91.301 Organization of Programming Languages
Prof. F. Martin
Problem Set 7: Object-Oriented Adventure Game
The programming assignment for this week explores two ideas: the simulation of a world in which objects are characterized by collections of state variables, and the use of object-oriented programming as a technique for modularizing worlds in which objects interact.
These ideas are presented in the context of a simple text-based adventure game like the ones available on many computers. Such games have provided an interesting sink of time for many computer lovers. In order not to sink too much of your own time, it is important to study the system and plan your work before starting to write any code.
This problem set begins by describing the overall structure of the simulation. The warm-up exercises in Part 2 will help you to master the ideas involved. Part 3 contains the assignment itself.
This assignment was developed by the Scheme group at MIT, adapted by Holly Yanco, and edited by Fred Martin.
- Read Chapter 3 introduction (Modularity, Objects, and State), Section 3.1 (Assignment and Local State), and Section 3.2 (Environment Model) from SICP.
- Read Additional Notes on Object-Oriented Programming.
Use the Module language in DrScheme for this assignment.
Part 1: The SICP Adventure Game
The basic idea of adventure games is that the user plays a character in an imaginary world inhabited by other characters. The user plays the game by issuing commands to the computer that have the effect of moving the character about and performing acts in the imaginary world, such as picking up objects. The computer simulates the legal moves and rejects illegal ones. For example, it is illegal to move between places that are not connected (unless you have special powers). If a move is legal, the computer updates its model of the world and allows the next move to be considered.
Our game takes place in a strange, imaginary world called UMass Lowell, with imaginary places such as a computer lab, a robot lab, and a department office. In order to get going, we need to establish the structure of this imaginary world: the objects that exist and the ways in which they relate to each other.
Initially, there are three procedures for creating objects:
(make-thing name) (make-place name) (make-person name birthplace restlessness)
In addition, there are procedures that make people and things and procedures that install them in the simulated world. The reason that we need to be able to create people and things separately from installing them will be discussed in one of the exercises later. For now, we note the existence of the procedures
(make&install-thing name birthplace) (make&install-person name birthplace restlessness)
Each time we make or make and install a person or a thing, we give it
a name. People and things also are created at some initial place. In
addition, a person has a restlessness factor that determines how often
the person moves. For example, the procedure
may be used to create the two imaginary characters,
andy, and put them in their places, as it were.
(define computer-lab (make-place 'computer-lab)) (define robot-lab (make-place 'robot-lab)) (define holly (make&install-person 'holly robot-lab 3)) (define andy (make&install-person 'andy computer-lab 2))
All objects in the system are implemented as message-accepting procedures.
Once you load the system on your machine, you will be able to control
andy by sending them
appropriate messages. As you enter each command, the computer reports what happens and where it is
happening. For instance, imagine we had interconnected a few places so that the following scenario is
(ask holly 'look-around) At robot-lab : holly says -- I see nothing () (ask (ask holly 'place) 'exits) (south) (ask holly 'go 'south) holly moves from robot-lab to west-hall #t (ask holly 'go 'east) holly moves from west-hall to elevator-lobby #t (ask holly 'go 'north) holly moves from elevator-lobby to computer-lab At computer-lab : holly says -- Hi andy #t (ask andy 'look-around) At computer-lab : andy says -- I see holly
In principle, you could run the system by issuing specific commands to each of the creatures in the world, but
this defeats the intent of the game since that would give you explicit control over all the characters. Instead,
we will structure our system so that any character can be manipulated automatically in some fashion by the
computer. We do this by creating a list of all the characters to be moved by the computer and by simulating
the passage of time by a special procedure,
clock, that sends a
move message to each creature in the list. A
move message does not automatically imply that the creature receiving it will perform an action. Rather, like
all of us, a creature hangs about idly until he or she (or it) gets bored enough to do something. To account
for this, the third argument to
make-person specifies the average number of clock intervals that the person
will wait before doing something (the restlessness factor).
Before we trigger the clock to simulate a game, let's explore the properties of our world a bit more.
First, let's create a
sicp-textbook and place it in the computer-lab (where
andy now are):
(define sicp-textbook (make&install-thing 'sicp-textbook computer-lab))
Next, we'll have
holly look around. She sees the textbook and
andy. The textbook looks useful, so we have
holly take it and leave.
(ask holly 'look-around) At computer-lab : holly says -- I see sicp-textbook andy (sicp-textbook andy) (ask holly 'take sicp-textbook) At computer-lab : holly says -- I take sicp-textbook #t (ask holly 'go 'south) holly moves from computer-lab to elevator-lobby #t
andy had also noticed the manual; he follows
holly and snatches the textbook away. Angrily,
holly sulks off to the
(ask andy 'go 'south) andy moves from computer-lab to elevator-lobby At elevator-lobby : andy says -- Hi holly #t (ask andy 'take sicp-textbook) At elevator-lobby : holly says -- I lose sicp-textbook At elevator-lobby : holly says -- Yaaaah! I am upset! At elevator-lobby : andy says -- I take sicp-textbook #t (ask holly 'go 'west) holly moves from elevator-lobby to west-hall #t (ask holly 'go 'south) holly moves from west-hall to network-closet #t
holly, beneath the
network-closet is an inaccessible dungeon, inhabited by a troll named
grendel. A troll is a kind of person; it can move around, take things, and so on. When a troll gets a
message from the clock, it acts just like an ordinary personunless someone else is in the room. When
grendel decides to
act, it's game over for
(ask grendel 'move) grendel moves from dungeon to network-closet At network-closet : grendel says -- Hi holly #t
After a few more moves,
grendel acts again:
(ask grendel 'move) At network-closet : grendel says -- Growl.... I'm going to eat you, holly At network-closet : holly says -- Dulce et decorum est pro computatore mori! holly moves from network-closet to heaven At network-closet : grendel says -- Chomp chomp. holly tastes yummy! *burp*
The simulator for the world is contained in two files. The first file,
game-plt4.ss, contains the basic object system, procedures to create
people, places, things and trolls, together with various other useful
procedures. The second file,
world-plt4.ss, contains code that initializes
our particular imaginary world and installs
You can find these files on the course web site. Load and evaluate the world file; this will load in the game file as well.
Part 2: Warm-up Exercises
Do these exercises before starting work on the computer. These exercises should be turned in with your assignment.
Exercise 1: Draw a simple inheritance diagram showing all the kinds of objects (classes) defined in the
adventure game system (
game-plt4.ss), the inheritance relations between them, and the methods defined for each
Exercise 2: Draw a simple map showing all the places created by evaluating
world-plt4.ss, and how they
interconnect. You will probably find this map useful in dealing with the rest of the problem set.
Exercise 3: Suppose we evaluate the following expressions:
(define pizza (make-thing 'pizza robot-lab)) (ask pizza 'set-owner holly)
At some point in the evaluation of the second expression, the expression
(set! owner new-owner)
will be evaluated in some environment. Draw an environment diagram, showing the full structure of
at the point where this expression is evaluated. Don't show the details of
robot-lab are names defined in the global environment that point off to some objects that you
draw as blobs.
Exercise 4: Suppose that, in addition to
pizza in Exercise 3, we define
(define pepperoni-pizza (make-named-object 'pizza))
pepperoni-pizza the same object (i.e., are they
holly wanders to a place where they both are and
looks around, what message will she print?
Solutions to these exercises should also be turned in with your solutions to the warm-up exercises.
Exercise 5: We suggest that you do this exercise before you start coding the assignment, because it illustrates a bug that is easy to fall into when working with the adventure game.
install is implemented as a method defined as part of
person. Notice that the person
puts the person on the clock list (this makes them
animated) then invokes the
mobile-object version on
which makes the
self is being installed aware that
self thinks it is in that place. That is, it makes the
birthplace consistent in their belief of where
self is. The relevant
details of this situation are outlined in the code excerpts below:
(define (make-person name birthplace threshold) (let ((mobile-obj (make-mobile-object name birthplace)) ...) (lambda (message) (cond ... ... ((eq? message 'install) (lambda (self) (add-to-clock-list self) ((get-method mobile-obj 'install) self) )) ; ** ...)))) (define (make-mobile-object name place) (let ((named-obj (make-named-object name))) (lambda (message) (cond ... ... ((eq? message 'install) (lambda (self) (ask place 'add-thing self))) ...))))
Louis Reasoner suggests that it would be simpler if we change the last line of the make-person version of the install method to read:
(ask mobile-obj 'install) )) ; **
Alyssa P. Hacker points out that this would be a bug. If you did
that, she says, then when you
holly moves to a new place, she'll thereafter be in two places at
once! The new place will claim that
holly is there, and
of birth will also claim that
holly is there.
What does Alyssa mean? Specifically, what goes wrong? You will likely need to draw an appropriate environment diagram to explain carefully.
Exercise 6: We do not expect you to have to make significant changes in the
game code, though you
may do so if you want to.
You have a
world buffer. Since the simulation model works by data mutation, it is possible to get
your Scheme-simulated world into an inconsistent state while debugging. To help you avoid this problem,
we suggest the following discipline: any procedures you change or define should be placed in your answer
file; any new characters or objects you make and install should be added to
This way, whenever you change some procedure you can make sure your world reflects these changes by simply re-evaluating the entire world.ss file.
Finally, to save you from retyping the same scenarios
repeatedlyfor example, when debugging you may want to create a
new character, move it to some interesting place, then ask it to
actwe suggest you define little test script
procedures at the end of the
world buffer that you can invoke to
act out the scenarios when testing your code.
See the comments in
world for details. After loading the files,
andy move around by repeatedly evaluating
(clock). (a) Which person is more restless? (b) How often do both
of them move at the same time?
Exercise 7: Implement
student, a special kind of
person. Define a
make-student that creates the student object. It should
person. A student has an instance variable,
that indicates whether or not the student has taken and passed
Initally, all students have not passed OPL. A student has a method
take-opl that changes the state of
#t. A student also
has a method
cheat-on-problem-set that changes the state of
#f. A student also has a method
#t if the student has passed OPL and
Exercise 8: Make and install a new character of a student type, yourself, with a high enough threshold (say, 100) so that you have free will and are not likely to be moved by the clock.
Place yourself initially in the
computer-lab. Also make and
install a thing called
late-homework, so that it starts in the
computer-lab. Pick up the
late-homework, find out where
holly is, go there, and try to get
holly to take the homework even
though she is notoriously adamant in her stand against accepting tardy
problem sets. Can you find a way to do this that does not leave you
upset? Turn in a list of your definitions and actions. If you wish,
you can intersperse your moves with calls to the clock to make things
more interesting. (Watch out for
Exercise 9: Define a procedure
make-advisor that makes a special kind of person. It should inherit from a person but have an additional method:
(ask 'advise Person). An advisor will enroll a student in OPL if they are in the same place and the student has not yet passed OPL. When enrolled, the student is sent to the OPL classroom (you can do this with
(ask Person 'move-to OPL)).
Note that you'll need to create the OPL place in your world. It's up to you whether or not to make this a place from which one can exit.
In addition, you should make the
clock-tick method for
an advisor automatically enroll any students who haven't passed OPL in
the place where the advisor is. If no students are in the current
location, the advisor will act like a normal person; that is, it should
invoke the super-class (person) clock-tick method. You may find the
other-people-at-place procedure (defined in game.ss) useful. Create
and install at least one advisor in your world in the elevator-lobby.
Exercise 10: Now you have the elements of a simple game that you
play by interspersing your own moves with calls to the clock. Your
goal is to leave the
computer-lab, gain access to the
robot-lab, and return, without being enrolled in OPL (start
yourself off as not having passed OPL, as you're still in the course
To make the game more interesting, you should also create some
student(s) besides yourself and set up the student
act method so
that a student will try to move around campus, collecting interesting
things that they find and occasionally leaving some of their
possessions behind when they move on.
Turn in your new student
act method and a demonstration that it
Required for Honor Students and Graduate Students; Extra Credit for Others
Exercise 11: Design a non-trivial extension to this simulated world. You can do what you want (so long as it is in good taste, of course). The extension can be as elaborate as you like, but don't go overboard; this is meant to be a problem set, not a term paper, after all.
Try to base your extended simulation around a theme. Possibilities include classes, labs, food, robots, etc. Use your imagination!
Whatever you choose to do, your simulation should include at least two
new kinds of persons, places, or things, using inheritance. For
example, you might implement a classroom as a new kind of place. Your
new objects should have some special methods or special properties in
relation to other objects. To continue the previous example, you might
make new kinds of people called
overworked-students, who go to sleep when
they enter classrooms. In answering this problem, you should turn in:
- One or two paragraphs explaining the story behind your simulation. Describe your new objects and their behaviors.
- An inheritance diagram showing your new classes.
- Listings of any new procedures you write, or old procedures that you modify.
- A transcript that shows your simulation in action.
Note that it's more impressive to implement a simple, elegant idea, than to just amass tons of new objects and places.