2017-06-30 23:48 GMT+02:00 Panicz Maciej Godek <godek.mac...@gmail.com>:
> > > 2017-06-30 22:33 GMT+02:00 Catonano <caton...@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)
