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Keyword reference for overview

Question 77 (2003):

Hi! I a find occam very difficult and was wondering if there is going to be an overview lecture or two soon? There is so much info to take in that I don't feel I have a proper grasp of it yet. I find working out whether the system will deadlock or not difficult to determine, and also when to use parallel and when to use sequential. Whenever I program something in occam I have countless errors even though I think I have computed it correctly. This worries me as in the exam we obviously will not have the benefit of a compiler. Can I suggest that some optional terminal sessions are set up almost as drop-ins, as a lot of people in my seminar group are finding it easy? Thanks.

Answer 77:

The remainder of my part of this course consists of the real-time material (chapter 8 of the slides and the paper: "Managing Hard Real-Time ...") and JCSP (which presents a Java class library giving exactly the same occam/CSP concurrency model). The JCSP material will review the fundamental ideas that have already been presented for occam. There are only a few such ideas and they are simple. But they have to be mastered.

Deadlock analysis is not particularly simple but, so long as you are fluent in the concurrency mechanisms of the occam/CSP model, it becomes not unreasonable. If you think it's hard in occam, try it for the standard Java threads model!

If you don't understand when to use PAR and when to use SEQ, then you have not understood anything about the concurrency model and you have to do something about it. Have you not been attending the weekly seminar groups where all the ideas are rehearsed as you discuss the assignments? We provide these seminar groups precisely to enable you to raise any issues - especially fundamental technical principles - with your seminar leader and group.

You should not be getting "countless errors" when compiling occam code. We provided 4 (or 5) weeks of terminal classes last term specifically to assist you in mastering the basic syntax and structure of the language and in understanding the compiler error messages. If you are now getting countless error messages, you do not have such basic mastery ... and how can you have got by the first three assessment exercises?

Basic syntax and structure is given on slides 4-22, 4-26 .. 4-30, 4-33 .. 4-35, 5-27 and 5-36. Some extra stuff is on 10-5 .. 10-6, 10-10 .. 10-22. Of course, smeantics is also discussed there. Everything is spelt out in words in the "occam2 Checklist" paper (17 pages). And there are examples everywhere else. But it all must be read and re-read and discussed and re-read. And you must draw/design lots and lots of process icons and network diagrams and write lots and lots of code.

We already provide - through the weekly seminars - far more help for this course than you will find available for many others. I do not think magic assitance at terminal classes will have much educational benefit at this stage in the course. If completely stuck in understanding why your program won't compile, ask a friend, your seminar leader or me (phw) directly. Only puzzle over the first few error messages from any compiler - fix those and then re-compile. View the occam compiler expecially as your friend - it just won't let you do anything silly and prevents you from making a great range of more subtle mistakes. It ensures you know about the scope of declarations and the scope of programming structures. Learn the very simple rules, let your programs compile and they will work ...

Finally, when marking exam scripts, minor errors in syntax are ignored - so long as the overall logic and structure can be seen. You are not being assessed in the exam on trivial mistakes that the compiler would put you right upon if presented with your code. Indented layout, however, needs to be (rouhgly) right because that shows up the logical structures. But indenting hand-written code costs no time and should be automatic.

Keywords: overview , exams