cs152 Project Phase 2: Parser Generation Using bison

Due Date: Wednesday, Feb 18, 2009 at 11:59p.m.

Grade Weight: 10% of total course grade

The project must be completed individually; you are not allowed to collaborate with your classmates. Any collaboration or code sharing (including code submitted in past quarters) will be considered a violation of academic integrity and dealt with as set forth in the academic integrity policy.


In the previous phase of the class project, you used the flex tool to create a lexical analyzer for the "MINI-L" programming language that reads in a MINI-L program (from a text file) and identifies the sequence of lexical tokens in the text file. In this phase of the class project, you will create a parser using the bison tool that will check to see whether the identified sequence of tokens adheres to the specified grammar of MINI-L. The output of your parser will be the list of productions used during the parsing process. If any syntax errors are encountered during parsing, your parser should emit appropriate error messages. Additionally, you will be required to submit the grammar for MINI-L that you will need to write before you can use bison.

[The MINI-L language is described in detail here. Pay particular attention to the syntax diagrams here that describe the appropriate structure of the various MINI-L statements.]
[The required output format for your parser is described here.]


Bison is a tool for generating parsers. Given a specified context-free grammar for a language, bison will automatically produce an LALR(1) bottom-up parser for that language. The parser will ensure that a given sequence of tokens are ordered in such a way that they adhere to the specified grammar of the language. If the tokens fail to parse properly, then appropriate syntax error messages are outputted.

Shift/Reduce and Reduce/Reduce conflicts:
When ambiguities exist in the specified grammar, bison will emit one or more conflicts when it is run. These conflicts do not prevent bison from generating the parser from your specification, but they may unexpectedly affect how your parser behaves. A shift/reduce conflict occurs when the parser may perform either a shift or a reduce. A reduce/reduce conflict occurs when there are two or more production rules that apply to the same sequence of input tokens. In general, reduce/reduce conflicts indicate errors in the grammar (or at least serious ambiguities) and should be eliminated by modifying the grammar specification as needed. Shift/reduce conflicts, on the other hand, are more difficult to completely eliminate, especially when using the special "error" token provided by bison to handle errors. Therefore, you should try to eliminate as many shift/reduce conflicts as you can, but some shift/reduce conflicts may remain as long as they do not adversely affect the behavior of your parser. You can run bison with the -v option to generate an output file containing detailed information about each conflict.

In our department, bison is installed and can be used on machine "bass".

[A brief introduction to bison can be found here.]
[The detailed manual for bison can be found here.]

Detailed Requirements

The following tasks will need to be performed to complete this phase of the project.
  1. First, you will need to write the grammar for the MINI-L language, based on the specification for MINI-L that we have provided for you. You must submit this grammar along with the other files required for this phase of the class project!
  2. Create the bison parser specification file using the MINI-L grammar. Ensure that you specify helpful syntax error messages to be outputted by the parser in the event of any syntax errors.
    Example: write the bison specification in a file named mini_l.y.
  3. Run bison to generate the parser for MINI-L using your specification. The -d flag is necessary to create a .h file that will link your flex lexical analyzer and your bison parser. The -v flag is helpful for creating an output file that can be used to analyze any conflicts in bison. The --file-prefix option can be used to change the prefix of the file names outputted by bison.
    Example: execute the command bison -v -d --file-prefix=y mini_l.y. This will create the parser in a file called y.tab.c, the necessary .h file called y.tab.h, and the informative output file called y.output.
  4. Ensure that your MINI-L lexical analyzer from the first phase of the class project has been constructed.
    Example: if your lexical analyzer specification is in a file called mini_l.lex, then use it with flex as follows: flex mini_l.lex. This will create the lexical analyzer in a file called lex.yy.c.
  5. Compile everything together into a single executable.
    Example: compile your parser into the executable parser with the following command: gcc -o parser y.tab.c lex.yy.c -lfl. The program parser should now be available for use.

Example Usage

Suppose your parser is in the executable named parser. Then for the MINI-L program primes.min, your parser should be invoked as follows:

cat primes.min | parser

The list of productions taken by your parser during parsing should then be printed to the screen. As one example, the output might look like this (you do not need to number each production or label each non-terminal with the corresponding production number). However, your output may be different due to different productions in your specification. The most important thing is that your parser should not output any syntax errors for syntactically correct programs, and your parser should output helpful syntax error messages (for at least the first syntax error) whenever syntax errors do exist in the inputted MINI-L program.

Submission instructions

Submit via Moodle. Turn in your source code and the Makefile (or a build script) in a .tar.gz or .zip file. Make sure you can uncompress and compile your submission on bass, otherwise your score for this project will be 0.

Extra credit for implementing your project in OCaml

Similar to project 1, if you implement this project in Objective Caml (an ML variant that supports functional, object-oriented, and imperative programming) you will receive 10% extra credit on this project's score.

We have installed OCaml on bass. It comes with its own parser generator, ocamlyacc (similar to bison), and an impressive array of libraries.

If you are having technical difficulties with OCaml, contact the instructor. TA's do not have time and resources to help students with the OCaml implementation.

OCaml resources

Implementing the project in a functional language other than OCaml

This might be possible, but check with the instructor first.