From: Panicz Maciej Godek <godek.maciek@gmail.com>
To: Catonano <catonano@gmail.com>
Cc: "guile-user@gnu.org" <guile-user@gnu.org>
Subject: Re: dynamic-wind
Date: Fri, 30 Jun 2017 23:48:23 +0200 [thread overview]
Message-ID: <CAMFYt2YEtzUWkqRveeJRR8GXwQ_Dom6t_saLYy8_=XsfOgy3QQ@mail.gmail.com> (raw)
In-Reply-To: <CAJ98PDyvUuTTTeSYBJVtRmskiqijLn+cLGCqD18LRboQTY7cbg@mail.gmail.com>
2017-06-30 22:33 GMT+02:00 Catonano <catonano@gmail.com>:
> 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
next prev parent reply other threads:[~2017-06-30 21:48 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-30 20:33 dynamic-wind Catonano
2017-06-30 21:48 ` Panicz Maciej Godek [this message]
2017-07-02 6:00 ` dynamic-wind Catonano
2017-07-02 6:01 ` dynamic-wind Catonano
2017-07-02 11:58 ` dynamic-wind Chris Vine
2017-07-05 6:14 ` dynamic-wind Catonano
2017-07-05 8:23 ` dynamic-wind David Kastrup
2017-07-08 20:03 ` dynamic-wind Amirouche Boubekki
2017-07-08 21:34 ` dynamic-wind Marko Rauhamaa
2017-07-09 7:21 ` dynamic-wind David Kastrup
-- strict thread matches above, loose matches on Subject: below --
2017-07-09 12:59 dynamic-wind Chris Vine
2017-07-09 14:09 ` dynamic-wind Vítor De Araújo
2017-07-09 14:49 ` dynamic-wind Chris Vine
2017-07-17 10:04 ` dynamic-wind Catonano
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