Custom atoms and operations

To extend EXS, define an atom for the values your expression language uses, define an operator implementation, then register its type and add an evaluation step. The CustomSolver example implements len(...) over string atoms and uses the built-in < operator to compare lengths.

The example allocates a new operator ID after the built-in IDs and derives a one-argument group operator. This excerpt uses the example’s CustomAtom:

enum CustomOperatorType { LENGTH_OPERATOR = snt::exs::NUM_OPERATOR_TYPES };

class OperatorLength : public snt::exs::OperatorGroup<1> {
public:
    OperatorLength()
        : OperatorGroup<1>("len", {"len", "(", ")", ","}, LENGTH_OPERATOR) {}

    void operate_group(snt::exs::TokenListBase* tokens) override {
        auto argument = tokens->get_left();
        auto* atom = static_cast<CustomAtom*>(argument.atom);
        atom->custom_length();
        tokens->put_left(argument);
    }
};

OperatorGroup<1> parses one argument between parentheses. The solver evaluates that argument first, then operate_group changes the atom to its length. The example’s CustomAtom derives from snt::exs::AtomBase<CustomAtom, AtomValueType>; it parses string operands with from_string and implements custom_length and comparison_less for the example’s value types. For a different operation shape, derive from OperatorBase and override operate_unary, operate_binary, or operate_ternary as appropriate. The chosen step category must match that override.

Register the new operator and place its group operation before the comparison step:

snt::exs::OperatorList operators;
operators.append(snt::exs::LESS_OPERATOR,
                 std::make_shared<snt::exs::OperatorLess>());
operators.append(LENGTH_OPERATOR, std::make_shared<OperatorLength>());

snt::exs::StepList steps;
steps.append(snt::exs::GROUP_OPERATION, {LENGTH_OPERATOR});
steps.append(snt::exs::BINARY_OPERATION, {snt::exs::LESS_OPERATOR});

snt::exs::Solver<CustomAtom> solver(operators, steps);
auto result = solver.eval("apple < len(hospital)");

The complete atom and operator implementations are in the CustomSolver source. The EXS examples also show arrays (ArraySolver) and atoms that own std::unique_ptr values (UniquePtrSolver). See Solver settings for passing application data to atom parsing and operators.