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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 : * |
---------------
My only (not super important) complaint is that, in the first picture, `acc`, in the global frame, should be pointing to the same procedure object as `dispatch` is pointing to, over in Frame 0.
Reply to aos:
You seems to lacking the reply to
Where is the local state for acc kept?
"The local state" is in E0.
And "(parameter, body)" of withdraw etc. are also "shared between 'acc' and 'acc2'" (Also see djrochford's comment where parameter body of acc and parameter body of dispatch are actually pointing to the same thing).
The above answer is great. I would just like to add an answer to the follow-up questions:
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.