[racket-dev] Fwd: Q. about "Directly Reflective" paper

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Wed Nov 17 07:15:37 EST 2010

IMO, this is what one should expect from someone when an error is
pointed out in their paper.

It is a bit surprising to me that something like that got thru at
HOSC, of all places, tho.


On Tue, Nov 16, 2010 at 11:41 PM, John Clements
<clements at brinckerhoff.org> wrote:
> Well, he's generous about it; here's what he had to say.
> John
> Begin forwarded message:
> From: Aaron Stump <aaron-stump at uiowa.edu>
> Date: November 16, 2010 5:58:42 PM PST
> To: John Clements <clements at brinckerhoff.org>
> Subject: Re: Q. about "Directly Reflective" paper
> Reply-To: astump at cs.uiowa.edu
> Hi, John.
> I think you are right about this.  Lambda abstractions evaluate to
> #procedures in Scheme R5RS, and so it is not possible to take a cdr or car
> of one of these.  I have no idea why I wrote this (four years ago -- there
> was a major lag between acceptance and publication at HOSC).  I will add a
> note to my web page about this, and possibly upload a revised version of the
> paper without this incorrect statement.
> Aaron
> On Tue, Nov 16, 2010 at 4:18 PM, John Clements <clements at brinckerhoff.org>
> wrote:
>> I'm reading your paper, "Directly Reflective Meta-Programming," and I got
>> stuck early on a remark of yours about Scheme:
>> > A meta-programming language is scope safe (or hygienic) iff variables
>> > may not be captured or escape their scopes during computation. Dynamic
>> > variables in Emacs LISP and Common LISP are a good example of a violation of
>> > scope safety [30, 24]. Scheme R5RS’s macro language is designed to be scope
>> > safe [21]. Other constructs in Scheme R5RS, however, enable violation of
>> > scope safety, even though the language does not have dynamic variables. For
>> > a violation of scope safety in spirit, though not technically, we have that
>> > (caddr ’(lambda (x) x)) evaluates to x. According to the R5RS language
>> > definition, ’(lambda (x) x) is a literal expression, and hence the
>> > occurrences of x in it are not variables at all, but just (unscoped) literal
>> > data. So in this example, a variable has been created (namely, the resulting
>> > unquoted x), but not by means of removing it from its scope. Using
>> > quasiquotation, however, the example may be modified to give a true
>> > violation of scope safety. The following expression extracts the variable x
>> > from its scope, by transforming the binding lambda expression into a piece
>> > of literal data, and then extracting and evaluating the quoted variable.
>> > ((lambda (y) (eval ‘(car (cdr (cdr ’,y))))) (lambda (x) x))
>> This looks pretty goofy to me.  Do you know of R5RS implementations that
>> actually allow you to peel apart a 3d value like this?  Racket (nee
>> MzScheme) certainly doesn't.
>> Thanks!
>> John Clements
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