I am trying to parse the following struct:
struct Selector {
std::string element;
std::string id;
std::vector<std::string> classes;
};
This struct is used to parse selectors in the form element#id.class1.class2.classn. These selectors always start with 1 or no elements, could contain 1 or no ids, and could contain 0 to n classes.
This gets even more complicated though, because classes and id can appear in any order, so the following selectors are all valid: element#id.class1, .class1#id.class2.class3, #id.class1.class2, .class1.class2#id. For this reason, I have not been able to use hold[], or at<T>() approaches described here, and I also have not been able to use BOOST_FUSION_ADAPT_STRUCT.
The only way that I have been able to synthesize this struct, is with the following rules:
auto element = [](auto& ctx){x3::_val(ctx).element = x3::_attr(ctx);};
auto id = [](auto& ctx){x3::_val(ctx).id = x3::_attr(ctx);};
auto empty = [](auto& ctx){x3::_val(ctx) = "";};
auto classes = [](auto& ctx){x3::_val(ctx).classes.insert(x3::_val(ctx).classes.end(), x3::_attr(ctx).begin(), x3::_attr(ctx).end());};
auto elementRule = x3::rule<class EmptyIdClass, std::string>() = +x3::char_("a-zA-Z") | x3::attr("");
auto idRule = x3::rule<class EmptyIdClass, std::string>() = ("#" >> +x3::char_("a-zA-Z")) | x3::attr("");
auto classesRule = x3::rule<class ClassesClass, std::vector<std::string>>() = *("." >> +x3::char_("a-zA-Z"));
auto selectorRule = x3::rule<class TestClass, Selector>() = elementRule[element] >> classesRule[classes] >> idRule[id] >> classesRule[classes];
What would be the best way to parse this struct? Is it possible to synthesize this selector struct naturally, using BOOST_FUSION_ADAPT_STRUCT, and without semantic actions?
It seems like everytime I think I am am getting the hang of Spirit X3, I stumble upon a new challenge. In this particular case, I learned about issues with backtracking, about an issue with using at<T>() that was introduced in Boost 1.70 here, and I also learned that hold[] is not supported by X3.
I've written similar answers before:
I don't think you can directly fusion-adapt. Although if you are very motivated (e.g. you already have the adapted structs) you could make some generic helpers off that.
To be fair, a little bit of restructuring in your code seems pretty nice to me, already. Here's my effort to make it more elegant/convenient. I'll introduce a helper macro just like BOOST_FUSION_ADAPT_XXX, but not requiring any Boost Fusion.
As always, I like to start with the basics. Understanding the goal is half the battle:
namespace Ast {
using boost::optional;
struct Selector {
// These selectors always
// - start with 1 or no elements,
// - could contain 1 or no ids, and
// - could contain 0 to n classes.
optional<std::string> element;
optional<std::string> id;
std::vector<std::string> classes;
friend std::ostream& operator<<(std::ostream& os, Selector const&s) {
if (s.element.has_value()) os << s.element.value();
if (s.id.has_value()) os << "#" << s.id.value();
for (auto& c : s.classes) os << "." << c;
return os;
}
};
}
Note that I fixed the optionality of some parts to reflect real life.
You could use this to detect repeat-initialization of element/id fields.
#include "propagate.hpp"
DEF_PROPAGATOR(Selector, id, element, classes)
We'll dig into this later. Suffice it to say it generates the semantic actions that you had to tediously write.
Now, we can simplify the parser rules a lot, and run the tests:
int main() {
auto name = as<std::string>[x3::alpha >> *x3::alnum];
auto idRule = "#" >> name;
auto classesRule = +("." >> name);
auto selectorRule
= x3::rule<class TestClass, Ast::Selector>{"selectorRule"}
= +( name [ Selector.element ]
| idRule [ Selector.id ]
| classesRule [ Selector.classes ]
)
;
for (std::string const& input : {
"element#id.class1.class2.classn",
"element#id.class1",
".class1#id.class2.class3",
"#id.class1.class2",
".class1.class2#id",
})
{
Ast::Selector sel;
std::cout << std::quoted(input) << " -->\n";
if (x3::parse(begin(input), end(input), selectorRule >> x3::eoi, sel)) {
std::cout << "\tSuccess: " << sel << "\n";
} else {
std::cout << "\tFailed\n";
}
}
}
See it Live On Wandbox, printing:
"element#id.class1.class2.classn" -->
Success: element#id.class1.class2.classn
"element#id.class1" -->
Success: element#id.class1
".class1#id.class2.class3" -->
Success: #id.class1.class2.class3
"#id.class1.class2" -->
Success: #id.class1.class2
".class1.class2#id" -->
Success: #id.class1.class2
Now, how did I generate those actions? Using a little bit of Boost Preprocessor:
#define MEM_PROPAGATOR(_, T, member) \
Propagators::Prop<decltype(std::mem_fn(&T::member))> member { std::mem_fn(&T::member) };
#define DEF_PROPAGATOR(type, ...) \
struct type##S { \
using T = Ast::type; \
BOOST_PP_SEQ_FOR_EACH(MEM_PROPAGATOR, T, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)) \
} static const type {};
Now, you might see that it defines static const variables named like the Ast types.
You're free to call this macro in another namespace, say
namespace Actions { }
The real magic is Propagators::Prop<F> which has a bit of dispatch to allow for container attributes and members. Otherwise it just relays to x3::traits::move_to:
namespace Propagators {
template <typename F>
struct Prop {
F f;
template <typename Ctx>
auto operator()(Ctx& ctx) const {
return dispatch(x3::_attr(ctx), f(x3::_val(ctx)));
}
private:
template <typename Attr, typename Dest>
static inline void dispatch(Attr& attr, Dest& dest) {
call(attr, dest, is_container(attr), is_container(dest));
}
template <typename T>
static auto is_container(T const&) { return x3::traits::is_container<T>{}; }
static auto is_container(std::string const&) { return boost::mpl::false_{}; }
// tags for dispatch
using attr_is_container = boost::mpl::true_;
using attr_is_scalar = boost::mpl::false_;
using dest_is_container = boost::mpl::true_;
using dest_is_scalar = boost::mpl::false_;
template <typename Attr, typename Dest>
static inline void call(Attr& attr, Dest& dest, attr_is_scalar, dest_is_scalar) {
x3::traits::move_to(attr, dest);
}
template <typename Attr, typename Dest>
static inline void call(Attr& attr, Dest& dest, attr_is_scalar, dest_is_container) {
dest.insert(dest.end(), attr);
}
template <typename Attr, typename Dest>
static inline void call(Attr& attr, Dest& dest, attr_is_container, dest_is_container) {
dest.insert(dest.end(), attr.begin(), attr.end());
}
};
}
A lot of the complexity in the propagator type is from handling container attributes. However, you don't actually need any of that:
auto name = as<std::string>[x3::alpha >> *x3::alnum];
auto selectorRule
= x3::rule<class selector_, Ast::Selector>{"selectorRule"}
= +( name [ Selector.element ]
| '#' >> name [ Selector.id ]
| '.' >> name [ Selector.classes ]
)
;
Is more than enough, and the propagation helper can be simplified to:
namespace Propagators {
template <typename F> struct Prop {
F f;
template <typename Ctx>
auto operator()(Ctx& ctx) const {
return call(x3::_attr(ctx), f(x3::_val(ctx)));
}
private:
template <typename Attr, typename Dest>
static inline void call(Attr& attr, Dest& dest) {
x3::traits::move_to(attr, dest);
}
template <typename Attr, typename Elem>
static inline void call(Attr& attr, std::vector<Elem>& dest) {
dest.insert(dest.end(), attr);
}
};
}
As you can see evaporating the tag dispatch has a beneficial effect.
See the simplified version Live On Wandbox again.
For posterity on this site:
test.cpp
//#define BOOST_SPIRIT_X3_DEBUG
#include <boost/spirit/home/x3.hpp>
#include <iostream>
#include <iomanip>
namespace x3 = boost::spirit::x3;
namespace Ast {
using boost::optional;
struct Selector {
// These selectors always
// - start with 1 or no elements,
// - could contain 1 or no ids, and
// - could contain 0 to n classes.
optional<std::string> element;
optional<std::string> id;
std::vector<std::string> classes;
friend std::ostream& operator<<(std::ostream& os, Selector const&s) {
if (s.element.has_value()) os << s.element.value();
if (s.id.has_value()) os << "#" << s.id.value();
for (auto& c : s.classes) os << "." << c;
return os;
}
};
}
#include "propagate.hpp"
DEF_PROPAGATOR(Selector, id, element, classes)
#include "as.hpp"
int main() {
auto name = as<std::string>[x3::alpha >> *x3::alnum];
auto selectorRule
= x3::rule<class selector_, Ast::Selector>{"selectorRule"}
= +( name [ Selector.element ]
| '#' >> name [ Selector.id ]
| '.' >> name [ Selector.classes ]
)
;
for (std::string const& input : {
"element#id.class1.class2.classn",
"element#id.class1",
".class1#id.class2.class3",
"#id.class1.class2",
".class1.class2#id",
})
{
Ast::Selector sel;
std::cout << std::quoted(input) << " -->\n";
if (x3::parse(begin(input), end(input), selectorRule >> x3::eoi, sel)) {
std::cout << "\tSuccess: " << sel << "\n";
} else {
std::cout << "\tFailed\n";
}
}
}
propagate.hpp
#pragma once
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/seq/for_each.hpp>
#include <functional>
namespace Propagators {
template <typename F> struct Prop {
F f;
template <typename Ctx>
auto operator()(Ctx& ctx) const {
return call(x3::_attr(ctx), f(x3::_val(ctx)));
}
private:
template <typename Attr, typename Dest>
static inline void call(Attr& attr, Dest& dest) {
x3::traits::move_to(attr, dest);
}
template <typename Attr, typename Elem>
static inline void call(Attr& attr, std::vector<Elem>& dest) {
dest.insert(dest.end(), attr);
}
};
}
#define MEM_PROPAGATOR(_, T, member) \
Propagators::Prop<decltype(std::mem_fn(&T::member))> member { std::mem_fn(&T::member) };
#define DEF_PROPAGATOR(type, ...) \
struct type##S { \
using T = Ast::type; \
BOOST_PP_SEQ_FOR_EACH(MEM_PROPAGATOR, T, BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__)) \
} static const type {};
as.hpp
#pragma once
#include <boost/spirit/home/x3.hpp>
namespace {
template <typename T>
struct as_type {
template <typename...> struct tag{};
template <typename P>
auto operator[](P p) const {
return boost::spirit::x3::rule<tag<T,P>, T> {"as"}
= p;
}
};
template <typename T>
static inline const as_type<T> as = {};
}