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

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.
>
> ____________________
>   Racket Users list:
>   http://lists.racket-lang.org/users
>
>
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