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Re: dynamic-wind

From: Vítor De Araújo
Subject: Re: dynamic-wind
Date: Sun, 9 Jul 2017 11:09:25 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Icedove/38.8.0

On 09/07/2017 09:59, Chris Vine wrote:
> On Sun, 09 Jul 2017 00:34:13 +0300
> Marko Rauhamaa <address@hidden> writes:
>> Hm. Python's try/finally has several uses in virtually every
>> program.
>> Trouble is, Scheme's continuations make it impossible to know when
>> something is really final.  
>> In fact, implementing coroutines and cooperative multitasking using
>> continuations almost guarantee a repeated back-and-forth through
>> dynamic-wind.
>> I strongly suspect Scheme's continuations are more trouble than they
>> are worth.  
> I disagree with that.  On the first point, you know that a
> dynamic-wind block can no longer be re-entered (if that is what you mean
> by "really final") when the continuation object concerned is no longer
> accessible.  At that point it, and all references to other objects
> encapsulated by the continuation, will be released in the ordinary
> way.  You also know the same when your continuation is only an escape
> continuation.

That helps the implementation know if a continuation will not be entered
again, but it does not help when you want to do the kinds of things you
do with unwind-protect or try/finally in other languages. For example,
with unwind-protect, you can open a port or another resource and ensure
it will be closed if control escapes the unwind-protect form. You can do
that with dynamic-wind, but it is less meaningful to do so because
control can be re-entered again. There is no language construct (as far
as I know – maybe there is in Guile) that can detect that flow has
exited the form and *will never enter it again*. So the presence of
continuations make operations like unwind-protect less meaningful. I
don't know what is the Scheme way to address these situations.

> Secondly, this is something of an irrelevance.  I have found it very
> rare that one would want to use dynamic-wind when implementing
> co-operative multi-tasking with coroutines (at any rate,
> only does so for thread pool
> thread counts, and that is to cater for exceptions in local code rather
> than for jumps via continuation objects).  Jumping out of a
> dynamic-wind block using a coroutine is generally inimical to the kind
> of asynchronous programming that coroutines are used for: you generally
> don't want to unset the state of the continuation, and then set it up
> again when you re-enter.  You normally want to leave it just as it was
> at the time you yielded.
> I may be mistaking you for another poster, but I think you have
> previously said that you prefer the inversion of control ("callback
> hell") style of asynchronous programming to using coroutines.  You
> would not usually think of using dynamic-wind there either, I hope.
> Scheme's continuations are very useful.  Guile's delimited
> continuations are even more so.  Dynamic-wind not so much, because it
> is a very blunt instrument.
> Chris

Vítor De Araújo

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