Re: Reminder: Review for Zach's parsing library starts on Feb 19th

From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2024-02-25 09:26:12

sob., 24 lut 2024 o 02:21 Zach Laine via Boost <boost_at_[hidden]>
napisał(a):

> On Thu, Feb 22, 2024 at 4:36 PM Christian Mazakas via Boost
> <boost_at_[hidden]> wrote:
> >
>
> [snip]
>
> > ### The Feedback
> >
> > The differences between Spirit.X3 and Parser need to be made much more
> > clear by the
> > documentation, by which I mean:
> > > * Spirit X3 has rules that do not compose well — the attributes
> produced
> > by a rule can change depending on the context in which you use the rule.
>
> I will add this to the docs as well, but I thought I'd post here an
> example of what I was referring to above.  Here is a complete program
> using X3:
>
> #include <boost/spirit/home/x3.hpp>
>
> #include <iostream>
> #include <set>
> #include <string>
> #include <vector>
>
>
> namespace x3 = boost::spirit::x3;
> using ints_type = x3::rule<class ints, std::vector<int>>;
> BOOST_SPIRIT_DECLARE(ints_type);
>
> x3::rule<class ints, std::vector<int>> ints = "ints";
> constexpr auto ints_def = x3::int_ % ',';
> BOOST_SPIRIT_DEFINE(ints);
>
> #define FIXED_ATTRIBUTE 0
>
>
> int main()
> {
>     std::string input = "43, 42";
>     auto first = input.begin();
>     auto const last = input.end();
> #if FIXED_ATTRIBUTE
>     std::vector<int> result;
> #else
>     std::set<int> result;
> #endif
>     bool success = x3::phrase_parse(first, last, ints, x3::space, result);
>     if (success) {
>         // We want this to print "43 42\n".
>         for (auto x : result) {
>             std::cout << x << ' ';
>         }
>         std::cout << "\n";
>     }
>
>     return 0;
> }
>
> IMO, the problem with this is that, even though I specified a vector
> as the rule's attribute type, I can feed the rule a set (#define
> FIXED_ATTRIBUTE 1), and it happily populates it with the parsed
> values.  Note that this sorts, and in this case reorders, the values.
> I think this looseness is important in many cases, but there's no way
> to turn it off in X3.  In Parser the way to turn it off is to use a
> rule instead of a parser.  My inability to disable this looseness
> resulted in wrong results in an X3 YAML parser I was working on.  I
> really loved so much of the design of X3, that my frustration with
> this problem was a pretty big part of the motivation for writing
> Parser.
>

I would recommend putting something like the above in the docs, where the
motivation for using rule parsers is listed. Currently it states "fix the
attribute type produced by a parser to something other than the default"
but what you sayin above is more: "prevent the parsers from doing things
that they would otherwise do".

I note that parsing a homogenous sequence to a std::set does work, at least
without rules:
https://godbolt.org/z/x57j1dq5T

I was surprised to find that, in the context of other precautions, feeding
parsed data to a collection is performed via `insert` rather than
`push_back`.

Regards,
&rzej;

>
> Zach
>
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