lemon--

An experimental fork of the lemon parser generator that hopes to be more compatible with c++11. Placement constructors and destructors are automatically generated, so you shouldn't need to deal with pointers as much (unless, of course, you want to!)

contrived example

###lemon--

%token_type {std::any} // c++17
%type number_list{std::vector<int>}
%type string_list{std::vector<std::string>}
// or use a std::shared_ptr or std::unique_ptr...

program ::= list.

list ::= number_list(L). {

	for (auto i : L) {
		printf("%d\n", i);
	}
}

list ::= string_list(L). {

    for (const auto &s : L) {
        printf("%s\n", s.c_str());
    }
}

number_list(L) ::= NUMBER(N). {
    L.push_back(std::any_cast<int>(N));
}

number_list(L) ::= number_list(L) NUMBER(N). {
	L.push_back(std::any_cast<int>(N));
}

string_list(L) ::= STRING(S). {
    L.push_back(std::any_cast<std::string>(S));
}

string_list(L) ::= string_list(L) STRING(S). {
    L.push_back(std::any_cast<std::string>(S));
}

###lemon

%token_type {std::any *)}
%token_destructor { delete $$ }

%type number_list{std::vector<int> *}
%type string_list{std::vector<std::string> *}
%destructor number_list { delete $$ }
%destructor string_list { delete $$ }

program ::= list.

list ::= number_list(L). {

	for (auto i : *L) {
		printf("%d\n", i);
	}
	delete L;
}

list ::= string_list(L). {

    for (const auto &s : *L) {
        printf("%s\n", s.c_str());
    }
    delete L;
}


number_list(L) ::= NUMBER(N). {
    L = new std::vector<int>();
    L->push_back(std::any_cast<int>(*N));
    delete N;
}

number_list(L) ::= number_list(L) NUMBER(N). {
    L->push_back(std::any_cast<int>(*N));
    delete N;
}

string_list(L) ::= STRING(S). {
    L = new std::vector<std::string>();
    L->push_back(std::any_cast<std::string>(*S));
    delete S;
}

string_list(L) ::= string_list(L) STRING(S). {
    L->push_back(std::any_cast<std::string>(*S));
    delete S;
}

Notes

I tried to make these examples fairly similar. Lemon-- automatically destructs all right-hand-side variables and automatically constructs the left-hand-side variable. Additionally, it uses placement constructors and destructors so you use non-POD data type instead of pointers.

Standard lemon only calls the %destructor for right-hand-side terms that don't have an alias, hence the manual deletion above.

Exceptions

Don't do that.

one(RHS) ::= two(A). {
    RHS = std::move(A);
    throw std::exception("oops");
}

In the above code, RHS's destructor will not be called and it will leak.
A will remain on the parse stack in a valid but unspecified state. Thus trying to continue parsing may or may not work (additionally, there may be other parser internals that are out of sync.) The destructor leak is fixable but due to the other reasons, you'd be better off remaining unexceptional.

Other enhancements

There is a new %header declaration. This is a block of code that will be written, verbatim, into the generated header file.

%header {
    void *ParseAlloc(void *(*mallocProc)(size_t));
    void ParseFree(void *p, void (*freeProc)(void*));
    void Parse(void *yyp, int yymajor, struct MyTokenType *yyminor);
}

Object Oriented?

test/lisp uses an experimental object-oriented approach. I find this works better in many cases.

The inheritance hierarchy is:

lemon_base<TokenType> : your_parser : yypParser

#define LEMON_BASE your_parser in your header and use the lempar.cxx template.

##Changes:

• %extra_argument is gone. Put anything you need in your_parser (public or protected so it's accessible).
• %syntax_error, %parse_failure, %parse_accept, and %stack_overflow may still be used but can also be moved into your class.
• ParseAlloc and ParseFree are gone. Use the normal constructors and destructors.

yypParser is hidden in an anonymous namespace, so it can only be created via a %code block in your parse definition. The lisp example exposes it as such:

%code {
  std::unique_ptr<your_parser> your_parser::create() {
    return std::make_unique<yypParser>();
  }
}

yypParser inherits all your constructors so you can pass in any appropriate arguments.

other versions for your consideration

Trying to make lemon work better with c++ is not a new idea, apparently. These are some other efforts. I have no knowledge of their suitability.

• http://sourceforge.net/projects/lemonxx/
• http://sourceforge.net/projects/lemonpp/