SICP练习3.20：序对过程实现下环境模型评估推演（无法绘图）

Since you mentioned eye discomfort prevents drawing a formal environment diagram, let's walk through the evaluation step-by-step using the procedural pair implementation from SICP, and map out how the environment structure evolves.

First, here's the core procedural definition of pairs we'll use (all bound in the global environment):

(define (cons x y)
  (define (dispatch m)
    (cond ((eq? m 'car) x)
          ((eq? m 'cdr) y)
          (else (error "Undefined operation -- CONS" m))))
  dispatch)

(define (car z) (z 'car))
(define (cdr z) (z 'cdr))

Let's use this example expression sequence to trace the environment model:

(define p (cons 1 2))
(car p)
(cdr p)

Step 1: Evaluating (define p (cons 1 2)) #

When we call (cons 1 2):

• A new local environment e1 is created, whose enclosing environment points to the global environment.
• In e1, the parameters x is bound to 1, and y is bound to 2.
• Next, we define the dispatch procedure inside cons: this creates another local environment e2, whose enclosing environment is e1. The dispatch procedure object stores a pointer to e2.
• The cons procedure returns the dispatch procedure object.
• Finally, the global environment binds the variable p to this dispatch procedure.

Step 2: Evaluating (car p) #

The car procedure is defined as (lambda (z) (z 'car)), so evaluating (car p) does the following:

• We call car with argument p (which is the dispatch procedure from Step 1). This creates a new environment e3 enclosing the global environment, with z bound to the dispatch procedure.
• The body of car is (z 'car) — this means we invoke the dispatch procedure with the argument 'car.
• Invoking dispatch creates a new environment e4, whose enclosing environment is e2 (since dispatch's procedure object points to e2). In e4, the parameter m is bound to 'car.
• The cond clause checks (eq? m 'car) which is true, so we look up the value of x. Following the environment chain: e4 → e2 → e1, we find x is bound to 1.
• The result of (car p) is 1.

Step 3: Evaluating (cdr p) #

This follows almost the same logic as (car p):

• (cdr p) calls the cdr procedure, which invokes (dispatch 'cdr).
• A new environment e5 is created (enclosed by e2), with m bound to 'cdr.
• The cond clause returns y, which we find in e1 bound to 2.
• The result of (cdr p) is 2.

Environment Chain Summary #

• Global Environment: Binds cons, car, cdr, and eventually p (to the dispatch procedure).
• e1: Created when cons is called; binds x=1, y=2; encloses global environment.
• e2: Created when dispatch is defined inside cons; encloses e1; referenced by the dispatch procedure object.
• e4: Created when dispatch is called for car; binds m='car; encloses e2.
• e5: Created when dispatch is called for cdr; binds m='cdr; encloses e2.

内容的提问来源于stack exchange，提问作者lightning_missile