在解析括号表达式时提升精神核心转储

Question

有一些应该解析终端文字序列的简化语法：id、'<'、'>' 和“:action”。我需要允许括号 '(' ')' 只做提高阅读。（完整的例子有 http://coliru.stacked-crooked.com/a/dca93f5c8f37a889 ）我的语法片段：

    start  = expression % eol;
    expression   = (simple_def >> -expression)
    | (qi::lit('(') > expression > ')');

    simple_def = qi::lit('<') [qi::_val = Command::left] 
    | qi::lit('>') [qi::_val = Command::right] 
    | key [qi::_val = Command::id] 
    | qi::lit(":action") [qi::_val = Command::action] 
    ;
    
    key = +qi::char_("a-zA-Z_0-9");

当我尝试解析时：const std::string s = "(a1 > :action)"; 一切都很顺利。但是当我用括号 "(a1 (>) :action)" 带来更多复杂性时，我得到了 coredump。仅供参考 - coredump 发生在 coliru，而 msvc 编译的示例仅演示失败解析。

所以我的问题是：(1) 括号有什么问题，(2) 如何将括号引入表达式。

p.s。它是简化的语法，实际上我有更复杂的情况，但这是一个最小的可重现代码。

Answer 1

你应该只处理期望失败：

terminate called after throwing an instance of 'boost::wrapexcept<boost::spir
it::qi::expectation_failure<__gnu_cxx::__normal_iterator<char const*, std::__
cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >
>'
  what():  boost::spirit::qi::expectation_failure
Aborted (core dumped)

如果您处理预期失败，程序将不必终止。

修正语法

您的 'nested expression' 规则只接受一个表达式。我认为

expression = (simple_def >> -expression)

旨在匹配“1 个或多个 `simple_def”。但是，替代分支：

     | ('(' > expression > ')');

不接受相同的：它只是在解析 `)' 后停止。这意味着根据语法，您的输入完全无效。

我建议通过表达意图来简化。您在语义类型定义方面走在了正确的道路上。让我们避免“狡猾地” Line Of Lines（甚至是是什么？）：

using Id     = std::string;
using Line   = std::vector<Command>;
using Script = std::vector<Line>;

并始终如一地使用这些 typedef。现在，我们可以表达我们“思考”的语法：

    start  = skip(blank)[script];
    script = line % eol;

    line   = +simple;
    simple = group | command;
    group  = '(' > line > ')';

看，通过简化我们的心智模型并坚持下去，我们避免了您很难发现的整个问题。

这是一个快速演示，其中包括错误处理、可选的调试输出、测试用例和封装船长，因为它是语法的一部分：Live On Compiler Explorer

#include <fmt/ranges.h>
#include <fmt/ostream.h>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>

namespace qi  = boost::spirit::qi;
namespace phx = boost::phoenix;

enum class Command { id, left, right, action };

static inline std::ostream& operator<<(std::ostream& os, Command cmd) {
    switch (cmd) {
        case Command::id: return os << "[ID]";
        case Command::left: return os << "[LEFT]";
        case Command::right: return os << "[RIGHT]";
        case Command::action: return os << "[ACTION]";
    }
    return os << "[???]";
}

using Id     = std::string;
using Line   = std::vector<Command>;
using Script = std::vector<Line>;

template <typename It>
struct ExprGrammar : qi::grammar<It, Script()> {
    ExprGrammar() : ExprGrammar::base_type(start) {
        using namespace qi;

        start  = skip(blank)[script];
        script = line % eol;

        line   = +simple;
        simple = group | command;
        group  = '(' > line > ')';

        command = 
            lit('<')       [ _val = Command::left   ] |
            lit('>')       [ _val = Command::right  ] |
            key            [ _val = Command::id     ] |
            lit(":action") [ _val = Command::action ] ;

        key = +char_("a-zA-Z_0-9");

        BOOST_SPIRIT_DEBUG_NODES((command)(line)(simple)(group)(script)(key));
    }

private:
    qi::rule<It, Script()>                 start;
    qi::rule<It, Line(), qi::blank_type>   line, simple, group;
    qi::rule<It, Script(), qi::blank_type> script;

    qi::rule<It, Command(), qi::blank_type> command;

    // lexemes
    qi::rule<It, Id()> key;
};

int main() {
    using It = std::string::const_iterator;
    ExprGrammar<It> const p;

    for (const std::string s : {
            "a1 > :action\na1 (>) :action",
            "(a1 > :action)\n(a1 (>) :action)",
            "a1 (> :action)",
        }) {

        It f(begin(s)), l(end(s));

        try {
            Script parsed;
            bool ok = qi::parse(f, l, p, parsed);

            if (ok) {
                fmt::print("Parsed {}\n", parsed);
            } else {
                fmt::print("Parsed failed\n");
            }

            if (f != l) {
                fmt::print("Remaining unparsed: '{}'\n", std::string(f, l));
            }
        } catch (qi::expectation_failure<It> const& ef) {
            fmt::print("{}\n", ef.what()); // TODO add more details :)
        }
    }
}

版画

Parsed {{[ID], [RIGHT], [ACTION]}, {[ID], [RIGHT], [ACTION]}}
Parsed {{[ID], [RIGHT], [ACTION]}, {[ID], [RIGHT], [ACTION]}}
Parsed {{[ID], [RIGHT], [ACTION]}}

奖金

不过，我认为使用 qi::symbols 命令可以大大简化这一切。事实上，看起来你只是在标记化（当你说括号不重要时你确认了这一点）。

    line   = +simple;
    simple = group | command | (omit[key] >> attr(Command::id));
    group  = '(' > line > ')';
    key    = +char_("a-zA-Z_0-9");

现在你根本不需要 Phoenix：Live On Compiler Explorer，打印

ok? true {{[ID], [RIGHT], [ACTION]}, {[ID], [RIGHT], [ACTION]}}
ok? true {{[ID], [RIGHT], [ACTION]}, {[ID], [RIGHT], [ACTION]}}
ok? true {{[ID], [RIGHT], [ACTION]}}

更简单？

因为我观察到您基本上是按行标记化，为什么不简单地跳过括号，并一直简化为：

    script = line % eol;
    line   = *(command | omit[key] >> attr(Command::id));

那是全部。再看Live On Compiler Explorer：

#include <boost/spirit/include/qi.hpp>
#include <fmt/ostream.h>
#include <fmt/ranges.h>
namespace qi = boost::spirit::qi;

enum class Command { id, left, right, action };
using Id     = std::string;
using Line   = std::vector<Command>;
using Script = std::vector<Line>;

static inline std::ostream& operator<<(std::ostream& os, Command cmd) {
    return os << (std::array{"ID", "LEFT", "RIGHT", "ACTION"}.at(int(cmd)));
}

template <typename It>
struct ExprGrammar : qi::grammar<It, Script()> {
    ExprGrammar() : ExprGrammar::base_type(start) {
        using namespace qi;
        start = skip(skipper.alias())[line % eol];
        line  = *(command | omit[key] >> attr(Command::id));
        key   = +char_("a-zA-Z_0-9");

        BOOST_SPIRIT_DEBUG_NODES((line)(key));
    }
private:
    using Skipper = qi::rule<It>;
    qi::rule<It, Script()>        start;
    qi::rule<It, Line(), Skipper> line;

    Skipper                 skipper = qi::char_(" \t\b\f()");
    qi::rule<It /*, Id()*/> key; // omit attribute for efficiency
    struct cmdsym : qi::symbols<char, Command> {
        cmdsym() { this->add("<", Command::left)
            (">", Command::right)
            (":action", Command::action);
        }
    } command;
};

int main() {
    using It = std::string::const_iterator;
    ExprGrammar<It> const p;

    for (const std::string s : {
            "a1 > :action\na1 (>) :action",
            "(a1 > :action)\n(a1 (>) :action)",
            "a1 (> :action)",
        })
    try {
        It f(begin(s)), l(end(s));

        Script parsed;
        bool ok = qi::parse(f, l, p, parsed);

        fmt::print("ok? {} {}\n", ok, parsed);
        if (f != l)
            fmt::print(" -- Remaining '{}'\n", std::string(f, l));
    } catch (qi::expectation_failure<It> const& ef) {
        fmt::print("{}\n", ef.what()); // TODO add more details :)
    }
}

版画

ok? true {{ID, RIGHT, ACTION}, {ID, RIGHT, ACTION}}
ok? true {{ID, RIGHT, ACTION}, {ID, RIGHT, ACTION}}
ok? true {{ID, RIGHT, ACTION}}

Note I very subtly changed +() to *() so it would accept empty lines as well. This may or may not be what you want

在解析括号表达式时提升精神核心转储

Boost spirit core dump on parsing bracketed expression

c++

boost

boost-spirit

修正语法

奖金

更简单？