = FeatureSet::from(vec![FEATURE_NEGATION, FEATURE_DISJUNCTION]);

assert!(features.supports(&FEATURE_NEGATION));
assert!(!features.supports(&FEATURE_COMPARISONS));

assert_eq!(features.to_string(), ".feature(negation, disjunction).");
}