sicp-ex-3.11



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(define acc (make-account 50))

global   _________________________________
env  -->| make-account :*                 |
        | acc : *       |                 |
         -------|-------|---^-----------^-
                |       |   |           |
                |     ( * , * )         |
                |       |               |
                        parameter: balance
                |       body: (define (withdraw ... ))
                |                       |
                |                -------Frame 0-      (parameter, body)
                |               | balance  : 50 |      |
                |           E0->| withdraw : *--|--> ( * , * )  
                |               | deposit  : *--|--> ( * , * ) 
                |               | dispatch : *--|--> ( * , * )     
                |                -------^----^--           |
                |    ___________________|    |_____________| 
                |   |
              ( * , * )
                |
                parameter : m           
                body      : (cond ((eq? m ... )))               

((acc 'deposit) 40)

Frame 1 is created when (acc 'deposit is evaluated).
Next, Frame 2 is created when (deposit amount). Since deposit is defined
in E0, Frame 2 pointer is to environment E0.

global   _________________________________
env  -->| make-account :*                 |
        | acc : *                         |
         -------|-----------------------^-
                |                       |
                |                -------Frame 0-
                |               | balance  : 50 |
              ( *, *-)--------->| withdraw : *  |
                                | deposit  : *  |<- E0
                                | dispatch : *  |
                                 -^-----^------- (make-account balance)
                         _________|     |
                        |        -------Frame 1-
                        |       | m : 'deposit  |<- E1 
                        |        --------------- (dispatch m)
                 -------Frame 2-
                | amount : 40   |<- E2
                 --------------- (deposit amount)

After ((acc 'deposit) 40) evaluation balance is set to 90 in Frame 0 and
Frames 1 and 2 are not relevant anymore.

global   _________________________________
env  -->| make-account :*                 |
        | acc : *                         |
         -------|-----------------------^-
                |                       |
                |                -------Frame 0-
                |               | balance  : 90 |
              ( *, *-)--------->| withdraw : *  |
                                | deposit  : *  |<- E0
                                | dispatch : *  |
                                 --------------- 

((acc 'withdraw) 60)

global   _________________________________
env  -->| make-account :*                 |
        | acc : *                         |
         -------|-----------------------^-
                |                       |
                |                -------Frame 0-
                |               | balance  : 90 |
              ( *, *-)--------->| withdraw : *  |
                                | deposit  : *  |<- E0
                                | dispatch : *  |
                                 -^-----^------- (make-account balance)
                         _________|     |
                        |        -------Frame 3-
                        |       | m : 'withdraw |<- E3
                        |        --------------- (dispatch m)
                 -------Frame 4-
                | amount : 60   |<- E4
                 --------------- (withdraw amount)

After ((acc 'withdraw) 60)

global   _________________________________
env  -->| make-account :*                 |
        | acc : *                         |
         -------|-----------------------^-
                |                       |
                |                -------Frame 0-
                |               | balance  : 30 |
              ( *, *-)--------->| withdraw : *  |
                                | deposit  : *  |<- E0
                                | dispatch : *  |
                                 ---------------

aos

The above answer is great. I would just like to add an answer to the follow-up questions:

How are the local states for the two accounts kept distinct? Which parts of the environment structure are shared between acc and acc2?

1. The local states for the two accounts each have their own environments.

2. The only part of the environment structure that is shared between 'acc' and 'acc2' is the global environment.