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

From: Tobia Conforto
Subject: Re: [Chicken-users] Threads and dynamic-wind still problematic
Date: Wed, 11 Mar 2009 00:44:21 +0100

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))

Or if you care for multiple values:

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

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


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