How can I add conditional expectation points in spirit X3 - c++

Question

I am currentl adding expectation points to my grammar in X3.
Now I came accross an rule, which looks like this.
auto const id_string = +x3::char("A-Za-z0-9_);
auto const nested_identifier_def =
x3::lexeme[
*(id_string >> "::")
>> *(id_string >> ".")
>> id_string
];
I am wondering how I can add conditional expectation points to this rule.
Like "if there is a "::" then there musst follow an id_string" or "when there is a . then there musst follow an id_string"
and so on.
How can I achieve such a behaviour for such a rule?

Answer 1

I'd write it exactly the way you intend it:
auto const identifier
= lexeme [+char_("A-Za-z0-9_")];
auto const qualified_id
= identifier >> *("::" > identifier);
auto const simple_expression // only member expressions supported now
= qualified_id >> *('.' > identifier);
With a corresponding AST:
namespace AST {
using identifier = std::string;
struct qualified_id : std::vector<identifier> { using std::vector<identifier>::vector; };
struct simple_expression {
qualified_id lhs;
std::vector<identifier> rhs;
};
}
LIVE DEMO
Live On Coliru
#include <iostream>
#include <string>
#include <vector>
namespace AST {
using identifier = std::string;
struct qualified_id : std::vector<identifier> { using std::vector<identifier>::vector; };
struct simple_expression {
qualified_id lhs;
std::vector<identifier> rhs;
};
}
#include <boost/fusion/adapted.hpp>
BOOST_FUSION_ADAPT_STRUCT(AST::simple_expression, lhs, rhs)
#include <boost/spirit/home/x3.hpp>
namespace Parser {
using namespace boost::spirit::x3;
auto const identifier
= rule<struct identifier_, AST::identifier> {}
= lexeme [+char_("A-Za-z0-9_")];
auto const qualified_id
= rule<struct qualified_id_, AST::qualified_id> {}
= identifier >> *("::" > identifier);
auto const simple_expression // only member expressions supported now
= rule<struct simple_expression_, AST::simple_expression> {}
= qualified_id >> *('.' > identifier);
}
int main() {
using It = std::string::const_iterator;
for (std::string const input : { "foo", "foo::bar", "foo.member", "foo::bar.member.subobject" }) {
It f = input.begin(), l = input.end();
AST::simple_expression data;
bool ok = phrase_parse(f, l, Parser::simple_expression, boost::spirit::x3::space, data);
if (ok) {
std::cout << "Parse success: ";
for (auto& el : data.lhs) std::cout << "::" << el;
for (auto& el : data.rhs) std::cout << "." << el;
std::cout << "\n";
}
else {
std::cout << "Parse failure ('" << input << "')\n";
}
if (f != l)
std::cout << "Remaining unparsed input: '" << std::string(f, l) << "'\n";
}
}
Prints
Parse success: ::foo
Parse success: ::foo::bar
Parse success: ::foo.member
Parse success: ::foo::bar.member.subobject