How the computer read the program

Question

It is a quite elementary but general question when I am reading SICP. I get puzzled about how the computer read the program. I understand the computer read the program as a layer or nested system, which means the lower layer can't read the item from upper layer but it is obvious not true when I encounter this program in SICP:

(define balance 100)
(define (withdraw amount)
        (if (>= balance amount)
            (begin (set! balance(- balance amount))
             balance)
         "Insufficient funds"))

As u can see, balance is some item I can treat on the upper layer, while I can see the lower layer program can still read that, so it is wrong, so I am wondering how the computer read the program

Answer 1

Both withdraw and balance are defined at the same level (balance is not in an "upper" level), and balance was defined before withdraw, so it's natural that withdraw can "see" balance.

The interpreter adds all top-level definitions to the same environment, and one definition can refer to another at the same level as long as it's been previously defined, for example:

(define x 10)
(define y (+ 6 x))

This applies to nested procedures as well. A nested procedure can "see" all the definitions above it. For example, this is valid:

(define x 42)

(define (f)      ; f and x are at the same level
  (define (g) x) ; g is in a level below f, but sees x
  (g))

(f)
=> 42

In the above example, g is internal to f, nothing outside f "sees" g, but g can "see" all that's above itself.