rql.h

#ifndef RQL_RQL_H
#define RQL_RQL_H

#include <functional>
#include <unordered_map>
#include "cpprest/json.h"

namespace rql
{
    // Resource Query Language
    // See https://github.com/persvr/rql for the extensible grammar
    // and https://doc.apsstandard.org/6.0/spec/rql/ for sometimes better documentation for a particular domain

    // Helper for parsing a Resource Query Language string into a json representation of the abstract syntax tree
    // The resulting json value follows the structure described in the RQL reference implementation for json-native
    // simple types (strings, numbers and booleans) and arrays, as well as in the representation of call-operators
    // being a json object with "name" and "args" fields, the latter an array containing at least one value (since
    // the ABNF for RQL doesn't support empty arrays or argument lists).
    // See https://github.com/persvr/rql#rqlparser
    // This implementation has one extension in that unrecognized RQL types are transformed to json objects with
    // "type" and (string) "value" fields. This allows typed values to be processed as appropriate by implementations.

    web::json::value parse_query(const utility::string_t& query);

    bool is_call_operator(const web::json::value& arg);
    bool is_typed_value(const web::json::value& arg);

    // Evaluate an RQL query using a resource property extractor function, and set of RQL call-operators

    struct evaluator;

    typedef std::function<bool(web::json::value& results, const web::json::value& key)> extractor;
    typedef std::unordered_map<utility::string_t, std::function<web::json::value(const evaluator& eval, const web::json::value& args)>> operators;

    struct evaluator
    {
        explicit evaluator(extractor extract); // with default call-operators
        evaluator(extractor extract, operators operators);

        web::json::value operator()(const web::json::value& arg, bool extract_value = false) const;

        extractor extract;
        rql::operators operators;
    };

    // Construct a set of RQL call-operators, using default json value comparison

    operators default_operators();
    operators default_any_operators(); // array-friendly variant

    // Construct a set of RQL call-operators, using specified json value comparison
    // This allows typed values to be processed as appropriate by implementations.

    // result should be value_true, value_false or value_indeterminate
    typedef std::function<web::json::value(const web::json::value& lhs, const web::json::value& rhs)> comparator;

    operators default_operators(comparator equal_to, comparator less);
    operators default_any_operators(comparator equal_to, comparator less); // array-friendly variant

    // Helpers for json value comparison, implementing three-valued (tribool) logic

    const web::json::value value_indeterminate = web::json::value::null();
    const web::json::value value_false = web::json::value::boolean(false);
    const web::json::value value_true = web::json::value::boolean(true);

    web::json::value default_equal_to(const web::json::value& lhs, const web::json::value& rhs);
    web::json::value default_less(const web::json::value& lhs, const web::json::value& rhs);
}

#endif