Re: Special variables to relax boxing

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

On Thursday, March 21, 2013 03:03:06 PM Mark H Weaver wrote:
> Stefan, you're still describing your proposal in terms of low-level
> implementation details such as stacks.  In the general case, we cannot
> store environment structures on the stack.  Furthermore, in the
> general case *all* variables in scheme are bound to locations, not
> values.  Only in special cases can we use stacks, and only in special
> cases can we avoid boxing variables.  These are only _optimizations_.
> 
> If you're serious about this proposal, please read sections 3.1 and
> 3.4 of the R5RS carefully.  Explain your proposed _semantics_ (not
> the implementation details) in those terms, where *all* variables are
> bound to _locations_, and where there is no stack at all (everything
> is conceptually stored in a garbage-collected heap).
> 
> We need to understand the *semantics* in the simplest possible terms
> before we even begin to think about how to implement it.
> 
>     Thanks,
>       Mark

Ok, the sematics for the simple version is are,

Assume k, the continuation associated with with a dynamic wind or
unwind assume that there is a map from each continuation (k,id) 
to a value and getting and setting of this value is done through
ref-get and ref-set, assume that the winder and the rewinder lambda 
takes a first argument k beeing the continuation under action, finally
make-id will make a unique object. then the semantic would be:

(define-syntax-rule (with-special (a) code)
  (let ((id (make-id)))
    (dynamic-wind 
       (lambda (k) (set! a (ref-get k id)))
       (lambda () code)
       (lambda (k)  (ref-set! k id a)))))

A possible refinment of this is
associate to k two predicates e.g.
 (do-wind? k kind) predicate and a (do-unwind? k kind) wich takes
a parameter kind, then use the semanics

(define-syntax-rule (with-special (a kind) code)
  (let ((id (make-id)))
    (dynamic-wind 
       (lambda (k) 
         (when (do-wind? k kind) 
           (set! a (ref-get k id))))
       (lambda () code)
       (lambda (k)  
         (when (do-unwind? k kind)
           (ref-set! k id a))))))

Hopes this helps!

/Stefan