Homework #9 Solution

Start with class-inherit.rkt, which combines class.rkt, inherit.rkt, and inherit-parse.rkt into a single file.

You may find it nicer to work with class.rkt, inherit.rkt, and inherit-parse.rkt as separate files, but you must combine them into a single file for handin. To combine the files: append them in the listed order, then remove the second and third #lang plait lines and remove all require forms.

Part 1 — Instantiating Object
In the starting code, {new Object} doen’t work, even though Object is supposed to be a built-in class with no fields and no methods. Fix the implementation to that {new Object} produces an instance of Object.

(test (interp-prog (list)
`{new Object})
`object)
(test (interp-prog (list `{class Fish extends Object
{size color}})
`{new Object})
`object)
Part 2 — Conditional via select
Add select:

<Exp = ...
| {select <Exp <Exp}
The first expression in select should produce a number result, while the second expression should produce an object. If the number is 0, select calls the zero method of the object. If the number is not 0, select calls the nonzero method of the object. In each case, select calls the method with the argument 0.

(test (interp-prog (list `{class Snowball extends Object
{size}
[zero {arg} this]
[nonzero {arg}
{new Snowball {+ 1 {get this size}}}]})
`{get {select 0 {new Snowball 1}} size})
`1)
(test (interp-prog (list `{class Snowball extends Object
{size}
[zero {arg} this]
[nonzero {arg}
{new Snowball {+ 1 {get this size}}}]})
`{get {select {+ 1 2} {new Snowball 1}} size})
`2)
The result is up to you when select gets a non-number result for its first subexpression, but a “not a number” error is sensible. Similarly, the result is up to you when the second subexpression’s result is not an object or does not have a zero or nonzero method.

You will probably find it easiest to extend and test the Exp layer, then then ExpI layer, and then the parse and interp-prog layer.

Part 3 — instanceof
Add instanceof:

<Exp = ...
| {instanceof <Exp <Sym}
An expression {instanceof <Exp <Sym} produces 0 if the result of <Exp is an object that is an instance of the class named by <Sym or any of its subclasses. It should produce 1 f the result of <Exp is any other object. Note that {instanceof <Exp Object} should produce 0 as long as the value of <Exp is an object.

Note that you will have to introduce a notion of superclasses into the Class layer, even though method inheritance remains the job of the ClassI layer.

Example:

(test (interp-prog (list `{class Fish extends Object
{size color}})
`{instanceof {new Fish 1 2} Object})
`0)
(test (interp-prog (list `{class Fish extends Object
{size color}})
`{instanceof {new Object} Fish})
`1)
(test (interp-prog (list `{class Fish extends Object
{size color}})
`{instanceof {new Fish 1 2} Fish})
`0)
(test (interp-prog (list `{class Fish extends Object
{size color}}
`{class Shark extends Fish
{teeth}})
`{instanceof {new Shark 1 2 3} Fish})
`0)
(test (interp-prog (list `{class Fish extends Object
{size color}}
`{class Shark extends Fish
{teeth}}
`{class Hammerhead extends Shark
{}})
`{instanceof {new Hammerhead 1 2 3} Fish})
`0)
The result is up to you if <Exp in {instanceof <Exp <Sym} does not produce an object or if <Sym is not the name of a class.