PA2 — The Lexer

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 may complete this assignment using OCaml, Haskell, JavaScript, Python or Ruby.

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 lexical analyzer, also called a scanner, using a lexical analyzer generator. You will describe the set of tokens for Cool in an appropriate input format and the analyzer generator will generate actual code. You will then write additional code to serialize the tokens for use by later interpreter stages.


You must create three artifacts:

  1. A program that takes a single command-line argument (e.g., That argument will be an ASCII text Cool source file. Your program must either indicate that there is an error in the input (e.g., a malformed string) or emit, a serialized list of Cool tokens. Your program's main lexer component must be constructed by a lexical analyzer generator. The "glue code" for processing command-line arguments and serializing tokens should be written by hand. If your program is called lexer, invoking lexer should yield the same output as cool --lex Your program will consist of a number of OCaml files, a number of Python files, a number of JavaScript files, or a number of Ruby 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. Testcases and The first should lex correctly and yield a sequence of tokens. The second should contain an error.
You must use ply or ruby-lex or ocamllex or jison (or a similar tool or library). Do not write your entire lexer by hand. Parts of it must be tool-generated from regular expressions you provide.

Line Numbers

The first line in a file is line 1. Each successive '\n' newline character increments the line count. Your lexer is responsible for keeping track of the current line number.

Error Reporting

To report an error, write the string

ERROR: line_number: Lexer: 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:

Backslash not allowed \

Example error report output:

ERROR: 1: Lexer: invalid character: \

The .cl-lex File Format

If there are no errors in your program should create and serialize the tokens to it. Each token is represented by a pair (or triplet) of lines. The first line holds the line number. The second line gives the name of the token. The optional third line holds additional information (i.e., the lexeme) for identifiers, integers, strings and types. For example, for an integer token the third line should contain the decimal integer value.

Example input:

Backslash not

Corresponding .cl-lex output:


The official list of token names is:

In general the intended token is evident. For the more exotic names:

The .cl-lex file format is exactly the same as the one generated by the reference compiler when you specify --lex. In addition, the reference compiler (and your upcoming PA3 parser!) will read .cl-lex files instead of .cl files.

Lexical Analyzer Generators

You must use a lexical analyzer generator or similar library for this assignment.

All of these lexical analyzer generators are derived from lex (or flex), the original lexical analyzer generator for C. Thus you may find it handy to refer to the Lex paper or the Flex manual. When you're reading, mentally translate the C code references into the language of your choice.

My personal opinion is that the OCaml and Python tools are a bit more mature (i.e., easier to use) than the Ruby and JavaScript tools for this particular project, but the difference is slight.


You can do basic testing with something like the following:

$ cool --out reference --lex
$ my-lexer
$ diff -b -B -E -w

For example, if you used OCaml:

$ cool --out reference --lex
$ ocamllex my-lexer.mll
$ ocaml
$ diff

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

Need more testcases? Any Cool file you have (including the one you wrote for PA1) works fine. The contents of should be a good start. There's also one among the PA1 hints. You'll want to make more complicated test cases — in particular, you'll want to make negative testcases (e.g., testcases with malformed string constants).

Video Guides

A number of Video Guides are provided to help you get started on this assignment on your own. The Video Guides are walkthroughs in which the instructor manually completes and narrates, in real time, the first part of this assignment — including a submission to the grading server. They include coding, testing and debugging elements.

If you are still stuck, you can post on the forum, approach the TAs, or approach the professor. The use of online instructional content outside of class weakly approximates a flipped classroom model. Click on a video guide to begin, at which point you can watch it fullscreen or via Youtube if desired.

Python + PLY
OCaml + ocamllex

Reminder: You can watch YouTube videos at 1.5x speed with full audio.

What To Turn In For PA2

You must turn in a zip file containing these files:

  1. readme.txt — your README file
  2. — a novel positive testcase
  3. — a novel negative testcase
  4. source_files — including
    • main.rb or
    • or
    • main.hs (and some_file.x, if applicable) or
    • main.js (and some_file.jison, if applicable) or
    • and some_file.mll
    If your regular expressions and lexer definition are in some other file (e.g., lexer.mll, lexer.jison, etc.), be sure to include them as well!

Your zip file may also contain:

Working In Pairs

You may complete this project 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 PA2 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 wrw6y are working together, ph4u would submit with a team.txt file that contains the word wrw6y. Then ph4u and wrw6y will both receive the same grade for that submission.

This seems picayune, 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).

Grading Rubric

PA2 Grading (out of 50 points):