What do you need to do, you ask? Well, you tell me: You must design and implement some non-trivial extension to the pl/0 language. Ideally, whatever you do will effect all phases of the compiler, from lex to codegen. Here's one half-baked example: add goto's. Lots of details need to be specified: syntax and scope of labels, syntax for goto's, can you jump into/out of a loop, a procedure, a function, how does that effect the stack, etc., etc. There are lots and lots of other possibilities (strings, array slices, pointers, exceptions, objects, multiple inheritence, first-class functions, logic programming, Real Time Streaming ActiveMediaX Beans, ...), but do think it through; some of those are a wee bit complicated.

Step 1: Prepare a written description of your proposed language change, together with a sketch of what needs to be changed in the compiler. (What new tokens are needed? What parts of the parser need to change and will the basic pseudo-LL(1) style parser suffice? What semantic checks need to be added? Will the symbol table change? What kinds of test cases will you need to test it? Etc.) This should run 2-3 pages, typed. Schedule a time to meet with me to review this before you begin implementation.

Step 2: Implement and test it.

Step 3: Turn in, both electronically and on paper:

1. Code, printouts, and test cases as usual.
2. Include a synopsis of what you had to change in each module.
3. Also turn in your documentation from step 1, modified if necessary to reflect changes you made and why ("Feature X turned out to be much harder to do than anticipated because of Y, so I only did the more limited X' statement...").
4. Finally, I'd like to see a page or so of your reflections on what you learned during this assignment. For example, did you learn things you hadn't seen before as a result of looking over the whole compiler at once, rather than a phase at a time? Was the open-ended nature of the assignment a good thing or not?