Re: dynamic-wind

[Panicz Maciej Godek]

2017-06-30 22:33 GMT+02:00 Catonano <address@hidden>:

> On the irc channel I was suggested that it might have been a good fit for
> my use case
>
> I took a look at it in the manual
>
> I'm perplexed. I don't understand it
>
> How is it supposed to be used ?
>
>
It's very simple (at least from the point of view of a user)
When it is tempting to write something like

(define (within-context action)
  (enter-context)
  (action)
  (leave-context))

you simply change it to

(define (within-context action)
  (dynamic-wind
    (lambda () (enter-context))
    action
    (lambda () (leave-context))))

The thing is, that in general (action) may transfer control outside of the
scope of that particular context (like, using call/cc or exceptions) -- and
in such situations, we would like the (leave-context) handler to be
invoked. If and we ever get back there, we wish that the (enter-context)
were invoked again.



> The provided example is somewhat contrived, I couldn't understand it
> anyway.
>
> My use case is basic, really.
>
> I have a scheme wrap around a C library for reading xls files, freexl.
>
> Freexl uses a pointer to a structure tha represents the opened xls file and
> its contents
>
> Each function writes/reads in the memory region pointed to such pointer.
>
> In the end, it requires to use a function that closes the file AND frees
> all the involved structures in memory.
>
> So my idea was that I would have gotten a simple macro, like this
>
> (with-xls-file "path/to/my/xls-file.xls" handler-ptr
>    (do-something handler-ptr)
>    (do-something-more handler-ptr))
>
> and this would have expanded to
>
> (freexl-open "path/to/my/xls-file.xls" handler-ptr)
> (freexl-do-something handler-ptr)
> (freexl-do-something-more handler-ptr))
> (freexl-close handler-ptr)
>
> Do I need dynamic-wind at all ?
>

If you don't use dynamic-wind, some of the possible use cases will not be
covered. Non-local transfers of control will break the system.

I believe you'd like to assume that there shouldn't be any non-local
transfers of control, but actually you can't know this. And the interface
to dynamic-wind is very straightforward, so there's no excuse for not using
it.

Here's a simple real life example in Scheme:

(define current-working-directory getcwd)(define change-directory chdir)

(define (with-changed-working-directory dir thunk)  (let ((cwd
(current-working-directory)))    (dynamic-wind (lambda ()
(change-directory dir))           thunk           (lambda () (change-directory
cwd)))))

HTH