Re: dynamic-wind

[Catonano <catonano@gmail.com>]

2017-06-30 23:48 GMT+02:00 Panicz Maciej Godek <godek.maciek@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
>
>

aahh I see now

Well. the manual is not clear enough, I think, in this regard

In fact, I think Amirouche got tricked too

When I was playing with his library for accessing a Wiredtiger based db,
there was a bug that when some query went wrong, the connection to the db
didn t get released and so the subsequent connections couldn t be
established

I had to quit Guile and start it again.

I was using

(with-context some-connection
  (query...))


The code for with-context is here (and it doesn t use dynamic-wind as far
as I can see)
https://framagit.org/a-guile-mind/guile-wiredtiger/blob/master/wiredtiger/extra.scm#L328


I think that the use case should be explicitly stated, in the manual, as
you did here

Also the example should be about some db or network connection, not about
call/cc

So that understanding dynamic-wind and its use case wouldn t require
understanding call/cc (which I still don t get)