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Re: A proper tail version of with-fluids

From: Stefan Israelsson Tampe
Subject: Re: A proper tail version of with-fluids
Date: Fri, 4 Jan 2013 11:58:43 +0100

You are correct Noha, but please play down the impressed part, we have not reached the 
goal yet.

1. I did not implement the proper full scale one where we actually can
(In case of knowing that no continuation have been stored, get proper tail calls)
The reason is that 1. It contains a significant overhead, 2. I'm yet a little ignorant
about the inner workings of guile fluids. But in principle, one add meta information
to the fluids so that one can know that it's safe to overwrite the old fluid.

2. The one I implemented was simply pushing things on the dynstack located 
on the heap. This still saves a lot of memory because the function stack was 
handled more optimal because we used the tail call mechanism. The trick was to
be able to unwind automatically in a correct way when we issue a return.

3. Although I coded a version that should mix well with continuations, I haven't tested 
it yet.


On Fri, Jan 4, 2013 at 12:44 AM, Noah Lavine <address@hidden> wrote:
I think with-fluids could at least be semi-tail-recursive. If you imagine a normal non-tail-recursive implementation, you might get to a point where your continuation is going to set a fluid back to a value, and then the *next* continuation is going to set that fluid back to *another* value. Since these are only set!s and have no side-effects, you could somehow notice that and not do the first set!. So your continuation only needs to have one set! for each fluid, no matter how many times that fluid is changed by with-fluids.

But that seems tricky to implement. I haven't looked at how Stefan did it, but I'm impressed.


On Thu, Jan 3, 2013 at 6:36 PM, Ludovic Courtès <address@hidden> wrote:
Hi Stefan,

Stefan Israelsson Tampe <address@hidden> skribis:

> (define f (lambda (n) (if (= n 0) (fluid-ref a) (with-fluids ((a n)) (f (-
> n 1))))))
> with the modified VM:
> scheme@(guile-user)> (f 10000000)
> $2 = 1
> with the old VM, it craches. It works!

Hmm, I can’t see how ‘with-fluids’ or ‘parameterize’ could be
tail-recursive given that it uses ‘dynamic-wind’.  Am I missing


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