Eleminating left recursion in parser rule of spirit x3

问题

I am currently stuck with a rule I am trying to parse using boost spirit x3. Here is te EBNF(using the % operator from spirit for lists) for what I am trying to parse:

type ::= class_type | lambda_type

lambda_type ::= more_arg_lambda | one_arg_lambda

more_arg_lambda ::= "(", type%",", ")", "=>", type

one_arg_lambda ::= type, "=>", type  <- here is the left recursion

class_type ::= identifier%"::", ["<", type%",", ">"]

usng boost spirit x3, I am trying to parse into the following struct/variant:

typedef x3::variant<
        nil,
        x3::forward_ast<LambdaType>,
        x3::forward_ast<ClassType>
    > Type;

struct LambdaType {
        std::vector<Type> parameters_;
        Type return_type_;
    };
struct ClassType{
        std::vector<std::string> name_; 
        std::vector<Type> template_args_;
    };

I have a live example of what I am currently trying here, which is not working, I also tried to change the order of the variant parser, wich doesnt help, I get endless recusion, or not the behaviour I would expect(or wish). Can anybody help me debugging this parser? I think I have some type of left-recursion in the parser, is there a chance to avoid this or is there no chance to rewrite the grammar? Is this gramar even parsable with boost spirit x3?

EDIT:

I managed to eleminate the left recursion in this grammar. Now te grammar is the following:

type ::= class_type | lambda_type

    lambda_type ::= more_arg_lambda | one_arg_lambda

    more_arg_lambda ::= "(", type%",", ")", "=>", type

    one_arg_lambda ::= class_type, "=>" type, A
                       | "(", type%",", ")", "=>", type, "=>", type, A

    class_type ::= identifier%"::", ["<", type%",", ">"]

    A::= "=>", type, A | eps

but now there is the next problem, how can I make boost spirit x3 to parse these rules into the given structs? I cannot imagine what the A or the one_arg_lambda parsers are returning now, the one_arg_lambda parser should parse into a LambdaType struct, but depending on what A parses into thats not necessary true now. So the question is now, how can I get a non-left recursive parser, which parses the grammar above into my structs using boost-spirit-x3?

EDIT II:

i would like => to be right associative so foo => bar => baz => baham
means foo => (bar => (baz => bahama))

回答1:

I solved this problem and the solution was incredibly easy. The trick is to change the grammar, so I dont have a left recursion, and it parses nice into my structs.

so I changed

type ::= class_type | lambda_type

lambda_type ::= more_arg_lambda | one_arg_lambda

more_arg_lambda ::= "(", type%",", ")", "=>", type

one_arg_lambda ::= type, "=>", type  <- here is the left recursion

class_type ::= identifier%"::", ["<", type%",", ">"]

to

type ::= class_type | lambda_type

lambda_type ::= more_arg_lambda | one_arg_lambda

more_arg_lambda ::= "(", type%",", ")", "=>", type

one_arg_lambda ::= class_type, "=>", type  <- here is the magic trick

class_type ::= identifier%"::", ["<", type%",", ">"]

this second grammar descripes the exaclty same language, but without left recursion, and without changing the structure of the grammar. This was actualy luck and doesnt work for every grammar obviously.

来源：https://stackoverflow.com/questions/39949480/eleminating-left-recursion-in-parser-rule-of-spirit-x3