Project Overview

You may do this assignment in OCaml, Python or Ruby. You must use each language at least once (over the course of PA2 - PA5); you will use one language (presumably your favorite) twice.

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. However, you must still satisfy the language breadth requirement (i.e., you must be graded on at least one OCaml program, at least one Ruby program, and at least one Python program).

Goal

You will also write additional code to unserialize the class and implementation maps produced by the semantic analyzer and the parse tree produced by the parser.

The Specification

1. A program that takes a single command-line argument (e.g., file.cl-type). That argument will be an ASCII text Cool class map, implementation map, and AST file (as described in PA4). Your program must execute (i.e., interpret) the Cool program described by that input. If your program is called interp, invoking interp file.cl-type should yield the same output as cool file.cl. Your program will consist of a number of OCaml files, a number of Python files, or a number of Ruby files.
   • You will only be given .cl-type files from programs that pass the semantic analysis phase of the reference interpreter. You are not responsible for correctly handling (1+"hello") programs.
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 test1.cl, test2.cl, test3.cl and test4.cl. The testcases should exercise interpreter and run-time error corner cases.

Error Reporting

class Main inherits IO {
  my_void_io : IO ; -- no initializer => void value
  main() : Object {
    my_void_io.out_string("Hello, world.\n")
  } ;
} ;

Example error report output:

ERROR: 4: Exception: dispatch on void

Commentary

You can do basic testing as follows:

• $ cool --type file.cl  
  $ cool file.cl >& reference-output
  $ my-interp file.cl-ast >& my-output
  $ diff my-output reference-output
  

Note that this time, whitespace and newlines matter for normal output. This is because you are specifically being asked to implement IO and substring functions.

You should implement all of the operational semantics rules in the Reference Manual. You will also have to implement all of the built-in functions on the five Basic Classes.

What To Turn In For PA5c

For the PA5c checkpoint you will only be tested on hello-world.cl. If you can interpret that, you pass the checkpoint. (You can "cheat" the checkpoint by hard-coding output for that single test case, but you're ultimately only hurting yourself!)

You must turn in a zip file containing these files:

1. source_files -- your implementation, including
   • main.rb or
   • main.py or
   • main.ml

• team.txt -- an optional file listing your other team member (see below -- if you are not working in a team, do not include this file)

What To Turn In For PA5

1. readme.txt -- your README file
2. test1.cl -- a testcase
3. test2.cl -- a testcase
4. test3.cl -- a testcase
5. test4.cl -- a testcase
6. source_files -- your implementation, including
   • main.rb or
   • main.py or
   • main.ml

• team.txt -- an optional file listing your other team member (see below -- if you are not working in a team, do not include this file)

Working In Pairs

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 PA5 zipfile. That submission should include the file team.txt, a one-line flat ASCII text file that contains exactly and only the email address of your teammate. Don't include the @virgnia.edu bit. Example: If ph4u and wrw6y are working together, ph4u would submit ph4u-pa5.zip with a team.txt file that contains the word wrw6y. Then ph4u and wrw6y will both receive the same grade for that submission.

Autograding

In each case we will then compare your output to the correct answer:

• diff testcase.out correct-answer.out

For error messages and negative testcases we will compare your output but not the particular error message. Basically, your interpreter need only correctly identify that there is an error on line X. You do not have to faithfully duplicate our English error messages. Many people choose to (because it makes testing easier) -- but it's not required.

We will perform the autograding on some unspecified test system. It is likely to be Solaris/UltraSPARC, Cygwin/x86 or Linux/x86. However, your submissions must officialy be platform-independent (not that hard with a scripting language). You cannot depend on running on any particular platform.

There is more to your grade than autograder results. See the Programming Assignment page for a point breakdown.

Language Hints

I was performing the computation 4 / -59. In a very strange turn of events, this computation returns 0 in a simple C program (and, apparently, in OCaml), but -1 in Python. Bwah?

Python's division truncation works differently from C's. This is more than a little annoying, and certainly a "gotcha".

I seem to have solved the problem by adding

from __future__ import division

to the top of my file, which makes the / operator always return a floating-point result, then casting the result of a/b to an integer via the int() constructor.