Our goals for this assignment are pretty simple:

1. Understand why subtyping is useful
2. Practice implementing subtypes in Racket
3. Understand why methods are useful
4. Practice implementing methods in Racket

In this tutorial, you’ll deal with these two concepts separately but in the associated exercise, you’ll have to deal with them at the same time while designing interactive quizzes.

Tutorial 8 Starter Files

Part 1. Fun at the zoo

Today we’ll be defining a whole zoo’s worth of animals. However, as we saw on Monday, we want to use subtyping to make our object design more efficient.

Activity 1.1

First define a abstract struct called animal that has the following fields:

• name
• age
• weight

Note: if you’re still struggling with structs, I HIGHLY recommend listing out all of the automagically defined functions you get as comments in your program like I did on Monday and in the demo video.

Activity 1.2

Now define a subtype of animal called cat which has an additional field:

• sleeping-spot

Activity 1.3

Now define a subtype of animal called dog that has an additional field:

• best-friend

Activity 1.4

Now define a subtype of animal called mouse that has an additional field:

• hiding-spot

Activity 1.5

Now write a function called feed-animal! that takes as input a single animal and mutates its weight by adding 2 to it.

Make sure to write at least a couple of tests to make sure its working the way you think!

Hint: Remember, that now that we have imperatives…writing tests is a little confusing. You need to take into account what the weight WAS in order to find out what the weight will be AFTER calling the function.

Activity 1.6 (Challenge)

Write a function called feed-animals/faves! that takes in a list of animals (loa for short) and uses feed-animal! for each animal in the list with one big caveat…

Bob is Gizmo’s best friend. He is also the zoo keeper! Bob wants to feed all of the animals in the zoo, but he will feed any animal twice who lists him as their best-friend.

We’ve included some tests for you on this one.

Part 2. A roster of students

In the Racket file you downloaded scroll down until you see “PART 2”. Here you’ll find two different struct definitions.

; A student is...
;  - (make-student string number string)

This struct does not have any methods.

; A roster is...
;  - (make-roster string string (listof student))
;    with...
;      #:methods
;      display : roster -> (void)

Th struct represents the roster of a course. Here are its fields:

It also has a single method:

Activity 2.1

Your first job is to implement a new method for roster with the following specification:

Activity 2.2

Next (and last) implement one final method: