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Re: dynamic-wind

From: Chris Vine
Subject: Re: dynamic-wind
Date: Sun, 9 Jul 2017 13:59:21 +0100

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

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.


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