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

25 Feb 2024

      sob., 24 lut 2024 o 02:21 Zach Laine via Boost 
napisał(a):
...
On Thu, Feb 22, 2024 at 4:36 PM Christian Mazakas via Boost
 wrote:
...
[snip]
...
### The Feedback
...
* Spirit X3 has rules that do not compose well — the attributes
The differences between Spirit.X3 and Parser need to be made much more
clear by the
documentation, by which I mean:
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 
#include <iostream>
#include <set>
#include <string>
#include <vector>
namespace x3 = boost::spirit::x3;
using ints_type = x3::rule;
BOOST_SPIRIT_DECLARE(ints_type);
x3::rule 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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Re: [boost] Reminder: Review for Zach's parsing library starts on Feb 19th

Andrzej Krzemienski