# Exercises for Partner A

## Question A3

You will notice that whenever a person goes to a new place, the place gets an 'enter message. In addition, the place the person previously inhabited gets an 'exit message. When the place gets the message, it calls each procedure on its list of entry-procedures or exit-procedures as appropriate. Places have the following methods defined for manipulating these lists of procedures: add-entry-procedure, add-exit-procedure, remove-entry-procedure, remove-exit-procedure, and clear-all-procs. You can read their definitions in the code.

Sproul Hall has a particularly obnoxious exit procedure attached to it. Fix sproul-hall-exit so that it counts how many times it gets called, and stops being obnoxious after the third time.

Remember that the exit-procs list contains procedures, not names of procedures! It's not good enough to redefine sproul-hall-exit, since Sproul Hall's list of exit procedures still contains the old procedure. The best thing to do is just to load adv-world.scm again, which will define a new Sproul Hall and add the new exit procedure.

## Question A4: Part 1

We've provided people with the ability to say something using the messages 'talk and 'set-talk. As you may have noticed, some people around this campus start talking whenever anyone walks by. We want to simulate this behavior. In any such interaction there are two people involved: the one who was already at the place (hereafter called the talker) and the one who is just entering the place (the listener). We have already provided a mechanism so that the listener sends an enter message to the place when entering. Also, each person is ready to accept a notice message, meaning that the person should notice that someone new has come. The talker should get a notice message, and will then talk, because we've made a person's notice method send itself a talk message. (Later we'll see that some special kinds of people have different notice methods.)

Your job is to modify the enter method for places, so that in addition to what that method already does, it sends a notice message to each person in that place other than the person who is entering. The notice message should have the newly-entered person as an argument. (You won't do anything with that argument now, but you'll need it later.)

Add the following to adv-world.scm:

(define singer (instantiate person 'rick sproul-plaza))

(ask singer 'set-talk "My funny valentine, sweet comic valentine")

(define preacher (instantiate person 'preacher sproul-plaza))

(ask preacher 'set-talk "Praise the Lord")

(define street-person (instantiate person 'harry telegraph-ave))

(ask street-person 'set-talk "Brother, can you spare a buck")


Try walking around to sproul-plaza and telegraph-ave to see if the messages are triggered.

You must include a transcript in which your character walks around and triggers these messages.

## Question A4: Part 2

So far the program assumes that anyone can go anywhere they want. In real life, many places have locked doors.

Invent a may-enter? message for places that takes a person as an argument and always returns #t. Then invent a locked-place class in which the may- enter? method returns #t if the place is unlocked, or #f if it's locked. (It should initially be locked.) The locked-place class must also have an unlock message. For simplicity, write this method with no arguments and have it always succeed. In a real game, we would also invent keys, and a mechanism requiring that the person have the correct key in order to unlock the door. (That's why may-enter? takes the person as an argument.)

Modify the person class so that it checks for permission to enter before moving from one place to another. If a person cannot enter, return an error. Then create a locked place and test it out.

Note: A locked-place should take one instantiation variable, its name.

(define warehouse (instantiate locked-place 'warehouse))


## Question A5

Walking around is great, but some people commute from far away, so they need to park their vehicles in garages. A vehicle is just a thing, but you'll have to invent a special kind of place called a garage. Garages have two methods (besides the ones all places have): park and unpark. You'll also need a special kind of thing called a ticket; what's special about it is that it has a number as an instantiation variable.

The park method takes a vehicle (a thing) as its argument. First check to be sure that the vehicle is actually in the garage. (The person who possesses the vehicle will enter the garage, then ask to park it, so the vehicle should have entered the garage along with the person before the park message is sent.) Then generate a ticket with a unique serial number. (The counter for serial numbers should be shared among all garages, so that we don't get in trouble later trying to unpark a vehicle from one garage that was parked in a different garage.) Every ticket should have the name ticket.

You'll associate the ticket number with the vehicle in a key-value table like the one that we used with get and put in Lesson 6. However, get and put refer to a single, fixed table for all operations; in this situation we need a separate table for every garage. The file tables.scm contains an implementation of the table Abstract Data Type:

constructor: (make-table) returns a new, empty table.

mutator: (insert! key value table) adds a new key-value pair to a table.

selector: (lookup key table) returns the corresponding value, or #f if
the key is not in the table.


You'll learn how tables are implemented in SICP 3.3.3 (pp. 266-268). For now, just take them as primitive.

Make a table entry with the ticket number as the key, and the vehicle as the value. Then ask the vehicle's owner to lose the vehicle and take the ticket.

The unpark method takes a ticket as argument. First make sure the object you got is actually a ticket (by checking the name). Then look up the ticket number in the garage's table. If you find a vehicle, ask the ticket's owner to lose the ticket and take the vehicle. Also, insert #f in the table for that ticket number, so that people can't unpark the vehicle twice.

A real-life garage would have a limited capacity, and would charge money for parking, but to simplify the project you don't have to simulate those aspects of garages.

Be sure not to name anything a "car"! This will mess up everything!

Notes:

• A ticket only has one instantiation variable, a serial number. (e.g., (instantiate ticket 120)).
• A ticket is a thing with the name 'ticket
• A garage takes one instantiation variable, its name. (e.g., (instantiate garage 'soda-garage)).
• Do NOT define a new class for vehicles. You can assume that park is called with the correct argument.
• Parking a vehicle that is not owned by anyone should return an error.
• Unparking a vehicle that is not parked should return an error.