Re: A proper tail version of with-fluids

[Stefan Israelsson Tampe]

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.

/Stefan

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.

Noah

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
something?

Ludo’.