[plt-scheme] The Scheme way of: Using 'global' variables ?

Anton Erdman wrote:
> On Sun, 16 Aug 2009 20:59:28 +0530
> Amit Saha <lists.amitsaha at gmail.com> wrote:
> 
>> Hello all,
>>
>> After C/Java/Python, I have been exclusively using Scheme
>> (plt-scheme) for my day-day programming fun. Now, I have started
>> facing situations where I simply am not sure the correct way to do
>> something in Scheme. The latest case was something like this:
>>
>> I needed to use a 'global' identifier in my program, where I had some 
>> procedures,  couple of which needed R/W access respectively and I
>> wanted the value to persist for the lifetime of the program- just the
>> way it happens for variables you define at the topmost level in say,
>> C.
>>
>> So, what I did in scheme is- I just defined using (define var 0) and 
>> used (set!..) to change value in one procedure and used the
>> identifier name itself in some other procedure.
>>
>> For eg,
>>
>> <code>
>>
>> (define var 0)
>>
>> (define (proc1) (set! var (+ 1 var)))
>>
>> ; some other procs
>> ; in
>> ; between
>> ;
>>
>> (define (proc2) (display var))
>>
>> </code>
>>
>> But, I think this is not the way this should be done in Scheme,
>> because global variables is just calling for trouble in large
>> programs. At the same time, I am not sure how to achieve the same
>> thing in a better way, if there is one..
>>
>>
>> PS: plt-scheme just has the necessary batteries to get started easily
>> in Scheme :)
>>
>> Thanks a lot.
>>
>> Best Regards,
>> Amit
>>
> 
> I think, You can use modules or objects to isolate your global variable,
> as it may be done in C/C++. PLT-scheme has rich set of functions
> that work with namespaces. If it's what You have in mind.
> 
> If the variable needs to be really global, than, I think, you wr
> 
> Also You, probably, can use a function to isolate the variable, that
> manipulates the value in different ways like this:
> 
> <code>
> (define the-function
>   (let ((the-variable 0))
>     (lambda (operation . values)
>       (case operation
>         ((set) (set! the-variable (car values)))
>         ((display) (display the-variable))))))
> 
> (the-function 'display) ;; => 0
> (the-function 'set 11)
> (the-function 'display) ;; => 11
> </code>
> 
> I hope I understand you right way.

For this particular use case I am faced with, your suggestion definitely 
works for me. I will need to put it to use and see how well it scales up 
generally. This looks like good abstraction.

For now, Thanks a lot :)

Best,
Amit
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