Parsing into a set using boost spirit x3 - c++

Question

I am interested in if there is a way to parse into a set using boost spirit x3. the background is I have a string of tokens, each token represents a enum value, now i want to create a parser which parses if every every token is at most once in the string, it would be a charm if I could get all the parsed tokens into a std::set while parsing.
To get the enums back from the parsed string I am using a symbol_table:
enum class foo{bar, baz, bla, huh};
struct enum_table : x3::symbols<foo> {
enum_table() {
add("bar", foo::bar)
("baz", foo::baz)
("huh", foo::huh);
}
} const enum_parser;

Answer 1

I am interested in if there is a way to parse into a set using boost spirit x3.
Spirit can parse into std::set<> out of the box (at least as of Boost 1.61.0), so the following already works with the types you've shown:
std::set<foo> foos;
x3::phrase_parse(
input.begin(), input.end(),
+enum_parser,
x3::space,
foos
);
Online Demo
To get your parser to fail upon encountering duplicates, this is most easily accomplished with semantic actions:
std::set<foo> foos;
auto insert_foo_or_fail = [&foos](auto& ctx) {
_pass(ctx) = foos.insert(_attr(ctx)).second;
};
x3::phrase_parse(
input.begin(), input.end(),
+x3::omit[enum_parser[insert_foo_or_fail]],
x3::space
);
Online Demo