Re: [Chicken-users] Threads and dynamic-wind still problematic

[Tobia Conforto]

John Cowan wrote:
> Tobia Conforto scripsit:
> > I wonder how other compilers do it.  For example, I find Java's  
> > (and Python's) try/finally syntax quite useful.  I've always  
> > thought dynamic-wind was Scheme's equivalent construct, but it  
> > would appear I was mistaken.
>
> Here's the relevant writeup from Taylor Campbell's (Riastradh's)  
> blag, slightly edited.

Thank you, very instructive.

> (UNWIND-PROTECT form protection-form ...), a special form, evaluates  
> the first form and returns its value, but before returning the value  
> also executes the protection forms.  Furthermore, if a throw in the  
> primary form transfers control to outside the whole UNWIND-PROTECT  
> form, this, too, will execute the protection forms.

This is a restricted definition of what unwind-protect does in CL. It  
doesn't just execute the protection forms before a normal return and  
an exception, but also before return-from, go, and other kinds of non- 
local exits that in Scheme would be coded with call/cc.

But if the definition above (executing the finalizers before normal  
returns and exceptions only) is all that matters, it's not hard to  
write:

(define (unwind-protect thunk finalizer)
  (let* ((original-e-h (current-exception-handler))
         (result (with-exception-handler
                   (lambda (exn)
                     (with-exception-handler original-e-h finalizer)
                     (original-e-h exn))
                   thunk)))
    (finalizer)
    result))

Or if you care for multiple values:

(define (unwind-protect thunk finalizer)
  (let ((original-e-h (current-exception-handler)))
    (call-with-values
      (lambda ()
        (with-exception-handler
          (lambda (exn)
            (with-exception-handler original-e-h finalizer)
            (original-e-h exn))
          thunk))
      (lambda vals
        (finalizer)
        (apply values vals)))))

I believe this is much more of a Chicken version of try/finally than  
dynamic-wind is.


Tobia