1.2 Monday Afternoon

How to represent a virtual cat, which has a number for position and a number for happiness? Just one number is possible, but awkward...

  (define-struct vcat (x happiness))

Explain geneated definitions. Show some examples. Demonstrate selectors. Talk about new reduction rules and how a “value” corresponds to “the simplest form of something’s name.”

We want to write things like

  ; vcat-happy-enough? : vcat -> boolean

but to use vcat as a contract, we need a data definition:

; A vcat is

;   (make-vcat number number)

(define-struct vcat (x happiness))

Now we can write the function. Follow the design recipe. For the body, observe that we need to pull out a field. We pretty much always need to pull out a field, and that leads to the structure design recipe.

Shape of the data → shape of the (template and) function.

Implement vcat-run, which moves the cat forward by 25.

Implement vcat-run-if-happy, which runs if the cat is happy enough.

We’d like to design a game where some kind of animal hatches, and the game then proceeds, where an animal can be a cat or chameleon. Might need to ask

  ; vanimal-happy-enough? : vanimal -> boolean

but what is a vanimal? Need a new kind of data definition:

; A vanimal is either

;   * a vcat

;   * a vcham

Start to write vanimal-happy?. At the template stage, do we have compound data? Sortof...

New design recipe and kind of template to go with a new kind of data definition. Again, template & function match the shape of the data definition.

Write

  ; vanimal-scare : vanimal -> vanimal

where scaring a cat makes it unhappy, while a chameleon turns orange. Three data definitions imply three templates, etc.