我如何在精神X3中添加条件期望点

How can I add conditional expectation points in spirit X3

本文关键字：添加 条件 期望 X3      更新时间：2023-10-16

我目前正在向X3中的语法添加期望点。现在我遇到了一个规则，它看起来像这样。

auto const id_string = +x3::char("A-Za-z0-9_);
auto const nested_identifier_def =
        x3::lexeme[
                *(id_string >> "::")
                >> *(id_string >> ".")
                >> id_string
        ];

我想知道如何在此规则中添加条件期望点。比如"如果有一个"::"那么一定有一个id_string "或者"如果有一个.那么一定有一个id_string "等等......对于这样的规则，我怎样才能达到这样的行为呢?

我会按照你想要的方式来写:

auto const identifier 
    = lexeme [+char_("A-Za-z0-9_")];
auto const qualified_id 
    = identifier >> *("::" > identifier);
auto const simple_expression // only member expressions supported now
    = qualified_id >> *('.' > identifier);

和相应的AST:

namespace AST {
    using identifier = std::string;
    struct qualified_id : std::vector<identifier> { using std::vector<identifier>::vector; };
    struct simple_expression {
        qualified_id lhs;
        std::vector<identifier> rhs;
    };
}

现场演示

Live On Coliru

#include <iostream>
#include <string>
#include <vector>
namespace AST {
    using identifier = std::string;
    struct qualified_id : std::vector<identifier> { using std::vector<identifier>::vector; };
    struct simple_expression {
        qualified_id lhs;
        std::vector<identifier> rhs;
    };
}
#include <boost/fusion/adapted.hpp>
BOOST_FUSION_ADAPT_STRUCT(AST::simple_expression, lhs, rhs)
#include <boost/spirit/home/x3.hpp>
namespace Parser {
    using namespace boost::spirit::x3;
    auto const identifier 
        = rule<struct identifier_, AST::identifier> {}
        = lexeme [+char_("A-Za-z0-9_")];
    auto const qualified_id 
        = rule<struct qualified_id_, AST::qualified_id> {}
        = identifier >> *("::" > identifier);
    auto const simple_expression // only member expressions supported now
        = rule<struct simple_expression_, AST::simple_expression> {}
        = qualified_id >> *('.' > identifier);
}
int main() {
    using It = std::string::const_iterator;
    for (std::string const input : { "foo", "foo::bar", "foo.member", "foo::bar.member.subobject" }) {
        It f = input.begin(), l = input.end();
        AST::simple_expression data;
        bool ok = phrase_parse(f, l, Parser::simple_expression, boost::spirit::x3::space, data);
        if (ok) {
            std::cout << "Parse success: ";
            for (auto& el : data.lhs) std::cout << "::" << el;
            for (auto& el : data.rhs) std::cout << "." << el;
            std::cout << "n";
        }
        else {
            std::cout << "Parse failure ('" << input << "')n";
        }
        if (f != l)
            std::cout << "Remaining unparsed input: '" << std::string(f, l) << "'n";
    }
}

打印

Parse success: ::foo
Parse success: ::foo::bar
Parse success: ::foo.member
Parse success: ::foo::bar.member.subobject

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