CS 37 -- Clab 16

CS 37

Clab 16: The Environment Model

Our goal will be to master the environment model of evaluation. The key material is the bulleted items on pages 238 and 240.

To evaluate a combination:

1. Evaluate the subexpressions of the combination.

2. Apply the value of the operator subexpression to the values of the operand subexpressions.

The above rules are essentially the same as those expressed on page 14 of our text.

The environment model of procedure application can be summarized by two rules:

A procedure object is applied to a set of arguments by constructing a frame, binding the formal parameters of the procedure to the arguments of the call, and then evaluating the body of the procedure in the context of the new environment constructed. The new frame has as its enclosing environment the environment part of the procedure object being applied.
A procedure is created by evaluating a lambda expression relative to a given environment. The resulting procedure object is a pair consisting of the text of the lambda expression and a pointer to the environment in which the procedure was created.

These last two rules are quite different from those presented on pages 14-15.

We will slowly go through section 3.2.

(define (square x) (* x x)) Syntactic sugar for: (define square (lambda (x) (* x x))) The function definitions below are also syntactic sugar for lambda expressions. (define (sum-of-squares x y) (+ (square x) (square y))) (define (f a) (sum-of-squares (+ a 1) (* a 2)))

Let's build the procedure objects and then evaluate:

(f 5) illustrating the environments constructed during the evaluation.

Now you do ex 3.9 for the recursive version showing the environment structures created by evaluating (factorial 4).

If I think there is time, we'll go over the environment construction for the sequence below. In any case, you can review it at home.

(define (make-withdraw balance) (lambda (amount) (if (>= balance amount) (begin (set! balance (- balance amount)) balance) "Insufficient funds"))) (define W1 (make-withdraw 100)) (define W2 (make-withdraw 200)) (W1 50)