Cleanest way to handle both quoted and unquoted strings in Spirit.X3 - c++

Question

Buon giorno,
I have to parse something such as:
foo: 123
"bar": 456
The quotes should be removed if they are here. I tried:
((+x3::alnum) | ('"' >> (+x3::alnum) >> '"'))
But the parser actions for this are of type variant<string, string> ; is there a way to make it so that the parser understands that those two are equivalent, and for my action to only get a single std::string as argument in its call?
edit: minimal repro (live on godbolt: https://gcc.godbolt.org/z/GcE8Pj4r5) :
#include <boost/spirit/home/x3.hpp>
using namespace boost::spirit;
// action handlers
struct handlers {
void create_member(const std::string& str) { }
};
// rules
static const x3::rule<struct id_obj_mem> obj_mem = "obj_mem";
#define EVENT(e) ([](auto& ctx) { x3::get<handlers>(ctx).e(x3::_attr(ctx)); })
static const auto obj_mem_def = ((
((+x3::alnum) | ('"' >> (+x3::alnum) >> '"'))
>> ':' >> x3::lit("123"))[EVENT(create_member)] % ',');
BOOST_SPIRIT_DEFINE(obj_mem)
// execution
int main()
{
handlers r;
std::string str = "foo: 123";
auto first = str.begin();
auto last = str.end();
bool res = phrase_parse(
first,
last,
boost::spirit::x3::with<handlers>(r)[obj_mem_def],
boost::spirit::x3::ascii::space);
}

Answer 1

I too consider this a kind of defect. X3 is definitely less "friendly" in terms of the synthesized attribute types. I guess it's just a tacit side-effect of being more core-language oriented, where attribute assignment is effectively done via default "visitor" actions.
Although I understand the value of keeping the magic to a minimum, and staying close to "pure C++", I vastly prefer the Qi way of synthesizing attributes here. I believe it has proven a hard problem to fix, as this problem has been coming/going in some iterations of X3.
I've long decided to basically fix it myself with variations of this idiom:
template <typename T> struct as_type {
auto operator()(auto p) const { return x3::rule<struct Tag, T>{} = p; }
};
static constexpr as_type<std::string> as_string{};
Now I'd write that as:
auto quoted = '"' >> +x3::alnum >> '"';
auto name = as_string(+x3::alnum | quoted);
auto prop = (name >> ':' >> "123")[EVENT(create_member)] % ',';
That will compile no problem:
Live On Coliru
#include <boost/spirit/home/x3.hpp>
#include <iomanip>
#include <iostream>
namespace x3 = boost::spirit::x3;
struct handlers {
void create_member(std::string const& str) {
std::cerr << __FUNCTION__ << " " << std::quoted(str) << "\n";
}
};
namespace Parser {
#define EVENT(e) ([](auto& ctx) { get<handlers>(ctx).e(_attr(ctx)); })
template <typename T> struct as_type {
auto operator()(auto p) const { return x3::rule<struct Tag, T>{} = p; }
};
static constexpr as_type<std::string> as_string{};
auto quoted = '"' >> +x3::alnum >> '"';
auto name = as_string(+x3::alnum | quoted);
auto prop = (name >> ':' >> "123")[EVENT(create_member)] % ',';
auto grammar = x3::skip(x3::space)[prop];
} // namespace Parser
int main() {
handlers r;
std::string const str = "foo: 123";
auto first = str.begin(), last = str.end();
bool res = parse(first, last, x3::with<handlers>(r)[Parser::grammar]);
return res ? 1 : 0;
}
Prints
create_member "foo"
Interesting Links
Spirit X3, How to get attribute type to match rule type?
Combining rules at runtime and returning rules
spirit x3 cannot propagate attributes of type optional<vector>
etc.