Programming assignments 2 through 5 will direct you to design and build an interpreter for Cool. Each assignment will cover one component of the interpreter: lexical analysis, parsing, semantic analysis, and operational semantics. Each assignment will ultimately result in a working interpreter phase which can interface with the other phases.

You will complete this assignment using Python and implement the parsing component of an interpreter.

You may work in a team of two people for this assignment. You may work in a team for any or all subsequent programming assignments. You do not need to keep the same teammate. The course staff are not responsible for finding you a willing teammate.


For this assignment you will write a parser using a parser generator. You will describe the Cool grammar in an appropriate input format and the parser generator will generate actual code (in OCaml, Python or Ruby). You will also write additional code to unserialize the tokens produced by the lexer stage and to serialize the abstract syntax tree produced by your parser.


You must create four artifacts:

  1. A program that takes a single command-line argument (e.g., That argument will be an ASCII text Cool tokens file (as described in PA2). The cl-lex file will always be well-formed (i.e., there will be no syntax errors in the cl-lex file itself). However, the cl-lex file may describe a sequence of Cool tokens that do not form a valid Cool program.

    Your program must either indicate that there is an error in the Cool program described by the cl-lex file (e.g., a parse error in the Cool file) or emit, a serialized Cool abstract syntax tree. Your program's main parser component must be constructed by a parser generator. The "glue code" for processing command-line arguments, unserializing tokens and serializing the resulting abstract syntax tree should be written by hand. If your program is called parser, invoking python should yield the same output as cool --parse Your program will consist of a number of Python files.

  2. A plain ASCII text file called readme.txt describing your design decisions and choice of test cases. See the grading rubric. A few paragraphs should suffice.
  3. A plain ASCII text file called references.txt providing a citation for each resource you used (excluding class notes, and assigned readings) to complete the assignment. For example, if you found a Stack Overflow answer helpful, provide a link to it. Additionally, provide a brief description of how the resource helped you.
  4. Testcases and The first should parse correctly and yield an abstract syntax tree. The second should contain an error.
You must use ply (or a similar tool or library). Do not write your entire parser by hand. Parts of it must be tool-generated from context-free grammar rules you provide.

Line Numbers

The line number for an expression is the line number of the first token that is part of that expression.


while x <= 
       99 loop 
  x <- x + 1 

The while expression is on line 5, the x <= 99 expression is on line 5, the 99 expression is on line 6, and the x <- x + 1 and x + 1 expressions are on line 7. The line numbers for tokens are present in the serialized token .cl-ast file.

Your parser is responsible for keeping track of the line numbers (both for the output syntax tree and for error reporting).

Error Reporting

To report an error, write the string

ERROR: line_number: Parser: message

to standard output and terminate the program. You may write whatever you want in the message, but it should be fairly indicative.

Example erroneous input:

class Cons inherits List + IO {
Example error report output:
ERROR: 70: Parser: syntax error near +

The .cl-ast File Format

If there are no errors in your program should create and serialize the abstract syntax tree to it. The general format of a .cl-ast file follows the Cool Reference Manual Syntax chart. Basically, we do a pre-order traversal of the abstract syntax tree, writing down every node as we come to it.

We will now describe exactly what to output for each kind of node. You can view this as specifying a set of mutually-recursive tree-walking functions. The notation "superclass:identifier" means "output the superclass using the rule (below) for outputting an identifier". The notation "\n" means "output a newline".

Example input:

class List {
  -- Define operations on lists.

  cons(i : Int) : List {
		(new Cons).init(i, self)

Example .cl-ast output with comments:
1                      -- number of classes                   
3                      --  line number of class name identifier
List                   --  class name identifier
no_inherits            --  does this class inherit? 
1                      --  number of features
method                 --   what kind of feature? 
6                      --   line number of method name identifier
cons                   --   method name identifier
1                      --   number of formal parameters
6                      --    line number of formal parameter identifier
i                      --    formal parameter identifier
6                      --    line number of formal parameter type identifier
Int                    --    formal parameter type identifier
6                      --   line number of return type identifier
List                   --   return type identifier
7                      --    line number of body expression 
dynamic_dispatch       --    kind of body expression 
7                      --     line number of dispatch receiver expression 
new                    --     kind of dispatch receiver expression  
7                      --      line number of new-class identifier 
Cons                   --      new-class identifier
7                      --     line number of dispatch method identifier
init                   --     dispatch method identifier
2                      --     number of arguments in dispatch 
7                      --      line number of first argument expression
identifier             --      kind of first argument expression
7                      --       line number of the identifier
i                      --       what is the identifier? 
7                      --      line number of second argument expression
identifier             --      kind of second argument expression
7                      --       line number of the identifier
self                   --       what is the identifier? 		

The .cl-ast format is quite verbose, but it is particularly easy for later stages (e.g., the type checker) to read in again without having to go through all of the trouble of "actually parsing". It will also make it particularly easy for you to notice where things are going awry if your parser is not producing the correct output.

Writing the code to output a .cl-ast file given an AST may take a bit of time but it should not be difficult; our reference implementation (OCaml, so not directly comparable) does it in 116 lines and cleaves closely to the structure given above.

Parser Generators

You must use a parser generator or similar library for this assignment. In class, we discussed Ply, a parser generator for Python. You will find the documentation to be particularly helpful.

There exist similar tools for other programming languages:

All of these parser generators are derived from yacc (or bison), the original parser generator for C. Thus you may find it handy to refer to the Yacc paper or the Bison manual. When you're reading, mentally translate the C code references.


You can do basic testing as follows:

Example testing
$ cool --lex
$ cool --out reference --parse
$ python
$ diff -b -B -E -w

You may find the reference compiler's --unparse option useful for debugging your .cl-ast files.


If you are failing every negative test case, it is likely that you are not handling cross-platform compatibility correctly on all of your inputs and outputs.

If you are still stuck, you can post on the forum, approach the TAs, or approach the professor.

Video Guides

Wes Weimer has developed a number of Video Guides that you might find helpful. The Video Guides are walkthroughs in which Wes manually completes and narrates, in real time, the first part of a similar assignment — including a submission to his grading server. They include coding, testing and debugging elements.

These videos are considered an outside resource for completing this assignment. Be sure to note these videos in your references.txt if you use them.

Note: Wes's videos use a different submission site from this class.

What to Turn In For PA3

You must turn in a zip file containing these files:

Your zip file may also contain:

Working In Pairs

You may complete this assignment in a team of two. Teamwork imposes burdens of communication and coordination, but has the benefits of more thoughtful designs and cleaner programs. Team programming is also the norm in the professional world.

Students on a team are expected to participate equally in the effort and to be thoroughly familiar with all aspects of the joint work. Both members bear full responsibility for the completion of assignments. Partners turn in one solution for each programming assignment; each member receives the same grade for the assignment. If a partnership is not going well, the teaching assistants will help to negotiate new partnerships. Teams may not be dissolved in the middle of an assignment.

If you are working in a team, exactly one team member should submit a PA3 zipfile. That submission should include the file team.txt, a one-line, one-word flat ASCII text file that contains the email ID of your teammate. Don't include the bit. Example: If ph4u and kaa2nx are working together, ph4u would submit with a team.txt file that contains the word kaa2nx. Then ph4u and kaa2nx will both receive the same grade for that submission.

This seems minor, but in the past we've had students fail to correctly format this one word file. Thus you now get a point on this assignment for either formatting this file correctly (i.e., including only a single word that is equal to your partner's UVA email ID) or not including it (and thus not working in a pair).

Legacy Grading

The legacy server has an older version of python and ply installed. Be careful of language features you choose to use!

Grading Rubric

PA3 Grading (out of 50 points):