[racket] obtaining the location of an identifier

I think that just identifiers and `free-id-table`s should work here.
Here's your example:

#lang racket

(require syntax/id-table (only-in racket [set! #%set!]))

(define global (make-free-id-table))

(define-syntax-rule (location-of id) #'id)

(define-syntax-rule (set! id expr)
  (let ([v expr])
    (dict-set! global (location-of id) v)
    (#%set! id v)))

> (define x 1)
> (set! x 2)
> (set! x 3)
> (for/list ([(k v) (in-dict global)]) (list k v))
'((.#<syntax:4:8 x> 3))

Also at https://gist.github.com/samth/7558673

Sam

On Mon, Nov 18, 2013 at 2:43 PM, Emina Torlak <emina at eecs.berkeley.edu> wrote:
> I'm using Racket to implement a language for which I need to track state
> updates---in particular, variable mutation using set!.  For example,
> consider this module definition:
>
> #lang racket
>
> (require (only-in racket [set! #%set!]))
>
> (define global (make-hash))
>
> (define-syntax-rule (location-of id)
>   (#%variable-reference id)) ; doesn't quite do the right thing
>
> (define-syntax-rule (set! id expr)
>   (let ([val expr])
>     (hash-set! global (location-of id) val)
>     (#%set! id val)))
>
> When I evaluate the following sequence of forms against the above
> definition, I would like the global hash map to contain just one binding
> that maps the location for 'x' to the value 2.  With the above
> implementation I get two map entries, since variable-reference doesn't quite
> do what I hoped it did:
>
>> (define x 0)
>> (set! x 1)
>> (set! x 2)
>> x
> 2
>> global
> '#hash((#<variable-reference> . 1) (#<variable-reference> . 2))
>
> Is there another construct in Racket that I could use for this purpose?  If
> not, can something like this be implemented and how much work would it
> entail?
>
> I have a purely macro-based solution that works for the most part, but it's
> fragile and there are corner cases for which it is just wrong.  So, before
> trying to fix that, I was wondering if there is a nicer way to solve it by
> somehow getting handles for variable locations that are comparable using eq?
> or equal?
>
> Thanks!
>
> Emina
>
>
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