Re: dynamic-wind

[Chris Vine <vine35792468@gmail.com>]

On Sun, 2 Jul 2017 08:00:58 +0200
Catonano <catonano@gmail.com> wrote:
> 2017-06-30 23:48 GMT+02:00 Panicz Maciej Godek
> <godek.maciek@gmail.com>:
> > 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.
[snip]
> > 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)))))
> 
> aahh I see now
> 
> Well. the manual is not clear enough, I think, in this regard
[snip]
> 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)

Possibly the wording of the manual about dynamic-wind at
https://www.gnu.org/software/guile/manual/html_node/Dynamic-Wind.html#Dynamic-Wind
could be improved.  Stating that "If, any time during the execution of
thunk, the dynamic extent of the dynamic-wind expression is escaped
non-locally, out_guard is called" is a somewhat incomplete description
of when the out guard is called, since it is also invoked when the body
of the dynamic wind returns normally.  If you think the wording could
be improved I suggest you file a bug with a suggested wording.  Having
said that, I think that you are getting unnecessarily tied up with
call/cc rather than concentrating on the principle.

dynamic-wind is much more general than just for dealing with database or
network connections, which I think would be a poor focus for the manual.
dynamic-wind is relevant whenever control leaves the dynamic wind block
in question, either normally or by an exception or by invocation of a
specific continuation object such as via call/cc, call/ec or
abort-to-prompt.  Many languages have something similar - it is
analogous in some sense to a C# or Java finally block, which executes
whenever control leaves the associated try block, whether normally or by
virtue of an exception or a return, break, continue or goto statement.

The somewhat more unusual feature of dynamic-wind it that, because
the scheme language permits a dynamic context to be re-entered by the
invocation of a continuation object (say, as obtained by call/cc),
there is also an in guard made available to deal with entry into the
dynamic wind block.

Having said all that, dynamic-wind is not the answer to all cases where
control leaves a block of code non-locally.  It is best suited to cases
where invocation of a continuation object or the raising of an
exception fall to be dealt with in the same way, say by releasing a
resource such as by unlocking a mutex or closing a port.  But that is by
no means always the case - the invocation of a continuation object is
usually a deliberate programmatic strategy, whereas the raising of an
exception is usually not so and instead represents an unexpected
failure event of some kind in the program.

Chris