Re: Special variables to relax boxing

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

On Thursday, March 21, 2013 11:35:19 AM Noah Lavine wrote:
> (lambda ()
>   (let ((x 5))
>     (set! x (compute-1 x))
>     (set! x (compute-2 x))
>     x))
> 
> becomes
> 
> (lambda ()
>   (let ((k1 (lambda (x) (k2 (compute-2 x))))
>         (k2 (lambda (x) x)))
>     (k1 (compute-1 x))))
> 
> However, this rewriting idea runs into trouble when you try to figure
> out what happens to mutable variables that have been captured by
> closures (as Mark said). I don't know what the right solution is
> here.

The semantic is that cpatured variables in lambdas will be restored to
the value when the continuation left the guard. And this is something
you want many times e.g. consider,

(let ((a 0)
      (f (case-lambda (() a) ((b) (set! a b)))))
 (with-special (a)
   (for-each (lambda (x) (if (blah x) 
                             (k x f)
			     (abort-to-prompt 'tag a)))
      a-list)
   (f)))

Assume that you would like to be able to go back to the tag abort many
times in a redo/undo sequence, then it is natural for the a referenced
in f to be restored as well. But as you know sometimes this is not
what we want. As I said, using dynwinds and fluids it's really
possible to get this behavior today but it's really a bloated
solution. Anyhow the good news with this semantic is as with the
current behavior of assigned variables it's easy to reason with the
code although one risk some severe bugs. 

A good question though is
what we should use as default for e.g. a python implementation where
prompts is not included per se, but which we might want to add as a
special scheme flavour of python using an import e.g. what is the
natural thing to expect for the variables that are assigned.


Also in the previous email I had a suggestion to for each with
variable, a, add two slots in the stack e.g.

a1
val1
s2
val2
...

But there is a good reason to asign a kind to this as well e.g.
a1
val
kind
a2
val2
kand2
...

then we could use a predicates when we rewind a continuation e.g.

at rewind:
(when (pred1 kind1)
   (set! a1 (ref-val val1)))
...

and and at wind
(when (pred2 kind1)
   (set-val val1 a1))
...

A simple version of this with a complex implementation is what I
actually use in guile-log. 

BTW after this deep realizations of the guile variable behavior I'm 
ready
to implement a much improved version of these special variables in
guile-log that is both efficient and featureful. Cool!

Have fun
/Stefan