6.037, IAP 2019|Project 4 1 MASSACHVSETTS INSTITVTE OF TECHNOLOGY Department of Electrical Engineering and Computer Science 6.037|Structure and Interpretation of Computer Programs IAP 2019 Project 4 Release date: 24 January, 2019 Due date: 1 February, 2019 at 2300h Code to load for this project: • The code for the evaluator: oo-eval.scm • There are also some files which your finished evaluator will be able to run: oo-util.scm, oo-types.scm, and oo-world.scm. You will not need these files at first. • All of these files can be retrieved from the course web site. You should begin working on the assignment once you receive it. It is to your advantage to get work done early, rather than waiting until the night before it is due. You should also read over and think through each part of the assignment (as well as any project code) before you sit down at the computer. It is generally much more efficient to test, debug, and run a program that you have thought about beforehand, rather than doing the planning \online." Diving into program development without a clear idea of what you plan to do generally causes assignments to take much longer than necessary. Word to the wise: This project can take a lot of time. You'll need to understand the general ideas of object-oriented programming and the implementation provided of an object-oriented pro- gramming system as discussed in lecture. You'll also need to have a firm grasp of the metacircular evaluator from the previous project, of which oo-eval.scm is a derivative. In truth, this assign- ment is much more an exercise in reading and understanding a software system than in writing programs, because reading significant amounts of code is an important skill that you must master. We strongly urge you to study the code before you try the programming exercises themselves | starting to program without understanding the code is a good way to get lost, and will virtually guarantee that you will spend more time on this assignment than necessary. In this project we will apply the concepts of object orientation to the evaluator which we built in the previous project. Rather than construct an object system on top of the language, like the code handout from lecture 6 did, we will instead build it into the language itself, adding additional special forms and data structures to make the concepts of \classes" and \objects" into first-class types and objects in the language. That is, while m-eval's goal was to mimic Scheme's semantics as closely as possible so that it could run itself, oo-eval intentionally adds entirely new syntaxes which will allow us to write simple code for oo-eval which would be considerably more complex in ordinary Scheme. Once you have finished implementing this Scheme variant, you will be able to run, and improve upon, the provided object-oriented text adventure game which uses the new syntax and semantics. One key thing to be careful of { symbols in DrRacket are case sensitive. For readability, we have tried to use upper case for names of object types and for names of methods. 6.037, IAP 2019|Project 4 2 The story so far. Cons of the Dead Later reports could never clearly nail down the exact source of the plague, but they all agree that late Saturday night of the 2018 Mystery Hunt puzzle-solving competition1, something snapped in some poor hunter. Instead of solving puzzles, he blindly went in search of more brains. The disease spread quickly through the ranks of the hunters, with one biting another until they were all shambling mockeries of their former selves. You were working late in lab, unplugged from the world and tooling away, blissfully unaware that anything was amiss, until Alyssa P. Hacker burst in and slammed the door shut. \Zombies!" she shouted. You looked through the window in the door and saw a slightly decomposing Ben Bitdiddle trying to figure out how to operate the numeric keypad on the combo lock. \That won't hold him for long, and there are more of them coming! Quick, help me barricade the door!" You both pile up printers, desks, and extra workstations against the door. \Now what?" you ask. Alyssa pulls a USB flash drive our of her pocket. \We hope the trouble I went through to get this was worth it. I hope you've got some food; this is going to be a long night." You nod, looking at the pile of snacks and soda you had brought with you. You were planning for an all-nighter, but not one like this. Can software save your mortal soul? Alyssa sits down at a computer and plugs in the drive. \Ben and I had been working for the last few hours on trying to determine the cause of the outbreak, with hopes we could solve it before it spread further. We hoped we might get lucky and find a way to restore these people before the National Guard shows up and starts roasting zombies. \We worked in separate locations (for improved fault tolerance) and the zombies found him first. The timing was particularly bad{ he had just Zephyred me that he'd had a break-through in the simulation he had written! And since he hadn't committed his changes yet to our git repository, I had no choice but to sneak through the zombie-infested halls over to where he was working and grab what I could off his computer." Alyssa looks through the contents of the drive and sighs, \Most of his code has been overwritten by nonsense words, probably as his mind faded away. I'll extract what I can..." After a few minutes, Alyssa looks at you, worried. \Ben wrote an object-oriented world simulator, so he could model various scenarios. I've recovered most of the code for his game world. But, this code isn't normal Scheme. It appears to have syntax for creating classes, instantiating objects, and so on. I know Ben had been working on adding an object system to his own personal scheme evaluator (b-eval), but that's in the part of his files that were corrupted." She sits back, dejected. You exclaim, \But I just finished writing a Scheme evaluator for 6.037! I can extend that to add an implementation of an object-oriented system in no time. Then we can run Ben's simulation and see what he discovered!" Test for success Alyssa tells you that Ben's object system has some complexity to it, but if you start with the basics and test each piece as you go, you should be able to build up to what you need, a layer at a time. 1http://web.mit.edu/puzzle/www/ 6.037, IAP 2019|Project 4 3 She says, \I'll help guide you as much as I can, including producing some test cases for you, but I need to focus on seeing if there are any patterns in this gibberish." She points at her screen. \Maybe Ben's zombie-addled subconscious had something to say{ I'll see if I can decipher any more code." If you just load and run oo-eval.scm, you'll find that it runs some test cases and produces an error message. This is because the file loads a series of test definitions, and then applies the run-all-tests procedure. You can comment-out that call if you like and run the individual tests as you go, but do occasionally run all the tests to make sure you don't have any regressions. \Think of this as test-driven development. except with the threat of zombies. That's not usually part of this methodology." She adds that though her tests will help, you still need to consider the requirements at each step and do your own testing in addition to what she cooks up. Getting started You start thinking about what you'll need to add to your evaluator: data abstractions internal to oo-eval for managing classes and instances, as well as the special forms or primitive procedures you'll expose to the oo-eval user to create classes, instantiate them, and invoke methods on the resulting instances. Alyssa interrupts your train of thought. \Hey, look at this. I was able to recover part of an older version of Ben's evaluator with his object system implementation. I should warn you, though{ he'd been feeling a little meta lately." Ben had decided on a simple implementation for instances: a tagged list of the symbol instance and an association list2 of all the instance's local state (slot names and current values). He would always include a special slot named :class which would refer to the class the instance was made from. A snippet of Ben's code looks like this: (define (instance? obj) (tagged-list? obj 'instance)) (define (instance-state inst) (second inst)) (define (instance-class inst) (read-slot inst ':class)) ; Given an object instance and slot name, find the slot's current value (define (read-slot instance slotname) (let ((result (assq slotname (instance-state instance)))) (if result (cadr result) (error "no slot named" slotname)))) 2A list of two-element lists, each containing a name and a value. 6.037, IAP 2019|Project 4 4 ; Create an object instance from a class ; Store list of slots' data (including class as :class) in a tagged-list ; All slots other than :class start out with the value 'uninitialized ; Need to call the constructor if there is one (define (make-instance class .