Method EvalPremise

engine/topdown.go:142–177

EvalPremise evaluates a single premise top-down. This is similar to PROLOG style SLD resolution: even though we have negated atoms, they are treated them as lookups (stratified semantics).

(premise ast.Term, subst unionfind.UnionFind)

// This is similar to PROLOG style SLD resolution: even though we
// have negated atoms, they are treated them as lookups (stratified semantics).
func (q QueryContext) EvalPremise(premise ast.Term, subst unionfind.UnionFind) ([]unionfind.UnionFind, error) {
	var solutions []unionfind.UnionFind
	switch p := premise.(type) {
	case ast.Atom:
		p, err := functional.EvalAtom(p, subst)
		if err != nil {
			return nil, err
		}
		decl := q.PredToDecl[p.Predicate]
		if decl != nil && decl.DeferredPredicate() {
			err := q.EvalQuery(p, decl.Modes()[0], subst, func(fact ast.Atom) error {
				newsubst, err := unionfind.UnifyTermsExtend(p.Args, fact.Args, subst)
				if err != nil {
					return err
				}
				solutions = append(solutions, newsubst)
				return nil
			})
			if err != nil {
				return nil, err
			}
			return solutions, nil
		}
		return premiseAtom(p, q.Store.GetFacts, subst)

	case ast.NegAtom:
		return premiseNegAtom(p.Atom, q.Store, subst)

	case ast.Eq:
		return premiseEq(p.Left, p.Right, subst)

	case ast.Ineq:
		return premiseIneq(p.Left, p.Right, subst)
	}
	return nil, nil
}