Function Decide

builtin/builtin.go:216–433

Decide evaluates an atom of a built-in predicate. The atom must no longer contain any apply-expressions or variables.

(atom ast.Atom, subst *unionfind.UnionFind)

// Decide evaluates an atom of a built-in predicate. The atom must no longer contain any
// apply-expressions or variables.
func Decide(atom ast.Atom, subst *unionfind.UnionFind) (bool, []*unionfind.UnionFind, error) {
	// Check for temporal predicates first
	if IsTemporalPredicate(atom.Predicate) {
		return DecideTemporalPredicate(atom, subst)
	}

	switch atom.Predicate.Symbol {
	case symbols.StartsWith.Symbol:
		fallthrough
	case symbols.EndsWith.Symbol:
		fallthrough
	case symbols.Contains.Symbol:
		fallthrough
	case symbols.MatchPrefix.Symbol:
		fallthrough
	case symbols.MatchPair.Symbol:
		fallthrough
	case symbols.MatchCons.Symbol:
		fallthrough
	case symbols.MatchEntry.Symbol:
		fallthrough
	case symbols.MatchField.Symbol:
		fallthrough
	case symbols.MatchNil.Symbol:
		ok, nsubst, err := match(atom, subst)
		if err != nil {
			return false, nil, err
		}
		if !ok {
			return false, nil, nil
		}
		return ok, []*unionfind.UnionFind{nsubst}, nil
	}
	switch atom.Predicate.Symbol {
	case symbols.Filter.Symbol:
		if len(atom.Args) != 1 {
			return false, nil, fmt.Errorf("wrong number of arguments for built-in predicate 'filter': %v", atom.Args)
		}
		evaluatedArg, err := functional.EvalExpr(atom.Args[0], subst)
		if err != nil {
			return false, nil, err
		}
		if evaluatedArg == ast.TrueConstant {
			return true, []*unionfind.UnionFind{subst}, nil
		}
		return false, nil, nil

	case symbols.Lt.Symbol:
		if len(atom.Args) != 2 {
			return false, nil, fmt.Errorf("wrong number of arguments for built-in predicate '<': %v", atom.Args)
		}
		nums, err := getNumberValues(atom.Args)
		if err != nil {
			return false, nil, err
		}
		return nums[0] < nums[1], []*unionfind.UnionFind{subst}, nil
	case symbols.Le.Symbol:
		if len(atom.Args) != 2 {