[racket] Use `set!' or not in this scenario?

You could also hide the data with a monad.
On Jul 19, 2013 9:35 AM, "Carl Eastlund" <cce at ccs.neu.edu> wrote:

> One common solution to this issue is a parameter:
>
> (define current-data (make-parameter #f))
>
> (define (f1 the-data ...)
>   (parameterize ([current-data the-data])
>     (f2 ...)))
>
> (define (f2 ...)
>   ... (f3 ...) ...)
>
> ...
>
> (define (fx ...)
>   ... (current-data) ...)
>
> The parameterize form in f1 means that calls to (current-data) will
> produce the-data during the dynamic extend of f1 (meaning from the time f1
> is called until it returns).  This isn't just a set! on enter and exit;
> different threads won't see this change, for instance, and if f1 is
> exited/re-entered using a continuation, (current-data) will only produce
> the-data while inside f1.  So while it's not purely functional, it's a much
> more disciplined kind of effect.
>
> Carl Eastlund
>
> On Fri, Jul 19, 2013 at 12:23 PM, Ben Duan <yfefyf at gmail.com> wrote:
>
>> Scenario: A piece of data is determined in the first function `f1', but
>> is only processed in a sub-sub-sub-… function `fx'.
>>
>> One way is to use pass `the-data' as arguments from `f1' through `f2' all
>> the way down to `fx':
>>
>>     (define f1 (the-data …)
>>       …
>>       (f2 the-data …)
>>       …)
>>
>>     (define f2 (the-data …)
>>       …
>>       (f3 the-data …)
>>       …)
>>
>>     …
>>
>>     (define fx (the-data …)
>>       … the-data …)
>>
>> But in the above way, the body of `f2', `f3', `f4' and so on doesn't use
>> `the-data'. It is only passed to the next function. And I still have to add
>> the argument `the-data'.
>>
>> Another way is to use `set!':
>>
>>     (define the-data …)
>>
>>     (define f1 (the-data …)
>>       …
>>       (set! the-data …)
>>       …
>>       (f2 …)
>>       …)
>>
>>     (define f2 (…)
>>       …
>>       (f3 …)
>>       …)
>>
>>     …
>>
>>     (define fx (…)
>>       … the-data …)
>>
>> But in this way, the benefits of being functional are lost. For example
>> there will be some problems writing tests for these functions.
>>
>> My question is, which way is better? Or are there other ways to solve
>> this problem?
>>
>> Thanks,
>> Ben
>>
>> P.S. This question is not about Racket. It's just a beginner's question
>> about how to program. Please let me know if it's not appropriate to ask
>> this kind of questions here. Thank you.
>>
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>>
>>
>
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