README.md

The limecc parser generator

limecc is a lexer and parser generator similar to other tools like yacc, bison and especially lemon from which limecc sources inspiration. Grammars are written in a language called Lime, which describes lexical tokens, grammar productions, and sematic actions. The generator produces C++ code for the corresponding lexer and parser.

Installation

Since limecc is written in pure Python, you can simply fetch it from PyPI.

easy_install limecc

Usage example

The Lime language was designed to be concise, yet self-descriptive. The following grammar description produces a simple calculator.

WS :: discard
WS ~= {\s+}
NUM :: {double}
NUM ~= {[0-9]+}(x) { return atoi(x.c_str()); }

expr :: {double}
expr ::= mul.
expr(E) ::= expr(E1) "+" mul(E2). { E = E1 + E2; }
expr(E) ::= expr(E1) "-" mul(E2). { E = E1 - E2; }

mul :: {double}
mul ::= term.
mul(E) ::= mul(E1) "*" term(E2). { E = E1 * E2; }
mul(E) ::= mul(E1) "/" term(E2). { E = E1 / E2; }

term :: {double}
term ::= atom.
term ::= "+" atom.
term(A) ::= "-" atom(E). { A = -E; }

atom :: {double}
atom ::= NUM.
atom(A) ::= "(" expr(E) ")".

Lexical tokens -- ofter referred to as the terminals symbols -- can be defined in one of two ways: using the ~= operator with a regular expression, or with a quoted string literal.

All symbols have types. The WS terminals have the special type of discard, which causes the lexer to throw them away. Such tokens cannot appear in a syntactic production as they are invisible to the parser. The terminal NUM has the C++ type double. The semantic action associated with the terminal converts the textual representation of the token to its numeric form. Quoted literals have the special type of void, which allows them to appear in the grammar, but makes them invisible to the semantic actions.

Semantic actions that are attached to production rules can refer to values of right-hand-side symbols using their name. Having names for symbols make the actions easier to read and understand.

Semantic actions can be omitted if exactly one right-hand-side symbol has a non-void type. In such case, reducing the rule will propagate that symbol's value to the left-hand-side symbol. For instance, the production term ::= atom. is equivalent to the production term(A) ::= atom(B). { A = B; }.

The first symbol on the left-hand-side of a production rule is considered the root of the grammar; in this case it the symbol expr.

You can compile the above grammar as follows.

limecc calc.y

This command will produce calc.hpp, a C++ file containing the parser class. Use the class as follows.

#include "calc.hpp"
#include <iostream>
#include <string>

int main()
{
    std::string line;
    while (std::getline(std::cin, line))
    {
        try
        {
            parser p;
            p.push_data(line.data(), line.data() + line.size());
            std::cout << p.finish() << "\n";
        }
        catch (...)
        {
            std::cerr << "error: Invalid syntax.\n";
        }
    }
}