[racket] obtaining the location of an identifier

I believe Robby's solution will work.  It's basically similar to what I had
in mind, but I was hoping I'd be able to avoid it :)

The crux of the issue is the need for unique identifiers for variable
locations.  I also track updates to vectors and structs, for example, and
this is easy precisely because I can use their identities to compute the
"location" of the update.  So, boxing values in "raw" variables will have
the same effect.

In terms of implementation, though, it'll be trickier.  As far as I can
tell, this approach involves whole-module analysis and transformation:

* Expand the module completely.
* Find all uses of set! and collect their targets.
* Rewrite the def and all uses of target variables to perform auto boxing
and unboxing.

If there are tricks/constructs that I should use for the above steps, I'd
love to hear them.   Or, of course, if there is some way to obtain variable
location identifiers from the runtime, that would be great to know as well.

Emina





On Wed, Nov 20, 2013 at 5:19 AM, Robby Findler
<robby at eecs.northwestern.edu>wrote:

> I didn't try it, but you it might work to use local-expand and then find
> all the binding forms and then rewrite them to use boxes (or some other
> source of uniqueness you might have around).
>
> Robby
>
>
> On Wed, Nov 20, 2013 at 12:53 AM, Emina Torlak <emina at eecs.berkeley.edu>wrote:
>
>> This is how my solution currently works, but unfortunately, it's not
>> quite right.  Here is a small example that demonstrates why, assuming the
>> implementation based on free-id-table:
>>
>> (define (cell init)
>>   (let ([x init])
>>     (case-lambda
>>       [() x]
>>       [(v) (set! x v)])))
>>
>> (define foo (cell 0))
>> (define bar (cell 1))
>>
>> > (foo 2)
>> > (bar 3)
>> > (foo)
>> 2
>> > (bar)
>> 3
>> > (dict->list global)
>> '((.#<syntax:18:17 x> . 3))
>>
>> In the above scenario, I need the global map to contain two bindings:
>>  one for the location of 'foo.x' and the other for the location of 'bar.x.'
>>
>>
>> Emina
>>
>>
>>
>> On Tue, Nov 19, 2013 at 10:31 PM, Sam Tobin-Hochstadt <
>> samth at cs.indiana.edu> wrote:
>>
>>> 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
>>> >
>>> >
>>> > ____________________
>>> >   Racket Users list:
>>> >   http://lists.racket-lang.org/users
>>> >
>>>
>>
>>
>> ____________________
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>>   http://lists.racket-lang.org/users
>>
>>
>
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