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The Review for Zach's parser library has concluded
by Marshall Clow 01 Mar '24

01 Mar '24

Thanks to everyone who participated, and especially to those to submitted reviews. (And Zach, of courser) I’ll have results soon. — Marshall

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Boost.Parser parsing length-prefixed strings efficiently
by Mohammad Nejati [ashtum] 01 Mar '24

01 Mar '24

Hi everyone, I have two questions about the following code, which attempts to parse length-prefixed strings using the proposed Boost.Parser: ``` std::string input = "[5]HELLO[5]WORLD"; unsigned int size = 0; auto action = [&size](auto & ctx) { size = bp::_attr(ctx); }; auto lps_parser = '[' >> bp::uint_[action] >> ']' >> bp::repeat(std::ref(size))[bp::char_]; auto result = bp::parse(input, *lps_parser); ``` 1. Is it possible to eliminate the need for the action and size variable by using some kind of placeholder in bp::repeat that utilizes the attribute of the first parser? 2. How can we make this part of the parser more efficient: bp::repeat(std::ref(size))[bp::char_] The current implementation seems to loop and execute bp::char_ parser on each character, while all it needs to do is chunk a portion of the input string. Regards, Mohammad Nejati

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New website is open-source!
by Vinnie Falco 29 Feb '24

29 Feb '24

The new website and documentation are provided under the BSL, and the source repositories have been transferred to the Boost GitHub organization: https://github.com/boostorg/website-v2 https://github.com/boostorg/website-v2-docs While we sort out the unresolved issues related to the licensing of artwork and logo, the new website has moved to this temporary location for now: https://boost.io Please continue to experience, enjoy, and most importantly test the new website. A gentle reminder: it is not “finished” and likely will not be “perfect” for a while. The launch of the new website is the beginning of the journey rather than the end. Issues opened here will be acted upon by our dedicated website development team: https://github.com/boostorg/website-v2/issues For documentation-related issues, please open and monitor issues here: https://github.com/boostorg/website-v2-docs/issues Pull requests are always welcomed! Thanks

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Boost.Parser questions
by Ruben Perez 29 Feb '24

29 Feb '24

Hi all, Sorry for jumping into this so late. I'll send my review tomorrow (that's still on time, isn't it?). I've been playing with the library and some questions popped up (please note I've no prior experience with Spirit): * What's the rationale for using operator[] for semantic actions (instead of a regular method like .action())? It looks like the parsing problem with inline lambdas could be easily worked around with this. * What's the rationale for using operator[] for directives? repeat(24)[p] looks like an over-complication, where a regular function repeat(24, p) would have just worked fine. * For non-recursive rules, what's the rationale on preferring the macro over something like: constexpr bp::rule<struct replacement_field_tag, std::string> replacement_field { "replacement_field", +bp::char_ }; * My use case involves parsing identifiers that can only contain ASCII lowercase, uppercase, digits and the underscore. I'd usually do this by defining a `bool is_identifier_char(char c)` function. This is what I could do with Boost.Parser - is there a better way (one that doesn't force me to list all valid characters)? constexpr const char name_chars[] = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_0123456789"; constexpr auto name_continuation_rng = std::span<const char>( name_continuation, sizeof(name_continuation) - 1 ); * What's the rationale behind the special naming convention for functions like _attr, _value, _globals and the like? Is there a reason why these can't be member functions of the context? * I've found really frustrating the generic lambda approach for semantic actions, since this completely inhibits tooling like clangd. Is there a reason why we can't have a public definition of the parsing context I could use in my functions? * I started my evaluation using clang-18 (Linux) but builds fail (seems to happen under all clang versions I tried). gcc-12 seems to work fine. Note that I'm using the develop branch, since I understand it contains the bug fixes for the issues that previous reviewers found. Regards, Ruben.

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Boost.Parser review
by Richard 29 Feb '24

29 Feb '24

- Do you think the library should be accepted as a Boost library? Yes, this would be a great addition to Boost as a successor to Spirit. ====================================================================== Details: Note: I've only had time to read through the documentation. I'm familar with Spirit Classic, Spirit V2 and X3 parsing libraries and have done presentations on them using my own written example code. - What is your evaluation of the design? It's very similar to Boost.Spirit in philosophy, but other than the (optional) connection to Boost.Hana feels very decoupled from all the other bits of boost at a user level. Spirit's direct connection to things like Phoenix and Fusion made Spirit more complicated to explain to other developers and more difficult to use IMO. I understand why such things were done at the time of Boost.Spirit's writing, but being a C++17 library frees Boost.Parser from all these digressions which is good. The only thing obviously lacking were the binary parsers from Spirit V2 (bug filed). - What is your evaluation of the implementation? I've only reviewed the documentation, so I haven't had any time to dig into the implementation before the review deadline. I will say that there are no comments in the header files and that could be improved. It means that without consulting the tutorial documentation, I have no information in the included headers to guide me. - What is your evaluation of the documentation? The documentation is well written and follows a logical progression. It could use a math expression example as this is one of the most common reasons for needing to write a parser (typical C style math operators and precedence rules). The JSON example is nice (shows recursive rule parsing), but doesn't deal with operator precedence at all, so another detailed example showing this would be helpful. There were some minor typos and mistakes; I filed github issues on those. The concepts and reference section are just raw code dumps. They could use some additional doxygen style comments to tie things together. - What is your evaluation of the potential usefulness of the library? I would find this library a great successor to Spirit. - Did you try to use the library? With what compiler? Did you have any problems? No, I only had time to read through documentation. - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? I carefully read through every page of the documentation in the Tutorial section and skimmed over the reference section. - Are you knowledgeable about the problem domain? Yes. -- "The Direct3D Graphics Pipeline" free book <http://tinyurl.com/d3d-pipeline> The Terminals Wiki <http://terminals-wiki.org> The Computer Graphics Museum <http://ComputerGraphicsMuseum.org> Legalize Adulthood! (my blog) <http://LegalizeAdulthood.wordpress.com>

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Boost.Parser review
by Ruben Perez 29 Feb '24

29 Feb '24

Hi all, This is my review of the proposed Boost.Parser. As in other occasions, I've taken a practical approach and used the library to (re)implement an existing, real feature in Boost.MySQL: client-side SQL formatting. This requires parsing a format string with replacement fields (similar to std::format strings). - What is your evaluation of the potential usefulness of the library? I think it's useful. Parsing is a common task, and this library simplifies it. I tend to prefer hand-crafted parsers in general, but libraries like the proposed tend to save time (and bugs). Also, many other users have manifested interest in it. I think the library is too heavyweight regarding compile times to be used in header-only libraries like Boost.MySQL, but it should be fine in compiled libraries and applications. - What is your evaluation of the design? I think it's pretty good in general. It allows you to express complex parsers using little code, which is good. In my opinion, it may be too generic (allowing any forward character range as input, for instance), which tends to increase complexity. I tend to prefer simpler approaches like the one Boost.URL uses - but I understand not everyone thinks like me, and there's a use case for each approach. As we discussed in previous posts, if I was writing this, I would have made some decisions differently (see https://lists.boost.org/Archives/boost/2024/02/256010.php) but I don't think there is any problem with the current design. The author has justified his decisions, and everything makes sense. - What is your evaluation of the implementation? I can tell it's really complex. I've attempted to debug a problem I had for which parser::trace::on wasn't enough (see below), and navigating the code was extremely difficult (reminds me of debugging Asio). This is probably inherent to the level of complexity of the library. This is not a criticism, but a thought I wanted to share. - Did you try to use the library? With what compiler? Did you have any problems? Yes, I did. As mentioned above, I re-implemented Boost.MySQL SQL formatting (see https://www.boost.org/doc/libs/master/libs/mysql/doc/html/mysql/sql_formatt…) using the proposed library. I've uploaded the code to a branch - you can check it here: https://github.com/boostorg/mysql/blob/66c99e31c9e548b6cf5b98e3d08255f22390… For reference, I've also pasted the grammar and the code at the end of this email. I've found the following problems: * Rule parsers using sequence + optional parsers yield unexpected results when the optional rule doesn't match. Making it a regular parser, rather than a rule parser, fixes the problem. The impact is that automatic replacement fields (i.e. `{}`) were being parsed as `{0}`. https://github.com/tzlaine/parser/issues/125 tracks this. * My clang-11 build fails. Looks like a concept is being used without proper protection, and the compiler doesn't support it. https://github.com/tzlaine/parser/issues/129 * My clang-16 build (and my local build attempts with clang 17, 18 and 19) fail because of a non-matching template redeclaration in parser/detail/text/transcode_view.hpp. I don't know if it's clang being picky or a clang bug. https://github.com/tzlaine/parser/issues/130 * My clang-14 build fails. This looks like it's hitting a clang + libstdc++ bug. See https://github.com/tzlaine/parser/issues/131. * My Apple clang 14 build fails. It also looks like an issue in clang or libc++. See https://github.com/tzlaine/parser/issues/133. * My MSVC 14.2 and 14.3 builds under C++17 fail (C++20 works fine). I've completely failed to parse what's the problem here. Description here: https://github.com/tzlaine/parser/issues/132 Some other minor problems: * As reported by other reviewers, enabling warnings=all or warnings=extra issues a lot of warnings. * [[no_unique_address]] (C++20) is used unprotected, which causes warnings under clang-7 (see https://github.com/tzlaine/parser/issues/128) * The CMakeLists.txt is not Boost-compliant (yet). * There is a #include <boost/hana.hpp>, which is not very friendly for compile-times - better replace it by the individual header that is needed. I've submitted the build logs together with all details I've been able to gather. I haven't been able to provide a minimal reproducible example for some of these (they were a lot and I had limited time). Note that all the builds referenced in the issues (except the OSX one) are run in Docker containers. If you have access to Docker, reproducing these environments should be trivial. Since I've done this in the past, should the author need any help with this, please let me know, I'm happy to help. Reading the CI files for the proposed library, I think compiler coverage is insufficient - for instance, there are no Linux clang builds, and the only Apple clang build uses clang 13 and C++17. - What is your evaluation of the documentation? The presentation is really good. I struggled to understand the concepts at first because I had no previous experience with parsers. I ended up reading the Bison concepts section (https://www.gnu.org/software/bison/manual/bison.html#Concepts) after which everything made perfect sense. I'd advise to expand the introduction a bit, or just point to other material for people like me. I fail to understand what RESOLVE means in https://tzlaine.github.io/parser/doc/html/boost_parser__proposed_/tutorial/…. Docs state "RESOLVE() is a notional macro that expands to the resolution of parse argument or evaluation of a parse predicate", but I'd say this resolution hasn't been explained yet - a link here would help. I've found the reference to be more confusing that helpful - it looks like a straight header dump. This is made worse by the concepts not being linked. Although it has many terrible things, the quickbook toolchain is (partly) capable of linking concepts - have a look at https://www.boost.org/doc/libs/master/libs/mysql/doc/html/mysql/ref/boost__…. This still has limitations, but I'd say it's better than what's currently there. If this is something you'd be interested in, please let me know and I will help you with it. - How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? I've spent around 2 full days of work reading about the problem domain, studying the docs, writing my use case, diagnosing the problems I reported and writing this review. - Are you knowledgeable about the problem domain? I've never used a parsing library like this before. I've read about the problem in the past, but wasn't very knowledgeable about it before the review. I've written small parsers like the one for format strings in the past, though. - My conclusion My recommendation is to REJECT the library in its current form. This is not a hard rejection - I completely think this library can be useful to people. But I found too many problems while using it - too many to get it into Boost in its current form. I'd be happy to see this library being proposed again in a couple of months, once all the issues found during the review get fixed. - Affiliation disclosure Please note that I'm affiliated with the C++ Alliance. I'd like to thank Zach for proposing the library, and Marshall for managing the review. Regards, Ruben. - Reference: parsing code in Boost.MySQL /** * format-str = *( text | literal-opening-brace | literal-closing-brace | replacement-field ) * text = *( any characters except {} ) * replacement-field = "{" [ replacement-id ] "}" * replacement-id = ushort | field-name * field-name = (character a-z, A-Z, _) *( any characters 0-9, a-z, A-Z, _ ) */ // Note: globals is a format_state structure, which has member functions // to append raw text and replacement fields to the output string // Actions constexpr auto action_text = [](auto& ctx) { _globals(ctx).append_text(_attr(ctx)); }; constexpr auto action_replacement_field = [](auto& ctx) { const std::optional<std::variant<unsigned short, std::string>>& val = _attr(ctx); format_state& st = _globals(ctx); if (val) { if (const auto* idx = std::get_if<unsigned short>(&*val)) { st.append_indexed_field(*idx); } else { st.append_named_field(std::get<std::string>(*val)); } } else { st.append_auto_field(); } }; constexpr auto action_opening_brace = [](auto& ctx) { _globals(ctx).append_text("{"); }; constexpr auto action_closing_brace = [](auto& ctx) { _globals(ctx).append_text("}"); }; // Character ranges constexpr std::array<const char, 63> name_chars{ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '_', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' }; constexpr boost::span<const char, 53> name_start_chars{name_chars.data(), 53}; // Parsers and rules constexpr auto field_name = parser::char_(name_start_chars) >> *(parser::char_(name_chars)); constexpr auto replacement_id = parser::ushort_ | field_name; constexpr auto replacement_field = ('{' >> -replacement_id) > '}'; constexpr auto literal_open_brace = parser::lit("{{"); constexpr auto literal_close_brace = parser::lit("}}"); constexpr auto text = +(parser::char_ - '{' - '}'); constexpr auto format_str_parser = *( text[action_text] | literal_open_brace[action_opening_brace] | literal_close_brace[action_closing_brace] | replacement_field[action_replacement_field] ); // Top-level parsing function inline void parse_format_str(string_view format_str, format_context_base& ctx, span<const format_arg> args) { // By default, parser prints to cerr, which we don't want parser::callback_error_handler handler([](const std::string&) {}); // Create a parsing state format_state st(ctx, args); // Invoke the actual parsing bool result = parser::parse( format_str, parser::with_globals(parser::with_error_handler(format_str_parser, handler), st) ); // If there was an error, report it if (!result) { ctx.add_error(client_errc::format_string_invalid_syntax); } }

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[release] The 1.85.0 release will close for major changes TOMORROW
by Marshall Clow 28 Feb '24

28 Feb '24

Calendar here: https://www.boost.org/development/ Next deadline: The master branch will close for the beta next Wednesday. — The release managers

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Boost.Parser review by Tobias
by tobi＠die-loews.de 27 Feb '24

27 Feb '24

Boost.Parser review Why do we need a DSEL-Parser library in Boost: I think nobody, who once has written a parser with a DSEL (like e.g. Spirit) in C++, ever want's to go back to out off source parser-generators like yacc/bison or ANTLR (or even hand written parsers): it is so convenient having the parser definition as a grammar within the code. Additionally, you can quite easily hook into the parser (e.g. Spirit's Phoenix functions). So, IMHO there is definitely need for such C++ libraries. On the other hand, the task of creating such a library is very challenging and requires high expertise and coding standards, as library-users have to rely on its correctness. So, Boost with its standards and peer-review process is definitely the place for such a library. (Personally, I wouldn't use a parser-library the hasn't gone through such a process.) Should accept Boost.Parser into Boost? There are already three parser libraries in Boost: Spirt and its successors Qi and X3 - Spirit: very outdated interface, IIRC C++98 compatible, superseded by Qi - Qi: Simply a revolution in parsing! It was blowing my mind, when I first used it. The way attributes were composed and semantic actions could applied with Phoenix was totally exciting. But of course, there was also the downside of (very) long compilation-times and frustrating long error-messages when something got wrong. And (again IIRC) Qi came to life before C++11, so it is also C++98. - X3: I only wrote one larger X3 parser, and to be frank, IMHO it never made it to production-level. There are several things that worked perfectly with Qi, which I never got running the same way in X3. Additionally, the Spirit libraries seem no longer to be in active development. Now, let's get to Boost.Parser: First, I don't want to go into the details of the library, as I think other reviewers did a very good job there. I've been following its evolving since I heard of it in 2022. Back then, I did some test with it and reported some Issues, which got resolved. During the last two years many additional features got added to it. So, Boost.Parser is definitely under active development. Furthermore, the author is an expert for Unicode, which is reflected in the built-in Unicode support of the library. It is written in modern C++ (requires C++17). It is compile-time friendly, supports std::ranges and concepts. It offers an option/abstraction to choose between std::tuples, if needed, or the more flexible hana-implementation. It's syntax is close to Qi, so transition for Qi-users should not be too hard. The documentation contains a lot of examples that help to understand how the library works and how it can be used. Finally, I really like the very easy to use trace-mode. Again, should accept Boost.Parser into Boost? Definitely Yes! Tobias

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Boost.Parser Review
by Phil Endecott 26 Feb '24

26 Feb '24

Boost.Parser Review =================== I've been using Spirit for almost 20 years. It has always been an amazing example of pushing the limits of what is possible in C++, even by the standards of Boost. So it has always left me thinking that we just need to wait a few more years until the language has the few more features needed to smooth over its remaining awkwardness. Now a few years have passed and we have, for example, lambdas, auto, constexpr, concepts. So I was excited to see what improvements Zach's proposed library has over Spirit. For example, when Metaparse was reviewed in 2015, compile-time parsing of BNF was discussed. Perhaps we should be able to write actual BNF in compile-time strings now, rather than re-use C++ operators? And now that we have constexpr, should we be able to convert the production rules to tables at compile time and do proper linear-complexity LALR parsing, rather than top-down? We do now have compile-time regular expression libraries than can do something like this. Initial Reaction ---------------- My first reaction when I looked at the docs was: - I'd like to congratulate Zach on choosing a sensible descriptive name for the library. - The rule syntax looks very much like Spirit. The docs look like the Spirit docs. It's more of an evolution than a revolution. - There's an ugly MACRO to define rules! That doesn't look very 2024 to me. Test Case --------- I decided to try to translate an exiting parser from Spirit v2 (qi) to the new library. It's a parser for latitude-longitude coordinates entered by a human, i.e. it is supposed to try to recognise a range of possible formats. Examples: 12.34 N, 56.78 E 56.78,-12.34 12d 30' n 45d 15' 7" w 12 30 45 N, 45 15 7 W 12d 30m 15s S 50d 59m 59s W There are some subtle issues with this grammar. Note for example that 's' could mean seconds or South. One advantage of Bison is that it gives compile-time diagnostics reporting some of these. Neither Spirit nor the new library can do this. The core of the Spirit code is as follows: (I will link to a .zip with the complete code at the end; I bet the mailing list messes up the long lines.) ///////////////////////////////////// degrees_symbol = lit(UTF8_DEGREES) | no_case[ lit("degrees") | lit("deg") | lit('d') ]; minutes_symbol = lit(UTF8_PRIME) | no_case[ lit('\'') | lit("minutes") | lit("min") | lit('m') ]; seconds_symbol = lit(UTF8_DOUBLE_PRIME) | no_case[ lit('"') | lit("seconds") | lit("sec") | lit('s') ]; uint_0_60 %= uint_ [ _pass = _1 < 60U ]; double_0_60 %= basic_udouble [ _pass = _1 < 60 ]; decimal_degrees %= basic_udouble >> -degrees_symbol; degrees_decimal_minutes = (uint_ >> -degrees_symbol >> double_0_60 >> -minutes_symbol) [ _val = _1 + _2/60.0 ]; degrees_minutes_seconds = (uint_ >> -degrees_symbol >> uint_0_60 >> -minutes_symbol >> double_0_60 >> -seconds_symbol) [ _val = _1 + _2/60.0 + _3/3600.0 ]; degrees %= degrees_minutes_seconds // Longest first. | degrees_decimal_minutes | decimal_degrees; northsouth %= no_case[ char_("ns") ]; eastwest %= no_case[ char_("ew") ]; latitude = (degrees >> northsouth) [( _pass = _1 <= 90, _val = if_else(_2=='S' || _2=='s', -_1, _1) )]; longitude = (degrees >> eastwest) [( _pass = _1 <= 180, _val = if_else(_2=='W' || _2=='w', -_1, _1) )]; signed_degrees %= basic_double >> -degrees_symbol; signed_latitude %= signed_degrees [ _pass = -90 <= _1 && _1 <= 90 ]; signed_longitude %= signed_degrees [ _pass = -180 <= _1 && _1 <= 180 ]; latlon = ((latitude >> longitude) [ _val = construct<g2d::Vector>(_2,_1) ]) | ((longitude >> latitude) [ _val = construct<g2d::Vector>(_1,_2) ]) | ((signed_longitude >> signed_latitude) [ _val = construct<g2d::Vector>(_1,_2) ]); ///////////////////////////////////// The complete Boost.Parser code is as follows: ///////////////////////////////////// #include "boost/parser/parser.hpp" #include <cassert> #include <iostream> #include <optional> #include <string> namespace bp = boost::parser; using namespace boost::hana::literals; namespace g2d { struct Vector { double x, y; }; }; bp::rule<struct uint_0_60, unsigned int> uint_0_60 = "uint_0_60"; bp::rule<struct double_0_60, double> double_0_60 = "double_0_60"; bp::rule<struct degrees_decimal_minutes, double> degrees_decimal_minutes = "degrees_decimal_minutes"; bp::rule<struct degrees_minutes_seconds, double> degrees_minutes_seconds = "degrees_minutes_seconds"; bp::rule<struct degrees, double> degrees = "degrees"; bp::rule<struct latitude, double> latitude = "latitude"; bp::rule<struct longitude, double> longitude = "longitude"; bp::rule<struct signed_latitude, double> signed_latitude = "signed_latitude"; bp::rule<struct signed_longitude, double> signed_longitude = "signed_longitude"; bp::rule<struct latlon, g2d::Vector> latlon = "latlon"; //const auto degrees_symbol = bp::no_case[ bp::lit("degrees") | bp::lit("deg") | bp::lit('d') ]; //const auto minutes_symbol = bp::no_case[ bp::lit('\'') | bp::lit("minutes") | bp::lit("min") | bp::lit('m') ]; //const auto seconds_symbol = bp::no_case[ bp::lit('"') | bp::lit("seconds") | bp::lit("sec") | bp::lit('s') ]; // FIXME those rules don't work and I don't know why. They work if the symbol is // the first alternative; it it's any of the other alternatives the double returned // by e.g. decimal_degrees below is always zero. // Use these instead: const auto degrees_symbol = bp::lit('d'); const auto minutes_symbol = bp::lit('m'); const auto seconds_symbol = bp::lit('s'); // Beware, 's' can also mean south. const auto uint_0_60_def = bp::uint_ [( [](auto ctx) { _pass(ctx) = _attr(ctx) < 60U; _val(ctx) = _attr(ctx); } )]; const auto double_0_60_def = bp::double_ [( [](auto ctx) { _pass(ctx) = _attr(ctx) < 60; _val(ctx) = _attr(ctx); } )]; const auto decimal_degrees = bp::double_ >> -degrees_symbol; const auto degrees_decimal_minutes_def = (bp::uint_ >> -degrees_symbol >> double_0_60 >> -minutes_symbol) [( [](auto ctx) { auto d = _attr(ctx)[0_c]; auto m = _attr(ctx)[1_c]; _val(ctx) = d + m/60.0; } )]; // FIXME this rule matches e.g. 150.5 as 150 .5; we want that input to // be matched by the decimal degrees rule as 150.5. In the qi code, which // has the same fundamental problem, it is avoided because the double_ // parser is configured to reject input with a leading decimal point. // It would be more correct to require that either a degrees symbol or // whitepace or both between the degrees and minutes numbers. That's // a bit complicated because the whitespace is handled by the skipper. // In a bottom-up parser I think the ambiguity is resolved by putting // decimal_degrees first in degrees_def below. That doesn't work for a // top-down parser. // FIXME for similar reasons, "150d 0.5m n" fails to parse, because // the degrees_minutes_seconds rule matches 150,0,.5 and then the next // rule up fails to parse the 'm'. const auto degrees_minutes_seconds_def = (bp::uint_ >> -degrees_symbol >> uint_0_60 >> -minutes_symbol >> double_0_60 >> -seconds_symbol) [( [](auto ctx) { auto d = _attr(ctx)[0_c]; auto m = _attr(ctx)[1_c]; auto s = _attr(ctx)[2_c]; _val(ctx) = d + m/60.0 + s/3600.0; } )]; const auto degrees_def = degrees_minutes_seconds | degrees_decimal_minutes | decimal_degrees; const auto northsouth = bp::no_case[ bp::char_("ns") ]; const auto eastwest = bp::no_case[ bp::char_("ew") ]; const auto latitude_def = (degrees >> northsouth) [( [](auto ctx) { auto d = _attr(ctx)[0_c]; auto ns = _attr(ctx)[1_c]; _pass(ctx) = d <= 90; _val(ctx) = ns=='S' || ns=='s' ? -d : d; } )]; const auto longitude_def = (degrees >> eastwest) [( [](auto ctx) { auto d = _attr(ctx)[0_c]; auto ew = _attr(ctx)[1_c]; _pass(ctx) = d <= 180; _val(ctx) = ew=='W' || ew=='w' ? -d : d; } )]; const auto signed_degrees = bp::double_ >> -degrees_symbol; const auto signed_latitude_def = signed_degrees [( [](auto ctx) { auto d = _attr(ctx); _pass(ctx) = -90 <= d && d <= 90; } )]; const auto signed_longitude_def = signed_degrees [( [](auto ctx) { auto d = _attr(ctx); _pass(ctx) = -180 <= d && d <= 180; } )]; const auto latlon_def = ((latitude >> longitude) [( [](auto ctx) { _val(ctx) = g2d::Vector{_attr(ctx)[1_c], _attr(ctx)[0_c]}; } )] ) | ((longitude >> latitude) [( [](auto ctx) { _val(ctx) = g2d::Vector{_attr(ctx)[0_c], _attr(ctx)[1_c]}; } )] ) | ((signed_longitude >> signed_latitude) [( [](auto ctx) { _val(ctx) = g2d::Vector{_attr(ctx)[0_c], _attr(ctx)[1_c]}; } )] ); BOOST_PARSER_DEFINE_RULES(uint_0_60); BOOST_PARSER_DEFINE_RULES(double_0_60); BOOST_PARSER_DEFINE_RULES(degrees_decimal_minutes); BOOST_PARSER_DEFINE_RULES(degrees_minutes_seconds); BOOST_PARSER_DEFINE_RULES(degrees); BOOST_PARSER_DEFINE_RULES(latitude); BOOST_PARSER_DEFINE_RULES(longitude); BOOST_PARSER_DEFINE_RULES(signed_latitude); BOOST_PARSER_DEFINE_RULES(signed_longitude); BOOST_PARSER_DEFINE_RULES(latlon); static std::optional<g2d::Vector> try_parse_latlon(std::string_view s) { auto input = latlon >> bp::eoi; return parse(s,input, bp::ws|bp::lit(',')); } int main(int argc, const char* argv[]) { assert(argc==2); auto opt_coords = try_parse_latlon(argv[1]); if (!opt_coords) { std::cerr << "parse error\n"; return 1; } else { std::cout << opt_coords->x << " " << opt_coords->y << "\n"; return 0; } } ///////////////////////////////////// That Ugly Macro --------------- bp::rule<struct latitude, double> latitude = "latitude"; const auto latitude_def = (degrees >> northsouth) .... ; BOOST_PARSER_DEFINE_RULES(latitude); Is a macro really the best way to do this? Spirit seems to manage without. Despite having a macro this is still very verbose. I've had to write the name of the rule FIVE times in three lines. Once there is a macro you might as well put everything inside it: BOOST_PARSER_DEFINE_RULE(latitude, double, (degrees >> northsouth) ....); Of course if you need to declare the rules before defining them due to recursion or other forward references you will need separate macros for declaration and definition. These rule declarations seem to need to be at global scope (maybe namespace scope?). Is that fundamental? Can't I put them in a class or a function? Semantic Actions ---------------- In Bison I can write semantic actions like this: { $$ = $1 + $2/60.0; } After expanding the $ variables that gets copied verbatim into the bison output, so it can include any code. Spirit clearly wanted to replicate that sort of functionality, but it pre-dates lambdas, so it includes a lot of amazing functionality that allows me to write semantic actions like: [ _val = _1 + _2/60.0 ] This is nice and concise but its limitation is of course that I cannot write arbitrary code in there. (BTW, are we strictly allowed to use identifiers starting with _ like _val?) What should this look like now that we have lambdas in the language? Of course it's going to be a bit more verbose because of the syntax overhead, but I was hoping that it would be something like this: [ [](auto deg, auto min) { return deg + min/60.0; } ] (Aside: That doesn't actually work because [[ can only appear in attributes even if there is a space, so it needs to be written [( [] ... )]. I did wonder if changing from operator[] to operator() might make sense?) That's longer than before, but I have given meaningful names to the parameters (which I can also do in Bison BTW) and I can include any code. But that's not the syntax that the current proposal needs. It needs this: [( [](auto ctx) { _val(ctx) = _attr(ctx)[0_c] + _attr(ctx)[1_c]; } )] Frankly I think that combines the worst of all the other options and adds some extra new verbosity of its own. One other advantage of having the lambda return a result could be improving how the semantic type (aka attribute type) of a rule is set. For example, if I have the rule degrees >> minutes where degrees and minutes return doubles, the semantic type of the combined rule is something like tuple<double,double>. But I'm going to add a semantic action that adds the two values as shown above, returning one double. If the return value of the semantic action's lambda were used as the semantic type of the rule then we could write: auto rule = (degrees >> minutes) [( [](auto d, auto m) { return d + m/60.0; } )]; and rule would return a double to its parent rule. As it is, the semantic action doesn't change the type and we have to explicitly and verbosely declare the rule as returning a double. Of course the context includes more than just the child rule semantic values and a slot for the result; for example it includes the _pass boolean that I use to clamp the range of values in my example: [( [](auto ctx) { _pass(ctx) = _attr(ctx)[0_c] <= 90; } )]; I suggest that either a special lambda signature is used to indicate that it is a predicate, or an additional "directive" e.g. predicate[ ... ] is added. Compilation Times and Errors ---------------------------- Compilation times for my example were: Boost.Parser: 28.6 s Boost.Spirit: 30.1 s but the grammar is not quite identical, so the small difference is not significant. Code size is somewhat larger for Parser than for Spirit. (g++ 12.2.0, -O3, Linux, aarch64.) I did not particularly notice improved error messages compared to Spirit, though they weren't worse. Consider for example this code from my example: return parse(s, input, bp::ws|bp::lit(',')); Originally I had this: auto skipper = bp::ws | bp::lit(','); return parse(s, input, skipper); But that failed to compile, and I got 15 screenfuls of inpenetrable error messages. I fixed it more-or-less by randomly changing things until it worked. If I now change it back I get only two screenfuls that accurately say that I can't return a bool from a function that's supposed to return an optional, but I still don't know why the return type of parse() changes when I move the declaration of the skipper. So I fear that the idea that compile times and error verbosity are improved compared to Spirit are not borne out. Bugs ---- I found a couple of bugs while writing this review which Zach has resolved. I suspect that this code has not had enough varied use to flush out all the issues. Other Issues ------------ The double_ parser is not configurable in the way that the Spirit double_ parser is. This is problematic; in my example 3e6n means "3 east 6 north", not "3000000 north", so I need a double_ parser that doesn't recognise exponents. I'd also like to ignore values with leading .s, i.e. 10.1 should not match int_ >> double_ as "10 0.1", and I really don't want a hacker to sneak a nan or inf into my code if it recognises those. The docs are not bad and are reminiscent of the Spirit docs. Personally I would prefer to see an example early on that takes a chunk of real BNF and translates it into a parser. I had issues with Apple Clang (15.0.0); it would not compile "#include <boost/parser.hpp>" until I turned off concepts. Has anyone tried with other Clangs? If this is not fixable maybe the library should detect this and use non-concepts mode automatically. Compiling with -Wall resulted in lots of unused parameter warnings. These can be fixed by commenting-out the parameter name, leaving just the type. Boost libraries should compile without warnings, some people use -Werror. Conclusion ---------- I was hoping to find a library that was an improvement on Spirit, but I regret to say that that's not what I've found here. The only notable improvement compared to Spirit.Qi that I've found is that no_case[] is propogated to nested rules, which isn't the case with Qi. Rules themselves are essentially the same as Spirit. The method for defining rules is more verbose and now needs an ugly macro. Semantic actions are less readable. It is possible that there are significant advantages compared to Spirit that I have missed, as a result of having used working Spirit code as my starting point, but I have read the doc's comparison with Spirit page. My conclusion is that this library should not be accepted. If others believe it should be accepted then I feel that should be conditional on many of the issues I've mentioned above being resolved, e.g. unused parameter warnings, configuration of the double_ parser, etc. File containing example: https://chezphil.org/tmp/parser_review.zip Regards, Phil.

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Doxygen+Quickbook and C++20 concepts
by Zach Laine 26 Feb '24

26 Feb '24

I've seen concepts just vanish from Doxygen+Quickbook docs in the past. I thought this was just a limitation. However, looking at the Quickbook docs, I see that there's now a conceptref reference link. So, I put this very simple concept in a header (parser.hpp) to test things: /** \concept non_void */ template<typename T> concept non_void = (!std::same_as<T, void>); Doxygen appears to process it ok, or at least it says in its output that it does. Then I see non_void show up in these files: bin/parser_reference-xml.doxygen bin/parser_reference-xml/index.xml bin/parser_reference-xml/parser_8hpp.xml (I put the concept in this header, parser.hpp) bin/parser_reference-xml/conceptboost_1_1parser_1_1non__void.xml However, when I put [conceptref non_void Non-void.] in a .qbk file, all that shows up in the rendered HTML is "Non-void." (it's not a clickable link). Also, the no_void concept is not in the Doxygen reference for that header. Most frustratingly, concept constraints disappear from constrained templates in the Doxygen reference docs. Is there some special sauce that makes Quickbook+Doxygen process concepts? Is it doing it, but I'm not looking in the right place? Zach

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