What is Hyacc | Features | Example | Download | License | References | Contact

What is Hyacc, and why it is special

Many people have used Yacc. It is a LALR(1) parser generator, often used with the lexical analyzer Lex to create compilers. There are many variations of Yacc, most famous ones are like Bison and Byacc (Berkely yacc). The problem with LALR(1) parser generators is that they contain "mysterious" reduce/reduce conflicts even for deterministic and unambiguous grammars, and these require the language designer to tweak the grammar which is often a time-consuming and painful process, and the resulted grammar may not be the same as before modification.

Hyacc is similar to Yacc in that it is a parser generator. It is different from yacc in that it is a LR(1) parser generator, which means it can accept all LR(1) grammars. This is more powerful than LALR(1) algorithm. A LR(1) parser generator also does not have the "mysterious reduce/reduce conflict" problem.

The general problem of LR(1) parser generation is that the original Knuth's canonical LR(k) algorithm was too expensive in time and space to be practical. Pager's algorithm [1] in 1977 based on the concept of "weak compatibility" reduces the size of the parsing table and makes canonical LR(1) parser generation practical. Hyacc shows that, with Pager's algorithm and proper use of data structures, the performance of a LR(1) parser generator can be close to LALR(1) parser generators.

Hyacc is pronounced as "HiYacc", means Hawaii Yacc.

Specifically, based on the original Knuth LR(1) algorithm, three optimizations are used:

1. Combine compatible states. [1]
2. Remove unit productions. [2]
3. Further remove repeated states after the previous step.

Hyacc tries the best to be compatible with Yacc and Bison in the format of input file and command line switches. So anyone used Yacc or Bison before should be able to start using Hyacc immediately.

Hyacc is ANSI C compliant, which makes it extremely easy to port to any platform.

Features

• Implements the original Knuth LR(1) algorithm.
• Combines compatible states using the concept of weak compatibility [1].
• Removes unit productions [2].
• Removes repeated states after removing unit productions.
• Allows empty production.
• Allows these directives: %token, %left, %right, %expect, %start, %prec.
• In case of ambiguous grammars, uses precedence and associativity to resolve conflicts. When unavoidable conflicts happen, in case of shift/reduce conflicts the default action is to use shift, in case of reduce/reduce conflicts the default is to use the production that appears first in a grammar.
• Is backward compatible to yacc and bison in the ways of input file format, ambiguous grammar handling, error handling and output file format.
• If specified, can generate a graphviz input file for the parsing machine.
• If specified, the generated compiler can record the parsing steps in a file.
• Works together with Lex and Flex.
• Is ANSI C compliant.
• Rich information in debug output.

• Hyacc requires that all the "%%" and "%..." directives start from the first column of the line. This restriction should be removed later.
• Hyacc is not reentrant.
• Hyacc does not support these Yacc directives: %nonassoc, %union, %type.
• Hyacc does not support mid-production actions.
• The optimization of removing unit productions is designed for LR(1) grammars only. Using it on ambiguous grammars can possibly lead to shift/shift conflicts, and thus should not be applied in such cases.

Example

Here is a yacc input file calc.y for an infix notation calculator, modified from an example from Bison's online document.

Type the command: hyacc calc.y -v -t -g -D0
-v asks for the generation of y.output
-D0 asks for printing all the relevant information into y.output
-t asks for generating y.parse that contains all the parsing details when the generated parser parses an input.
-g asks for generating y.gviz that can be used as input file to graphviz.

Download

Hyacc can be compiled and used in Unix, Linux and Windows, or any system that supports ANSI C.

Documentations (included in the source packages): readme.pdf, hyaccmanpage

Source package: Hyacc project page. Or directly go to the download page.

If you used Hyacc on a complex grammar, you can help me by providing these information.

License

Hyacc source code is released under the GPL license.

The only exception is the parser engine file "hyaccpar", which is released under the BSD license. This guarantees that the generated parser can be used in both open source and commercial softwares. A similar situation was addressed by Richard Stallman in the release notes of Bison 1.23 and 1.24.