Re: Some questions about concurrency (mostly)

[Chris Vine]

On Fri, 6 Nov 2020 17:20:04 +0100
"Jörg F. Wittenberger" <Joerg.Wittenberger@softeyes.net> wrote:
> Am Thu, 05 Nov 2020 23:22:09 +0100
> schrieb Fredrik Appelberg <fredrik@appelberg.me>:
> 
> > 3. I'm new to dynamic-wind. If I wanted to create a general form for
> >    executing a thunk protected by a mutex, would this be a good idea?
> > 
> >      (define (with-lock mutex thunk)
> >        (dynamic-wind
> >          (lambda () (mutex-lock! mutex))
> >              thunk
> >              (lambda () (mutex-unlock! mutex)))))
> > 
> >    I read somewhere that the before- and after-guards might execute
> >    multiple times, but then again I'm not really sure under what
> >    circumstances so I might be way off.
> 
> This approach is bound to fail badly.
> 
> It works just as long as there are a) no exceptions raised in `thunk`
> b) no code, not even in a library does any `call/cc`.  Including
> `call/cc` hidden in exception handlers (srfi-12, srfi-34 etc.)

For my elucidation, why?  The indentation of the code isn't ideal but
the whole purpose of dynamic-wind is to handle code leaving the thunk in
case of exception, application of a continuation object or regular
return.

Admittedly it would be more usual for this to be implemented as a macro
rather than a function with a thunk, but that is a minor matter.