class strstr_function : public function_expression {
public:
        explicit strstr_function(const args_list& args)
        : function_expression("strstr", args, 2, 2)
        {}
private:
        variant execute(const formula_callable& variables) const {
            const std::string& haystack = args()[0]->evaluate(variables).as_string();
            const std::string& needle = args()[1]->evaluate(variables).as_string();

            const size_t pos = haystack.find(needle);

            if(pos == std::string::npos) {
                return variant(0);
            } else {
                return variant(pos + 1);
            }
        }
};