Activity: Local state in Scheme

main.scm contains a version of the make-account function, always setting balance to 0 to start. Try running it, and make sure you can draw the same type of diagram that I drew to understand where balance is being stored when I do something like:

(define account (make-account))

We could imagine two ways of modifying the code to take in an initial balance parameter, allowing us to start off with some amount of money in the account:

• Add a parameter and bind it in the let (this is what I demonstrated in class):

(define make-account
    (lambda (initial-balance)
        (let ((balance initial-balance))
            (lambda (amt)
                    (set! balance (- balance amt))
                    balance))))

• Add a parameter and remove the let:

(define make-account
    (lambda (balance)
        (lambda (amt)
                (set! balance (- balance amt))
                balance)))

How is the diagram you'd draw for a line like (define account (make-account 30)) different for each of these? With the folks at your table, draw the two versions of the environment diagram on the whiteboards. Then, modify the code to add in the parameter balance in one of the two ways above.

We said that we could see make-account as operating similar to a class, with one instance variable. Try adding a second instance variable called max-withdrawal that is initialized to 5; different account holders should be able to have different values for max-withdrawal, as we might later add a method to modify this variable for an account holder based on special request. Modify the code to ensure that if the person tries to withdraw more than the max-withdrawal, no withdrawal occurs. On paper, draw a diagram of the environment if we ran the following:

(define my-acct (make-account 30))
(define cats-acct (make-account 400))
(my-acct 100) ;; doesn't withdraw because > 5
(my-acct 5)
(cats-acct 20)

In the previous section, my-acct and cats-acct had separate max-withdrawal variables, allowing for the potential to set them separately for different account holders. Imagine additionally that we want to have an absolute-max-withdrawal value for all account holders: no one ever gets to withdraw more than 100 at once. Modify make-account so that it declares a variable for absolute-max-withdrawal that will be shared across account holders, similar to a static variable in a class in Java, and always initialize absolute-max-withdrawal to 100.

Now, we'll try adding methods, callable via message passing. I'd like to be able to write code like:

(define my-acct (make-account 30))
((my-acct "deposit") 15) ;; returns 45, the new amount in my account
((my-acct "withdraw") 3) ;; returns 42

Modify make-account to add these options. Note that equal? will check if two strings are equal.

If you have extra time, consider how to modify your code to allow changing of the shared absolute-max-withdrawal variable as well as how to set the individual max-withdrawal variables. Draw a diagram for what's happening when you make these changes.