Is local state possible in shen?

[Boring Coder]

This is an example from SICP:  3. Modularity, Objects, and State  

is there a way to code this in Shen? and if not, why was the decision made to make all variables global?

(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 100))  

  (W1 50) 

 50 

 (W2 70) 

 30  

  (W2 40) 

 "Insufficient funds" 

 (W1 40) 

 10  

[dr.mt...@gmail.com]

The two reasons for not supporting this feature
are portability and math'l purity.  let is treated
as shorthand for a value (as in maths) which keeps
the type theory and formal reasoning simple.

Regards implementing the Scheme example in Shen,
probably the easiest is to use property lists.

(0-) (define make-withdraw
       WithDraw Account -> (let Balance (get Account balance)
                                                    NewBalance (- Balance WithDraw)
                                                    (if (> NewBalance 0)
                                                         (put Account balance NewBalance)
                                                         (error "Insufficient funds"))))
(fn make-withdraw)

(1-) (put w1 balance 100)
100

(2-) (put w2 balance 100)
100

(3-) (make-withdraw 50 w1)
50

(4-) (make-withdraw 60 w2)
40

(5-) (make-withdraw 45 w1)
5

(6-) (make-withdraw 45 w2)
Insufficient funds

M.

[Boring Coder]

I thought of that , but my understanding was that once you use state it didn't matter if it was local of global, the side effects are the same. 

(define create-object

    Name _          -> (error "Object defined" Name) where (bound? Name)

    Name Attributes -> (do

                         (set Name (vector (* (length Attributes) 2)))

                         (initialize-object Name Attributes)

                         Name))

(define initialize-object

    Name [] -> Name

    Name [[Key Value] | Attributes] -> (do (put Key value Value (value Name))

                                             (initialize-object Name Attributes))

  _    _ -> (error "Something wrong with your input")) 

[dr.mt...@gmail.com]

with vectors

(2+) (define withdraw
   {number --> (number --> number)}
   Balance -> (let Account (@v Balance <>)
                           (/. Withdrawal (let CurrBalance (<-vector Account 1)
                                               NewBalance  (- CurrBalance Withdrawal)
                                               (if (>= NewBalance 0)
                                                   (do (vector-> Account 1 NewBalance) NewBalance)
                                                   (error "Insufficient funds"))))))
(fn withdraw) : (number --> (number --> number))

(9-) (set a (withdraw 100))
#<FUNCTION (LAMBDA (Z18380) :IN withdraw) {252032FD}>

(10-) ((value a) 30)
70

(11-) ((value a) 30)
40

(12-) ((value a) 30)
10

(13-) ((value a) 30)
Insufficient funds

M.

[Boring Coder]

Yes, that is the one,. Now the state is hidden from the user and can only be accessed through the interface we provide.

I need to relearn vectors to have the correct mental modal, I didn't know I could do that.

[Raoul Duke]

...Might there then be a good summary explanation of scoping in Shen? 

(eg in JavaScript things like hoisting rules and TDZ are all complicated, but there are zillions of tutorials and actual reference docs available.)